Microbial fossils from the lower Yudoma Suite, earliest Phanerozoic, eastern Siberia

Microbial fossils from the lower Yudoma Suite, earliest Phanerozoic, eastern Siberia

Precambrian Research, 13 (1980) 109--166 109 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands MICROBIAL FOSSILS FR...

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Precambrian Research, 13 (1980) 109--166

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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

MICROBIAL FOSSILS FROM THE LOWER YUDOMA SUITE, EARLIEST PHANEROZOIC, EASTERN SIBERIA*

SU-CHU C. LO

Biogeology Clean Laboratory, University of California, Santa Barbara, CA 93106 (U.S.A.) (Received August 7, 1979; accepted January 15, 1980)

ABSTRACT Lo, S.C., 1980. Microbial fossils from the lower Yudoma Suite, earliest Phanerozoic, eastern Siberia Precambrian Res., 13: 109--166. Well-preserved nannofossils are here described from black chert lenses in dolomitic and carbonaceous limestones of the lower Y u d o m a Suite from the Khanda (or Belaya) and Aldan Rivers in eastern Siberia. In this area, Y u d o m a rocks conformably underlie Lower Cambrian and unconformably overlie Upper Proterozoic strata. Radiometric ages based mainly on glauconite from weakly metamorphosed strata date the Y u d o m a as 675 ± 25 M a to 570 ± 10 M a ago. A K-Ar date for one of the localitieshere studied is 650 Ma. The Y u d o m a nannofossils are preserved in the chert as light amber to dark organic residues. The m o d e of preservation is similar to microbial assemblages in cherts from the older and n o w classic Gunflint and Bitter Springs formations. Sixteen categories of nannofossils are identified,including 12 species and 4 morphogroups of filamentous and coccoid microorganisms assigned to 11 genera. Of these, three genera and eight species are new. Six of 16 categories are identical to or closely comparable with forms in the roughly 850 M a old Bitter Springs microbiota. Seven of the 16 categories are identical to or closely comparable with forms in the roughly, 1,900 M a old Belcher Islands microbiota. Most of the elements of this well-preserved microbiota appear to be cyanophytic, with coccoid forms dominant over filamentous. One form, Micrhystridium, m a y be planktonic; the others are probably benthic.

Age, stratigraphic relations, and microflora all are most consistent with an Ediacarian and Early Phanerozoic age (i.e., pre-Cambrian or sub-Cambrian, but not Precambrian in the sense of conventional usage).

INTRODUCTION

It is apparent that many problems concerning the origin of lifeand biological evolution require data from the study of the pre~ambrian nannofossil record. During the past two decades, many systematic investigationsof preCambrian microbiotas were completed, and they provide nannofossil evidences for a better understanding of pre-Cambrian lifein the informal sense. *Contribution Number 102, Biogeology Clean Laboratory, University of California, Santa Barbara, California 93106, U.S.A.

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(Because sub-Cambrian rocks equivalent to the Ediacarian System are here considered to belong properly to the beginning of the Phanerozoic Eon and Paleozoic Era, and n o t to the Precambrian of conventional usage; the latter term is here avoided in favor of the expression pre-Phanerozoic as discussed by Cloud, 1976a, b.) Most of the reported pre-Cambrian nannofossils have been referred to as cyanophytes or other simple algae with a fair degree of certainty. The exact processes involved in their preservation, however, are poorly understood, and little is known of their morphological changes during diagenesis. In this study, the chief nannofossils of the Yudoma microbiota are formally described and their t a x o n o m y is discussed. The purpose of this report is to document the Yudoma microbiota f o r future reference and to contribute to a more complete pre-Cambrian nannofossil record. Samples on which this study is based were collected by Preston Cloud of the U.S. Geological Survey during the summer of 1975 in field work made possible by arrangement between the U.S. and U.S.S.R. Academies of Sciences and with field guidance from M.A. Semikhatov of the Institute of Geology, U.S.S.R. Academy of Sciences. The first report of well-preserved nannofossils in the Y u d o m a Suite was that of Cloud et al. (1977). GEOLOGY

Regional relationship The Yudoma (or Udoma) Suite is one of the most widespread lithostratigraphic units on the Siberian Platform - - t h e Soviet terms " S u i t e " and "Subsuite" being approximately equivalent to Formation and Member in American practice. Outcrops of the Y u d o m a Suite are b o u n d e d by the Tyry River basin to the north, the upper Uchur and Maymakan Rivers to the south, the upper Maya River to the east, and the upper Aldan River to the west (Fig. 1). Structurally, this area includes part of.the Aldan shield in the west, the Uchur-Maya block in the center, and the YudomaoMaya trough in the east. Throughout this area, the Yudoma Suite is generally little deformed, dipping gently north in the Uchur-Maya block and the eastern and northeastern flanks of the Aldan shield. In the Yudoma-Maya trough, however, Yudomian rocks are folded with dips of 15~--30 ° and are overturned near large faults (Semikhatov et al., 1970). Attention was first drawn to the most characteristic Yudomian rocks -"light-colored, saccharoidal dolomites and calcareous dolomites" - - b y V.N. Zverev in 1913. Since then, similar light~olored limestones and dolomites have been described from many locations in the Uchur-Maya region and correlated from one place to another. The Yudoma Suite was first named as an independent subdivision in 1940 by Yu.K. Dzevanovsky in the northeastern part of the Aldan shield.

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For a long time it was incorrectly thought that this suite was very uniform throughout the entire Uchur-Maya region, consisting exclusively of lightcolored dolomites and limestones with a few intervals of sandstone and conglomerate near its base, and its age was considered to be early Cambrian. It was not until the late 1950's and early 1960's that the facies variability of

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Fig.1. Index map of e a s t e r n Siberia; shading encompases outcrops of the Yudoma Suite. Modified from map by Cartographic Division, National Geographic Society, 1960.

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the Yudoma Suite was recognized. Silty carbonaceous and other siliceous rocks were found, in addition to the light-colored dolomites, limestones, and occasional quartz sandstones (Semikhatov et al., 1970). Then B.M. Keller and B.S. Sokolov (1962) dated the Yudoma Suite as Vendian and removed it from the Cambrian system. A.G. Losev and V.I. Tatarinov (1958) recognized a two-member structure of the Yudoma Suite in the Bolshoi (or Great) Aim basin. Subsequent studies by numerous geologists confirmed this two-member division (Semikhatov et al., 1970). The upper Yudoma Suite is characterized by light-colored dolomites, mottled near the top while the lower Yudoma Suite is characterized by oncolitic dolomites, argillites, and sandstones. The boundary between the two subsuites is placed at the top of a cherty, highly carbonaceous limestone which overlies a thin layer of mudstone. The stromatolites of the Yudoma Suite are also seen as supporting this twofold division, with the stromatolites of the lower Yudoma resembling the upper Riphean forms, while those of the upper Yudoma are more similar to known Cambrian forms (Krylov, 1967b, 1968, in Semikhatov et al., 1970). Six reference sequences of the Yudoma Suite have been defined and correlated in the Uchur-Maya block and in the Yudoma-Maya trough (Semikhatov et al., 1967). These are as follows. (1) Yudomian-type sequence: located in the northern part of the marginal zone of the Yudoma-Maya trough, adjacent to the boundary between the Uchur-Maya depression and the Yudoma-Maya trough. (2) Suordakh sequence: developed in the deeper parts of the Yudoma-Maya trough, in the AUakh Yun' and Belaya (or Khanda) basins. (3) Middle Maya sequence: found on the eastern side of the Uchur-Maya block, in the middle Maya and upper Uchur basins. (4) Uchur sequence: located in the western part of the Uchur-Maya block (Uchur River, upper reaches of the Bloshoi Aim). (5) Aldan sequence: developed where the Yudoma Suite rests directly on Archean rocks at the crest and flanks of the Aldan shield. (6) Upper Maya sequence: found in the southern Yudoma-Maya trough. Detailed descriptions of these sequences are found in Semikhatov et ah (1967, 1970). According to these authors, all sequences are characterized by: (1) a specific stromatolite assemblage of similar compositions, (2) separation by a basal unconformity from more ancient underlying rocks of various ages, and (3} conformable succession by the "Variegated Suite", which contains at its base fossils of early Tommotian (Early Cambrian) age. These characteristics are consistent with a transition between Yudomian strata and Tommotian strata, and a Paleozoicage for both. Several other suites of Siberian rocks are correlated by Soviet geologists with the Yudoma Suite of the Uchur-Maya area. These are: (1) the Starorechensk Suite of the Anabar massif, (2) the Khorbuson Group of the Olenek uplift, (3} the Kharayuetekh Suite of the Kharaulakh salient, and (4) other strata of adjacent regions in northern Siberia (Semikhatov et al., 1970).

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All of these strata are unified by Soviet geologists as a single stratigraphic complex: the Yudoma Complex, typified by the Yudoma Suite.

Regional stratigraphic variation, age, and correlation V.A. Yarmolynk (1946) first recognized that the Yudoma Suite transgressively overlaps more ancient deposits throughout the Uchur-Maya region. Yudomian rocks rest on different suites of Riphean age (pre-Phanerozoic), beginning above with the upper Riphean Kandyk Suite and ending below with the lower Riphean Gonam Suite, as dated by radiometric numbers and stromatolite assemblages. In the area of the Yudoma-Maya trough, from which samples studied in this report were collected, Yudomian rocks are conformably overlain by the "Variegated Suite", a greenish~ gray, rarely pale yellowish and pink limestone with local glauconite (Semikhatov et ai., 1970). This "Variegated Suite" has also been called the Pestrotsvel Formation (e.g., Semikhatov et al., 1967; Cowie and Rozanov, 1973; Matthews and Missarzhevsky, 1975). In the eastern part of the Yudoma-Maya trough, thickly bedded fossiliferous limestones of the "Variegated Suite" conformably overlie light gray and brownish, fine,rained dolomites of the Yudoma Suite which lack a skeletal fauna. Archaeocyathids and hyolithids are abundant at the base of the "Variegated Suite". V.V. Missarzhevsky (in Semikhatov et al., 1970) refers these variegated limestones with their shelly fauna to the lower Tommotian Ajacicyathus sunnaginicus--Tiksitheca licis zone. Rocks of this upper "Variegated Suite" contain shelly fossils of middle Tommotian age, the Turkutheca annae and Dokidocyathus regularis zone, In the western part of the Yudoma-Maya trough, the limestones of the "Variegated Suite" contain only fragments of hyolithids, primitive cupshaped gastropods, and worm tubes. In this area, the fossfliferous limestones of the "Variegated Suite" also conformably overlie light~olored, massive, fine,rained Yudomian limestones that lack a typical skeletal fauna but locally contain the tubular problematical Metazoan known as Anabarites trisul. catus Missarzhevsky. In general, Yudornian rocks rest unconformably on paiaeontologically and radiometrically distinctive Riphean (pre-Phanerozoic) sediments but commonly grade upward to the overlying "Variegated Suite" or Pestrotsvel Formation (basal Cambrian). This stratigraphic setting makes study of the outcrops of the Yudoma Suite critical to a proper understanding of the transition from Proterozoic to Phanerozoic. In terms of Siberian stratigraphy the Pestrotsvel Formation is referred to the Tommotian stage and the Yudoma Suite to the Yudomian stage. Yudomian, Tommotian and Atdabanian are stages in the pre-Cambrian to Cambrian transition of the basal Phanerozoic bed of the Siberian Platform. The Tommotian stage precedes the Atdabanian stage which contains abundant archaeocyathids and trilobites. Although the base of the lower Cambrian

114 is still not agreed upon, the presence of trilobites in the Atdabanian is taken as evidence of a Cambrian age for it. The preceding Tommotian stage lacks trilobites (Rozanov, 1975), but is characterized by an abundant assemblage of skeletal fossils: archaeocyathids, sponges, hyolithids, primitive gastropods, brachiopods and numerous "algae" (Rozanov, 1975). The onset of this abundant fauna of shelly Metazoa is now taken by an increasing number of geologists as denoting the beginning of Cambrian history and therefore a basal Cambrian age for the Tommotian shelly fauna (e~., Rozanov, 1967, 1975; Missarzhevsky and Rozanov, 1968; Matthews and Missarzhevsky, 1975; Zhuravleva, 1970; Glaessner, 1972; Cowie and Cribb, 1978; provisionally, Cloud, 1976). If the base of shelly fauna is defined as the lower boundary of the Cambrian, then the preceding Yudomian is, strictly speaking, pre-Cambrian (but not Precambrian or Proterozoic) in age (see Cloud, 1976b) -- except where the clearly Cambrian "Nemakit-Daldyn" biota occurs, if the latter is truly Yudomian. For the "Nemakit-Daldyn" beds of northern Siberia and their equivalents, which are referred to the upper Yudomian stage by Soviet geologists, contain not only shelly fauna and tubular problematical forms, but also the distinctively Cambrian algae Epiphyton and Renalcis (Rozanov, 1975; Savitsky, 1975; Cloud, 1976b). But what is the relation of Yudomian to Paleozoic and Phanerozoic? Many Soviet geologists consider the Yudomian to be a terminal interval of the Riphean and somehow (in part or entirely) equivalent to the Vendian. The term Vendian of Sokolov (1972) was originally referred to the youngest complex of pre-Cambrian strata on the Russian Platform, which immediately precedes the oldest conventional Cambrian with shelly fauna and which locally contains non,shelly metazoan fossils of Ediacarian aspect. Sokolov (1972), like Cloud (1976b) considers the Vendian to be both pre~ambrian and earliest Paleozoic. Significantly, perhaps, there is a reported coincidence between the dating of rocks near the transition from Vendian to Cambrian on the Russian Platform and between Yudomian and Cambrian rocks on the Siberian Platform (Rozanov, 1975). However, Semikhatov et al. (1970) state that there are no convincing data demonstrating coincidence of their lower limits. Today, some geologists (e.g., Semikhatov et al., 1970; Keller, 1971) consider the Yudomian as preferable to Vendian as a unit of the standard stratigraphic scale of the terminal Riphean of the USSR, while Sokolov (in Cowie and Glaessner, 1975) considers the Yudomian merely as one of the regional facies of the Vendian and limits the term Vendian to strata younger than a prominent pair of diamictites (and probable tillites). On the other hand, based on the presence in these rocks of Ediacarian types of Metazoan fossils, Cloud (1968, 1976a) further concluded that in Sokolov's restricted sense, the Vendian and Correlative parts of the Yudomian should be considered as basal Paleozoic and basal Phanerozoic (as well as pre-Cambrian in the informal sense). Indeed, since Termier and Termier (1960) proposed the term Ediacarian as that of a new basal Paleozoic system

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and period and Cloud (1968) marshalled evidence supporting such a view, other geologists have also recognized that strata containing trilobites, archaeocyathids, hyolithids and other primitive shelly fossils are not u n c o m m o n l y preceded without obvious interruption of sedimentation by strata that contain elements of the Ediacarian fauna. The reported features of the Tommotian-Yudomian transition are of this nature. This evidence has most recently been reviewed by Stanley (1976). A number of "Ediacara".type fossils have now also been found from the strata which are stratigraphically correlatable to the Yudoma Suite of the Yudomian stage on the Siberian Platform. For instance, an Ediacarian fossil identified by B.S. Sokolov (per Savitsky, in Keller, 1963) as Rangea sp. is known from t h e K h a t y s p y t Formation, Khorbuson Group of the Yudoma Complex on the northwest flank of the Olenek Uplift, northern Siberia.Another fossil similar to Cyclomedusa plana Glaessner et Wade of the type Ediacarian is known from the Bernashov Beds of the Vendian Complex in the southwestern Ukraine (Zaika-Novatsky et al., 1968). Further reports of Edicarian fauna in the USSR have recently been elaborated by Fedonkin (1977) from upper pre-Cambrian deposits (Vendian, Valdai Series) of the Onega Peninsula in the White Sea area. Thus, the Yudomian seems to be properly assignable to the Ediacarian System, and the Ediacarian, characterized by the near-global representation of the oldest known soft-bodied Metazoan fauna, is biologically and stratigraphically transitional to the Cambrian, Phanerozoic and Paleozoic in its biological characteristics, and ought to be generally accepted as a basal Phanerozoic system as supported by Termier and Termier (1960), Cloud (1968, 1976a, b), and Sokolov (1972). To summarize, the following three features of the Yudomian in particular imply a Phanerozoic and Ediacarian affinity: (1) the presence of Ediacariantype fossils in Yudomian and Vendian strata; (2) the presence of premonitory shelly Metazoa, the tapering tubular Anabarites trisulcatus Missarzhevsky, in the Yudomian rocks in the Yudoma-Maya trough; and {3) the fact that upper Yudomian stromatolites are more like Cambrian than Riphean forms (I.N. Krylov, in SemiKhatov et al., 1970). Moreover, recognition of such a separation is assisted on a regional scale by the unconformable contact which sharply separates Yudomian strata from underlying Riphean sediments and other characters of paleogeography (the distribution of Tommotian rocks in the Uchur-Maya area is coincident with that of Yudomian rocks), paleoecology, and sedimentology that imply a closer relation to succeeding Cambrian than to the underlying Riphean strata (cf., Cloud, 1976a). Radiometric age determinations based primarily on K--Ar datings of glauconites give a time span for the Yudoma of 675 ± 25 Ma to 570 +- 10 Ma ago, according to Semikhatov et al. (1970). Similarly, Sokolov (1972) gives an age range for the Vendian from 680 ± 20 Ma to 570 + 10 Ma, and Cloud (1976) cites a range for Vend, Yudomian, or "Vendomian" as between %675 to 680 and % 570 Ma ago. Therefore, the Yudoma seems to record a historical

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interval of approximately 100 million years at the beginning of the Phanerozoic and immediately preceding the Cambrian. OCCURRENCE OF NANNOFOSSILS

Description of fossiliferouschert The nannofossils described in this study are preserved in black cherts collected from four differentlevelsof the lower Y u d o m a Suite. These fossiliferous cherts are non~tromatolitic and occur in thin layers and small lenses in limestone or dolomitic limestone. In this respect they differfrom the more commonly stromatoliticassociationsdescribed for most pre-Cambrian cherty nannofossil assemblages. These fossiliferouscherts are black, aphanitic,and non-laminated. Most of them display a waxy luster on fresh,broken faces. A few are dull black to gray and intersected by very fine,randomly oriented, quartz and calcitefracture fillings.Frequently, dark organic-richzones of cherts are surrounded by gray, less-fossiliferousor barren zones. Contacts of the chert lenses with surrounding carbonaceous rocks are sharp. The mineralogy of whole-rock and separated grains (~2/~m) of the fossiliferous cherts was established through X-ray diffractometry. Only quartz and calcite are recognized from the X-ray diffractometer patterns. In thin section, under microscopic examination, the chert is composed of: (1) predominantly cryptocrystallineand microcrystallinequartz; (2) lesseramounts of lengthfast,fibrous chalcedony; (3) minor yellow, brown and black amorphous organic matter and structuraUy well-preserved nannofossils; (4) minor opaque fine,rained pyrite crystalsto orange hematite framboids; and (5) occasional disseminated rhombs or veinletsof calcite or quartz. No evaporitic minerals or their pseudomorphs have been found. The chert matrix is dominanted by cryptocrystallineand microcrystalline quartz and lesseramounts of fibrous chalcedony. The lack of opal and the presence of fibrous chaloedony and microcrystallinequartz illustratethe gradational diagenetic change of silicain the Yudomian localities. Cryptocrystalline quartz is observed in a generally homogeneous random fabric while microcrystaUine quartz is shown in mosaic interlockingpatterns in the matrix. Remarkably, many radiatingchalcedony fibers,50--200 # m in diameter, occur in spheroidal habit and are surrounded by cryptocrystalline quartz, creating a fossil-likeeffect.The concentrically banded spheroidal structures of fibrous chalcedony, are probably due to the effect of the physical conversion from opal to chalcedony as they illustrate,the dehydration shrinkage effect (Pettijohn,1975). Small, irregularpatches of radiating clusters of chalcedony fibers are also found in the mosaic matrix of micro crystallinequartz. These patches measure about 0.25--0.39 m m in size and probably indicate crystallizationof silicain open cavities.Cavitiesin rocks at the time of silicificationare normally lined with chalcedony; the fibers

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stand normal to the cavity walls and serve to outline original voids (Williams et al., 1954). Consider the evidence for derivation of these bedded cherts from originally carbonate rocks. Under the microscope, euhedral rhombs and replaced relicts of calcite are observed floating in the matrix of cryptocrystalline quartz. Idiomorphic rhombs of carbonate are common in cherts associated with carbonaceous rocks. Commonly the rhombs consist of dolomite. However, from X-ray diffractometer patterns the author recognized only calcite in these cherts. Many fracture fillings and veinlets of calcite are found cutting through thechert. Irregular patches of calcite are also found, showing calcite twinnings. Frequently, brown to opaque, amorphous organic residues form the boundary zone between the calcite area and siliceous area (Fig.2). The presence and decomposition of organic matter probably prevented the dissolution of calcite (cf. Purdy, 1963). This may explain the unusual abundance of calcite in these fossiliferous cherts. If the calcite, protected by organic matter, did not entirely dissolve, the replacement by authigenic silica would not be complete.

Calcite appearing as euhedral rhombs, veinlets, and irregular patches showing twinning Fibrous chalcedony Organic material appearing on boundary of calcite and quartz

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Cryptocrystalline

quartz in the matrix

Fig.2. Petrology of chert from Cloud's locality 2 of 10/8/75. Diameter of the field of view is about 1.5 ram. Organic residues form the boundary zone between the calcite area and siliceous area.

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Many calciterhombs show irregular solution contacts with chalcedony. Some veinlets of calcite have been transected and replaced by cryptocrystalline quartz and chalcedony. Some irregular patches of uniformly oriented calcite touch one another, forming a mesh, but are enclosed and partly replaced by cryptocrystalline quartz. In general, evidence of silicareplacement of carbonate is strong. In thin sections of samples from locality 3 of 11/8/75, most calcite rhombs and small irregular patches are concentrated along the numerous veinlets of calcite (Fig.3). This m a y be due to the weathering of the chert. Chertification probably occurred at the same time or soon after the lithificationof the carbonaceous rocks. Then the weathering process produced the fractures in chert and calcite was introduced by interstitialwater and precipitated along the veinlets. T w o randomly chosen thin sections of the fossiliferouschert have been studied under cathodoluminescent light. Quartz matrix and calcite rhombs show homogeneous structure. N o evidence of overgrowth of detrital quartz or different compositional zoning of calcite has been recognized.

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Calcite appearing as rhombs or i r r e g u l a r patches aggregating along fracture zones Fibrous chalcedony Organic residues

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Fig.3. Petrology of chert from Cloud's locality 3 of 11/8/75. Diameter of the field of view is about 1.5 ram. Many calcite rhombs and small irregular patches are concentrated along numerous veinlets of calcite.

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Source and mechanics

of silica

Oehler (1976) suggested two diagenetic sequences to explain the differences between biogenic and inorganic cherts:

Biogenic deep-seacherts: Amorphous biogenic silica

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dissolution

Sx"1"meous solutions (? colloidal)

dehydration, crystallization Opal-CT

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Inorganic cherts: Amorphous silica gel

dehydration ~ crystallization ~

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Sp erulitm quartz chert

~ recrystallization Anhedral microcrystalline quartz chert The presence of sphemlitic quartz (chalcedony) in the Yudomian fossiliferous cherts suggests the inorganic type of silica of Oehler's silica diagenetic sequence rather than silica of biogenic origin. Actually, it is unlikely that any biogenic SiO2 was precipitated before %2 Ga (Ga is used for 109 years) and perhaps not before % 1.4 Ga Ago because all known precipitators of silica are eucaryotes and older eucaryotes are not known (P. Cloud, pers. comm., 1977). After %2--1.4 Ga, biologic silica secretion and precipitation might have occurred, although we have no direct evidence of it in the form of siliceous tests or frustrules until early Cambrian. Since no volcanic materials are known to be associated with the Yudomian rocks, weathering may have been the source of the silica in the contemporaneous sea water and interstitial fluids. In the absence of biogenic precipitation, ancient sea water, supersaturated with silica, could have been a source of silica

120 gel at or beneath the sea floor in favorable local environments, like the ancient region where the Yudomian rocks deposited. The mechanics of silica concentration of chert lenses in carbonate rocks is still controversial although many geologists have addressed the problem. Biggs (1957) suggested that replacement cherts are "epigenetic concretions formed by metasomatic processes operating during diagenesis and involving the aggregation of silica that originally had been deposited syngenetically with, and dispersed through the host rocks". Ramberg (1952) proposed the theory of diagenetic differentiation to explain why solution and reprecipitation of disseminated silica should occur in limestones. Putting these hypotheses together, a likely sequence is the following: (1) Primary precipitation of silica gel from seawater penecontemporaneous with carbonate. (2) Disseminated silica in host rocks. (3) Silica dissolved during diagenesis. (4) Diagenetic segregation or metasomatism replacing parts of the carbonate matrix with cryptocrystalline SiO2. As discussed above, the primary silica gel might have precipitated directly from seawater during ancient times. Thus, process (1) was a possible one for the Yudomian chert. In process (2), but probably not during Yudomian time, radiolarians, diatoms, sponges, or detrital silt particles could play the role of silica-keeper to store silica in carbonate rocks. Diagenesis of carbonate rocks begins immediately after deposition. Calcite can be dissolved at low temperature and pressure. Silica contained in carbonate rocks is as easily effected as calcite. Petrographic data suggest a replacement origin of the Yudomian fossiliferous cherts involving processes (3) and (4). The good preservation of abundant nannofossils implies early diagenetic silicification at low temperature and pressure.

Organic preservation When viewed in transmitted light, thin sections (about 30 ~m thick) of the Yudomian fossiliferous chert vary from dark brown to almost colorless. Most nannofossils are found in the darker parts of the chert. Amorphous organic matter, structurally well-preserved nannofossils, and pyritic or hematitic globular framboids are light~unber, brown, or black in color. Where the chert is barren of organic matter, pyrite or hematite globular framboids are light-gray. Thus, the appearance of the Yudomian cherts supports the widely held view that "the black cherts owe their color to an abundance of fine,rained pyrite and, more particularly, to the presence of amorphous organic matter, which is finely disseminated throughout t h e . . , matrix". (Barghoorn and Tyler, 1965a). The nannofossils of th~Yudoma chert tend to be preserved where the grain size of the quartz and chalcedony is small (generally < 15 ~m). Very little organic matter is preserved in the area constituted by coarse grains of

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mosaic quartz and chalcedony, and only a few broken pieces of filamentous nannofossils with some irregular organic particles are preserved in the areas with abundant ooids and grapestone structures. Where carbonates mineralized, the organisms lost their structures and primary morphology; only irregularly shaped organic residues are preserved. Apparently, there is a close relationship between the abundance of organic matter and the texture of the chert. The Yudomian nannofossils are light-amber, brown, or black organic residues of two types: (1) stain-like or film-like, amorphous, uniform, and diffuse, commonly light amber to brown; (2) fine-grained solid, black or opaque. Preservation, thus, is similar to that exhibited by the microbial assemblages of cherts from the Gunflint Iron Formation (Barghoorn and Tyler, 1965a), Bitter Springs Formation (Schopf, 1968), and Beck Springs Dolomite (Licari, 1978). The black, opaque, solid particles of the organic residues differ from opaque pyrite particles in their irregular, granular shape. The pyrite in cubic crystal forms, globular framboids, or small polygonal particles is commonly associated with organic remains (it may have been formed when hydrogen sulfide from decaying organic matter reacted with iron). Many globular framboids or cubic crystals are reddish brown or orange in color, owing to the replacement of pyrite by hematite. Several lines of evidence support the indigenous nature of the Yudomian nannofossils. None of them is located within fracture fillings. On comparing the organic structures in normal transmitted light with the same in polarized transmitted light, it is seen that these three~limensionally preserved organic structures are restricted to solid siliceous matrix. Chalcedony and quartz grains cut across the outlines of the delicate structures without disturbing the morphology of the organisms. The quality of preservation is very good by comparison with other known microbiotas observed in pre-Cambrian cherts. SUMMARY AND SIGNIFICANCE OF THE YUDOMA MICROBIOTA

The rich nannofossil assemblage here described provides an important record of a silicified benthic community of microorganisms having a K--At age of about 650 Ma. The Yudoma microbiota, including 16 categories (Fig.4) is largely cyanophytic, with coccoid forms dominating over filamentous. It includes 3 species of probable osciUatoriacean a~finity, 9 species of probable chroococcacean affinity, 1 species of probable entophysalidacean affinity, and 3 morphogroups of uncertain affinity. In their preserved characteristics, most Yudomian nannofossils are morphologically comparable to those of the perhaps 850 Ma old Bitter Springs microbiota and the roughly 1,900 Ma old Belcher Islands microbiota (Table I), as well as to living cyanophytes at the family or generic levels. Among these the rare presence of spiny acritarchs (Plate IV, 19 21--23) is most consistent with the earlier suggested basal Phanerozoic (Ediacarian) age.

122 CLASSIFICATION

SIZE For

D O M A I N P R O C A R Y O T A ChaSten 1938 (emend C l o u d ; M o o r m o n ~ B~Pierce 1 9 7 5 ) KINGDOM M O N E R A H a e c k e l 1866 ( e m e n d H u t c h i n s o n 1 9 6 7 } P H Y L U M M Y X O M O N E R A W h i t t a k e r 1 9 6 9 (emend C l o u d , M o o r m o % & Pierce 1975' s U e P H Y L U M C Y A N O P H Y T A Sachs 1874 ( e m l n d C l o u d , M o o r m a n , ~ Pierce 1975)

TAXA

Close H O R M O G O N E A E T h u r e t 1875 Order OSCILLATORiALE6 Elenkin 1949 Family OSCILLATORIACEAE Cumortier

OCCURRENCE

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ellipsoids eters

of

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and

: diem

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inner

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Kirchner

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1898

ClOl! COCCOGONOPNYCEAE Klrchner 1890 (emend Bourrelly 1970) Order CHNOOCOCCALE9 Wetlstein 1924 Family ENTOPHYSALIOACEAE atelier 1925

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Family

iNCERTAE

CHNOOCOCCACEAE

N~oeli

1849

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SEDIS

9 5

Eomycetopsls? s/ber/iensis n.

sp

.~.:::~i:?i.::~::~':..'..::';" ==,=~.~ Eomycetopsis? compylomHus n.

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Eoentophysol/syudomatica

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~rachyplegononkhandanum n. gen., n. sp. Aphetospora euthenio n. gen.,

(~

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Eozygionsp.

Schopf.a

Blacic,

n. sp.



1971

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Myxococcoides inornata

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Belcher Islands Supergroup(3) %1900 Ma

Myxoeoccoides inornata

Bitter Springs Formation(I,2) %850 Ma

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Barghoorn and Tyler

Huroniospora sp.

Gunflint Iron Formation(4) %2000 Ma

t~ O0

Relative abundances of taxa in other mierobiotas have been inferred from the following publications: 1, Schopf (1968); 2, Sehopf and Blacic (1971); 3, Hofmann (1976); 4, Barghoorn and Tyler (1956). A = abundant; C = c o m m o n ; R = rare; G = genus present only.

INCERTAE SEDIS Eosphaera? sp.

Eomyeetopsis? siberiensis n. sp. Eomycetopsis? eampylomitus n. sp.

Oscillatoriacese

Caryosphaeroides? sp.

Chroococcaceae (?)

Myxococcoides staphylidion n. sp. Eozygion sp.

n. sp.

Aphetospora euthenia n. gen.,

Chroococcaceae

Eoentophysalis yudomatica n. sp.

CYANOPHYTA Entophysalidaceae

Lower Yudoma Suite %650 Ma

Comparison of Yudomian taxa with similar taxa and their comparative abundances o f other pre-Cambrian microbiotas.

TABLE I

~L t~

125

The most prominent element in the Yudoma microbiota is Eoentophysalis yudomatica n. sp. (Plate II, 4--8), characterized by multiple-enveloped, celllike units and tabular colonies. It most closely resembles living Entophysalis Kiitzing of the Family Entophysalidaceae. The slender, tubular nannofossils of Eomycetopsis? siberiensis n. sp. (Plate I, 1--8) are the most abundant filamentous elements in the Yudoma microbiota. Eomycetopsis? siberiensis resembles the firm sheaths of Phormidiumlike or Lyngbya-like oscillatoriacean cyanophytes. Some unusual and perhaps stratigraphicaUy distinctive forms appear in this microbiota. Eight spiny spheroidal bodies (Plate IV, 19, 21--23) found in Yudomian cherts resemble acanthomorph acritarchs (the Acanthomorphitae is a subgroup in the acritarch classification o f D o w n i e et al., !963), and their size range (5.5--8.6/~m in diameter) fits the genus Micrhystridium Deflandre. According to Downie (1967), acanthomorph acritarchs larger than 20 ~m in diameter appear first in the Lower Cambrian. Acanthomorphs smaller than 20 ~m, that is Micrhystridium, were at that time considered to appear first in the upper Palaeozoic. In general, spiny acritarchs are not considered to appear earlier than Cambrian. However, according to G. Vidal (oral communi-

PLATEI Transmitted-light micrographs of filamentous nannofosalls in thin sections of black chert from the lower Yudoma Suite. Thin-section coordinates are identified by the distance listed in m m above ("y" coordinate); and mm to the right ("x" coordinate) of the center of a reference "× "which is scratched in the lower left-hand corner of the slide. Bar scale = 10 ~m except where otherwise indicated. 1--8. Eomycetopsis? siberiensis n. sp. 1. Type specimen, locality 2 of 10/8/75, C568(2), 28 ram(x)/19.8 ram(y). 2. Typical colony-like cluster, locality 2 of 10/8/75, C568(4), 23.7 ram(x)/9.6 ram(y). 3. Sparsely granular form, locality 6 of 5/8/75, C571(10), 35.4 ram(x)/8.6 mm(y). 4. Densely granular form showing a pattern similar to the preservation of multicellular or septate (?) structures, locality 1 of 10/8/75, C659(1), 39.4 ram(x)/-1.8 ram(y). 5. Mat-like cluster, some tubular specimens in this cluster are observed with inner threadlike inclusions, fig. 7 exhibiting a few of them, locality 6 of 5/8/75, C571(9), 30.9 ram(x)/ 1.5 ram(y). 6. Tubular form, locality 6 of 5/8/75, C571(9), 30.8 ram(x)/1.5 ram(y). 7. Tubular forms enclosing thread-like inclusions, locality 6 of 5/8/75, C571(9), 30.7 ram(x)/1.7 ram(y). 8. Cluster of granular forms, locality 3 of 11/8/75, C579(1), 13.4 ram(x)/15.8 ram(y). 9--11. Eomycetopsis? campylomitus n. sp. 9. Type specimen, locality 6 of 5/8/75, C571(9), 23.7 mm(x)/8.9 ram(y). I0. Exhibiting irregularly spaced sub-micron sized dark inchmions of some specimens, locality dark inclusions of some specimens, locality 6 of 5/8/75, C571(1), 38.7 ram(x)/ 15.4 mm(y). 11. Entangled cluster, locality 6 of 5/8/75, C571(9), 10 ram(x)/12.8 ram(y).

126

127

cation to P. Cloud, 25 Oct., 1978), the oldest spiny acritarchs occur in the upper Eleonore Bay Group (uppermost Riphean, pre-tillite)in east central Greenland. In the present study, the spiny acritarchs described are in the lower Yudomian stage, that is below trilobitesand shelly fauna, but probably above the level of tillitesin equivalent Vendian (Ediacarian) rocks. Their presence is one more (inconclusive but suggestive) point in favor of a Phanerozoic (and Paleozoic) age for this microbiota. A few branching tubular microstructures with distinctdark ellipsoidalinclusions (Plate IV, 20) appear in the Yudomian cherts. These tubular microstructures might represent branching filamentous cyanophytes, or cylindrical hyphae of fungi. They have no counterparts among previously described nannofossils. Another unusual form in this microbiota is the wiener-shaped rod, Brachypleganon khandanum n. gen., n. sp. (Plate II, 9--12). These short rods have morphologic characteristics similar to the extant chroococcacean cyanophytes Rhabdoderma Schmidle and Lauterborn and Gloeothece N~igeli.

PLATE II Transmitted-light micrographs of filamentous, coccoid, and short rod~haped narmofossils in thin sections of black chert from the lower Yudoma Suite. Thin-section coordinates are identified by the distance listed in mm above ( " y " coordinate), and nun to the right ( " x " coordinate) of the center of a reference "× "which is scratched in the lower left-hand corner of the slide. Bar scale = 10 ~m. 1--3. Euryculidion ¢ylindratum n. gen., n. sp. 1. Type specimen, broad tubular form, locality 2 of 10/8/75, C568(4), 30 ram(x)/10.2 ram(y). 2. Exhibiting dark brown, filamentous inner inclusion, partly segmented, locality 3 of 11/8/75, C670(2), 26.1 ram(x)/7.3 mm(y). 3. Sinuous form, locality 2 of 10/8/75, C568(4), 29.6 ram(x)/0.9 ram(y). 4--8. Eoentophysalis yudomatica n. sp. 4. Type specimen, locality 2 of 10/8/75, C568(4), 26.6 ram(x)/13.8 ram(y). 5. Linear arrangement in a colony-like cluster, locality 1 of 10/8/75, C659(1), 40 mm(x)/ 15.6 ram(y). 6. Colony-like cluster, locality 2 of 10/8/75, C568(2), 28 ram(x)/19.1 ram(y). 7. Degradational form, locality 2 of 10/8/75, C568(3), 32.1 ram(x)/9 ram(y). 8. Locality 2 of 10/8/75, C568(5), 15.3 ram(x)/5.6 mm(y). 9--12. Brachypleganon khandanum n. gen., n. sp. 9.10. Exhibiting randomly attached cilium-like structures. 9, locality 3 of 11/8/75, C670(2), 25.1 ram(x)/4.5 ram(y). 10, locality 3 of 11/8/75, C670(2), 19.7 ram(x)/10.2 mm(y). 11. Exhibiting circular transversal section, locality 3 of 11/8/75, C670(2), 17.6 ram(x)/ 1.9 mm(y). 12. Type specimen in the aggregated cluster, locality 3 of 11/8/75, C670(2), 18.4 mm(x)/ 1.9 ram(y).

128

P L A T E III T r a n s m i t t e d - l i g h t m i c r o g r a p h s of c o c c o i d n a n n o f o s s i l s in t h i n s e c t i o n s o f b l a c k c h e r t f r o m t h e l o w e r Y u d o m a Suite. T h i n s e c t i o n c o o r d i n a t e s are i d e n t i f i e d b y t h e d i s t a n c e listed in m m a b o v e ( " y " c o o r d i n a t e ) , a n d m m t o t h e f i g h t ( " x " c o o r d i n a t e ) of t h e c e n t e r o f a refere n c e " x " w h i c h is s c r a t c h e d in t h e l o w e r l e f t - h a n d c o r n e r o f t h e slide. Bar scale = 10 ~m. 1--5. Tetraphycus conjunctum n. sp.

129 Many elements of the Yudoma microbiota are smooth spheroids and cylindrical tubes. In all cyanophytes studied, the cellular structures deform more than their extracellular envelopes; the envelopes with their pigmentation often remain preserved after the cells have completely disintegrated. Therefore, the Yudomian smooth spheroids and tubes might represent empty envelopes or sheaths of cyanophycean cells in the final stages of degradation (Knoll and Barghoorn, 1975; Golubic and Barghoom, 1977). Gradational surface morphologies, from smooth to densely granular, are observed in Yudomian nannofossils and are interpreted as different preservation stages resulting from post-mortem changes in a single taxon. Two types of dark inner inclusions are observed within cell-like units of a Caryosphaeroides-like species from the Yudoma microbiota. One type is composed of multiple ~m~ized dark granules (Plate IV, 3). These granules, spheroidal to irregular in shape, cluster into sub~pheroidal bodies, about 3 ~m in diameter, and lie near the center or at the periphery of cell-like units. This type of inner inclusion is comparable to those of Melasmatosphaera Hofmann from the Belcher Islands microbiota (Hofmann, 1976) and Glenobotrydion Schopf from the Bitter Springs microbiota (Schopf, 1968). The other type of inner inclusion is a massive, spheroidal to oblong body, 3.9--8 ~m in longest dimension (Plate IV, 4--7). Usually one to three massive bodies are situated in the center or at the periphery of cell-like units. This type of inner inclusion resembles the "degraded internal structures" of Caryosphaeroides Schopf from the Bitter Springs microbiota (Schopf, 1968; Schopf and Blacic, 1971). These inner inclusions might represent either shrunken and degraded protoplasm of coccoid cyanophytes or preserved organelles, like nuclear residues of eucaryotic algae. In general, the Yudoma microbiota is composed of coccoid and filamentous nannofossils which commonly form mat-shape, colony-like clusters and are

1. Type specimen, cluster of planar tetrads and a few triads, dyads, locality 3 of 11/8/75, C670(1), 20.4 mm(x)/13 mm(y). 2, 3. Overlappedplanar tetrads, locality 3 of 11/8/75, C670(3), 21.1 mm(x)/7.8 mm(y). 4. Planar tetrad surrounded by a common envelope, locality 3 of 11/8/75, C670(2), 27 mm(x)/2.2 ram(y). 5. Locality 3 of 11/8/75, C670(1), 20.4 mm(x)/13 mm(y). 6--9. Myxococcoides staphylidion n. sp. 6. Grape-like duster, locality 2 of 10/8/75, C568(2), 25 mm(x)/10.5 ram(y). 7. Type specimen, locality 2 of 10/8/75, C568(3), 37.8 mm(x)/11.9 mm(y).

8. Net-likeag~egation, locality 6 of 5/8/75, C571(1), 42.4 ram(x)/14.9 ram(y). 9. Globular cluster surrounded by a halo-like common envelope,locality 2 of 10/8/75, C568(5), 27.7 mm(x)/10.2 ram(y). 10--12. Aphetospora euthenia n. gen., n. sp. 10. Type specimen, locality 1 of 10/8/75, C659(1), 36.9 mm(x)/14.5 mm(y). 11. Locality 3 of 11/8/75, C670(1), 25.4 ram(x)/16.8 mm(y). 12. Uniseriate aggregatesof two to three cell-like units long, comparable to chained specimeus of Myxococcoides inornata Schopf. Locality 3 of 11/8/75, C670(1), 26.7 ram(x)/1.5 ram(y).

130

PLATE IV Transmitted light micrographs of coccoid nannofossils, branched filamentous structures and acritarchs in thin sections of black chert from the lower Yudoma Suite. Thin section coordinates are identified by the distance listed in mm above ( " y " coordinate), and mm to the right ( " x " coordinate) of the center of a reference "× " which is scratched in the lower left-hand corner of the slide. Bar scale = 10 #m.

131

presumably benthic. Although the Y u d o m a microbiota is comparable to other microbiotas reported from stromatolites,it occurs in non~tromatolitic cherts and its constituent nannofossils are not responsible for stromatolite formation in the Y u d o m a Suite. This report on the Y u d o m a non-stromatolitic microbiota, then, not only describes new fossiltypes but adds preservational information to the study of pre-Cambrian microbiota. TAXONOMY

Taxonomic treatment of pre-Cambrian nannofossils is in a state of flux. Disagreement prevails as to what types and what quantities of data might

I, 2. Eozygion sp. 1, 2. Locality 1 of I018175, C659(I), 29.3 mm(x)/7.4 mm(y). 3--7. Caryosphaeroides? sp, 3. A pair of undivided spheroids surrounded by a c o m m o n envelope, each spheroid with inner inclusions of multiple dark granules, locality 2 of 10/8/75, C568(4), 24.1 ram(x)/ 2.9 mm(y). 4, 5. Cluster of undivided pairs and single spheroids, each spheroid with inner inclusions of massive, multiple bodies, locality 3 of 11/8/75, C670(1), 24.1 ram(x)/8.5 ram(y). 6. Undivided pair with inner inclusion of massive bodies, locality 1 of 10/8/75, C659(1), 33.8 ram(x)/16.2 mm(y). 7. Undivided pair with inner inclusions of three overlapped massive bodies, locality 3 of 11/8/75, C670(1), 24.4 ram(x)/14 mm(y). 8--11. Morphotypes of spheroids in triads. 8. Locality 1 of 10/8/75, C659(1), 38.2 mm(x)/O.1 ram(y). 9. Locality 3 of 11/8/75, C670(2), 26 ram(x)/5 ram(y). 10. Locality 3 of 11/8/75, C670(2), 25.2 ram(x)/6.8 mm(y). 11. Locality 3 of 11/8/75, C670(2), 25.5 ram(x)/5.8 ram(y). 12--15. Diplococcus-shaped nannofossils. 12. Cluster of Diplococcus-shaped nannofossils, locality 2 of 10/8/75, C568(2), 22.5 ram(x)/19.8 ram(y). 13. Locality 1 of 10/8/75, C659(1), 29.6 ram(x)/5.7 ram(y). 14. Locality 3 of 11/8/75, C670(3), 35.9 mm(x)/9.8 mm(y). 15. Locality 3 of 1118/75, C670(3), 19 ram(x)/10.3 mm(y). 16, 17. Spheroids in linear arrangement. 16. Six spheroids in a uniseriate aggregation, locality 3 of 11/8/75, C670(2), 26.4 ram(x)/ 8 ram(y). 17. Four spheroids in linear arrangement and a pair of the same kind of spheroids associated together, locality 3 of 11/8/75, C670(1), 24.5 ram(x)/8.5 ram(y). 18. Eosphaera? sp. Locality 2 of 10/8/75, C568(4), 23.7 ram(x)/9.6 ram(y). 20. Morphotype of branched tubes with ellipsoidal inclusions, locality 2 of 10/8/75, C568(3), 27.1 mm(x)/6 ram(y). 19, 21--23. Acritarcha, Micrhystridium sp. Deflandre. 19. Exhibiting numerous, short, cilium-like spines, locality 3 of 11/8/75, C670(3), 28.4 mm(x)/8.6 ram(y). 21, 22. Exhibiting two micrhystridia closely associated together, both bear radiating thornlike spines, longer but less numerous than specimen exhibited in fig. 19, locality 3 of 11/8/75, C670(3), 20.7 ram(x)/5.2 mm(y). 23. Exhibiting surface feature of fig. 22, locality 3 of 11/8/75, C670(3), 20.7 ram(x)/ 5.2 mm(y).

132 best be used for classification of pre-Cambrian nannofossils which occur in cherts, shales, or in some other matrix. There is no consistent approach. Procedures adopted in the taxonomic treatment of the Yudomian nannofossils are as follows: (1) The fossil microorganisms are taxonomically grouped on the basis of their morphology as seen by transmitted-light microscope. In other words, taxonomic decisions are based on the author's best evaluation of the available evidence, such as gross morphology, size, shape and type ,of clustering, suggesting possible patterns of cell-division or colony formation, comparison with modern analogues, and the likely effects of degradational processes on morphology in a c o m m o n genus or in a particular species. Such taxa of morphotypes are c o m m o n in paleobotanical studies. (2) Within the framework of botanical classification, nannofossil genera are allocated to systematic groups according to their likely biological affinities. Those nannofossil genera of uncertain affinity are classified provisionally as incertae sedis, a group from which t h e y may be removed if their relationships are subsequently determined. (3) Although there is little doubt that most of these nannofossils are remains of procaryotes, the very simple morphology of these nannofossils and the gross morphologic similarities among several algal taxa and among cyanophytes and bacteria make it difficult to assign them with certainty to or within a taxonomic scheme. Within the procaryotes, however, size alone would indicate their affinity with the cyanophytes (or "cyanobacteria") rather than (as is traditional) cyanophytes, this would seem to imply taxonomic evaluation according to criteria conventionally used in the classification of bacteria -- i.e., the biochemistry of the cell wall and interior, impractical for study of fossils. Considered as cyanophytes, however, their classification is based on morphology (Geitler, 1932; Fritsch, 1935; Desikachary, 1959; Golubic, 1976), which provides the only practicable grounds for an approach to the classification of the Yudomian nannofossils. (4) Because reproductive stages and structures are rarely preserved, vegetative cell morphology is used for the classification of nannofossils. This morphological limitation of fossil materials must be understood in considering the taxonomic assignment of Yudomian nannofossils based on morphological comparisons with extant cyanophytes. (5) In addition to the descriptive data, statistical and quantitative data are here presented for comparisons with similar taxa and for analyzing fossil populations. In the taxonomic treatments, dimensions are considered to be secondary in significance to the other morphologic criteria discussed. However, significant differences between size modes may be used as the sole or primary criterion for the classification of species (Desikachary, 1959). (6) Morphologic variability of extant cyanophytes, owing to their particular stage in a life cycle or to post-morte.m degradation, is considered in interpreting taxonomic affinities of the Yudomian microbiota. Wherever numbers of specimens permit, taxa are considered as populations. Data such as size

133

distribution and probable colonial organization are collected from populations and clustering patterns. (7) Since systematic pre-Cambrian paleomicrobiology is a relativelynew field,it is important to report data from initialstudies in reasonably complete detail, and that practice is followed here. METHODS Samples were studied in three types of preparation: thin~ectioned material, macerated material, and small freshly fractured chips of chert lenses under the scanning electron microscope (S.E.M.). Thin sections were examined under the Zeiss Photomicroscope II. First, the petrographic nature of the material, the degree of preservation, the aggregate geometry and the gross characteristics of morphologic entities were studied at 125× and 312X in polarized and normal transmitted light. Morphologic entities of nonbiologic origin were identified from their petrographic characteristics: for example, concentric and radial structures of pyrite and hematite, pseudofilamentous crystals, and mineral trails. Then, morphologic entities of probable biologic and possible biologic origin were studied at higher magnifications 500× and 1250× under oil immersion. Confidence in a biologic origin of three
134

Slides were prepared from the stained organic residues of each sample and were examined in transmitted light. Broken segments of filamentous nannofossils were observed. These filamentous nannofossils c o m m o n l y appear solitary, brown to opaque, cylindrical, with consistent width, widths measured 3.2--15.8 #m (11 specimens). They are probably the same kind o f filamentous nannofossils as those observed in thin,sectioned slides. No spheroidal or ellipsoidal nannofossils were found in these residues. Some macerated material was prepared for examination under the S.E.M. Filamentous and spheroidal clumps of organic and mineral matter of questionable biologic origin were observed. Photographs were taken with 70 mm Kodak Tri-X pan film. A few contaminants of m o d e m filaments and collapsed m o d e m spores were found and identified by their morphologic characteristics and their distinct surface textures. Freshly fractured chips of rock from Cloud's localities 3 of 11/8/75 and 2 of 10/8/75 were prepared for examination under the S.E.M. No fossils or impressions of fossils like those seen in thin sections were observed. MEASUREMENTS

In this study, size data was gathered from direct measurement using an eyepiece micrometer in the transmitted-light microscope of a Zeiss Photomicroscope II. Whenever material was sufficient,measurements were made of at least 100 randomly selected individuals of each category including at least 10 individuals within each aggregation. For consistency and clarity,measurements were made only at 500)< and 1250)< under oil immersion. Some measurements used in this study for size data are illustratedin Fig. 5. For spheroidal nannofossils, diameters are measurements of the innermost wall structure (Fig. 5, A--F). The innermost wall structure probably represents: (1) original ceil wall, or (2) the preservable interlayered material between nonpreservable ceil membrane and mucilaginous sheath, or (3) the firstor the second or the third or the n'th lameila of an originally lameUated sheath. To date, it is difficultto be sure of its original nature. However, the innermost wall structure is considered to mark the closest approximation to the diameter of the original cell wall. The diameter of the outermost envelope of the Yudomian multi-enveloped spheroidal nannofossils is difficultto measure because of its great variabilityin shape and size and its faint outline. If the specimens showing the feature were available,the separation between the innermost wall structure and the next adjacent envelope was measured (Fig. 5, G). For filamentous nannofossils, widths are measurements of the widest diameter (Fig. 5, H--J). Because the inner structures are c o m m o n l y absent or distorted, and because some filamentous specimens appear shrunken, the widest diameter is considered to mark the approximate width of the original organism.

135

For short rod-shaped nannofossils, lengths are measurements of the linear distance between two round ends (Fig. 5, K--M). Where curved, as most are, the measured lengths are shorter than true lengths. For the majority of specimens measured, however, the curvature of the rod was small; the error, therefore, insignificant, and the measurement the simplest to make. The measurements were made to a precision of I + ~0.1 ~m. Size of measurements are presented in size-frequency histograms for most taxa. The ordinate shows the number of specimens measured. The abscissa shows size data of specimens in ~m. Size classes were selected on the basis of discrimination by ocular micrometer scales.

Diameter

Diameter ~ .~ !

Diameter

0

Diameter

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Length of Spines A.

B.

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Diameters

E.

Diameters

Separation

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G. Width

Width

Width

t

I.

H.

|

Length

Length

K.

D.

L.

Fig.5. Explanation of measurements.

t J.

Length

M.

136 DESCRIPTION OF FOSSIL LOCALITIES Well-preserved nannofossils described in this study occur in non~stromatolitic cherts of the lower Y u d o m a Suite, from four localities along the Khanda (or Belaya) and Aldan Rivers in Yakutia, eastern Siberia. Details of the Yudoma Suite in this area have been described by Semikhatov et al. (1967, 1970). Localities yielding the richest nannofossils are found in stratigraphic sequences 78 and 80 of Semikhatov et al. (1970). These localities are at the north end of Kyllakh Ridge and to the east along the Belaya (or Khanda) River (Fig. 6). Here the Yudomian reference sequence is exposed in a narrow belt along a fault zone which separates the YudomaMaya trough from the Uchur-Maya block. The four localities which yielded the richest nannofossils are briefly described in the following paragraphs from published data supplementedly by Cloud's field notes. The stratigraphic columns are shown in Fig.7. (1) Three of Cloud's localities, 1 and 2 of 10/8/75 and 3 of 11/8/75, occur in stratigraphic sequence 78 of Semikhatov et al. (1970). Sequence 78 is located on the right bank of the Aldan River, about 12 km upstream from the m o u t h of the Khanda (or Belaya) River. Here, the Yudoma strata unconformably overlie pink and gray microgranular to very fine-grained dolomites of the middle Tsipanda Suite (middle Riphean), and conformably underlie red- and greenish-gray glauconitic limestone containing a shelly Tommotian fauna. The samples of Cloud's locality 3 of 11/8/75 come from small lenses of black chert which occur in the lower part of carbonaceous limestone toward the t o p of the lower Yudoma Suite. The level of these chert lenses is about 110--120 m below beds with shelly Tommotian fauna and a b o u t the same distance above the base of the Yudoma. The samples of Cloud's locality 1 of 10/8/75 come from black carbonaceous chert lenses which occur at the base of black calcareous shale (through ~ 0 . 5 m of beds) of the uppermost lower Yudoma Suite. The level of these chert lenses is about 80--90 m below beds with shelly Tommotian fauna and a b o u t 135--145 m above the base of the Yudoma. Black chert lenses of Cloud's locality 2 of 10/8/75 occur in a black limestone part of the same unit as 1 of 10/8/75, but about 3 m higher in the sequence. (2) Cloud's locality 6 of 5/8/75 is in stratigraphic sequence 80 of Semikhatov et al. (1970). This sequence outcrops near the m o u t h of the Tarynnakh River, along both slopes of the Belaya (or Khanda) River valley, Gornostak anticline. At this place the Yudoma Suite is intermediate in character between sequences 78 and 79 in Semikhatov et al. (1970). It consists mainly of limestone (some dolomitic) and with more sandstone than sequence 78. The upper Yudoma Suite unconformably overlies the lower Yudoma Suite in this sequence. The lower Cambrian deposits and the upper part of the upper Yudoma Suite are absent (Fig.7).

Fig.6. Index maps showing four fossil localities: 1 of 10/8/75, 2 of 10/B/75, 3 of 11/B/75 and 6 of 6/B/75. The upper map is a composite from: (1) Army Map Service, Corp of Engineers, U.S. Army, Washington (D.C.), 1961, and (2) Operational Navigation Chart, Defense Mapping Agency, Aerospace Center, St. Louis Air Force Station, Missouri, 1974. The lower map is a modification of map by Cartographic Division, National Geographic Society, 1960.



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I THICKNESS METERS

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ITHOLOGY

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,

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m

50 m

s?

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FOSSIL THICKNESS LOCATION (METERS) COLUMN

i

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DOLOMITE

LITHOLOGY

r

~

Upper Yudoma Suite

UNCONFORMITY,

Middle to U p p e r RIPHEAN

Lower Yudoma Suite

Cambrian

Fig.7. Stratigraphy of the Yudoma Suite, northern Yudoma-Maya trough, eastern Siberia. (Modified from Semikhatov et al., 1970)

l/Ar 6SO My, X

3 of iI/s 75

Scale in meters

50m. 40m. 30m. 20m. IOm" Om'

of 10 S 7S

AGE

I of 10 8 7S

E

K-At

~ LOCATION FOSSIL

80

SEQUENCE Along K h a n d a ( o r Belaya ) R i v e r valley, near the mouth of the Tarynnakh River

SEQUENCE 78

Right bank of the Aldan River, a b o u t 12 km upstream from the mouth of the K h a n d a (or B e l a y a ) R i v e r

o.Z 4C

Z < ~

v

Co 0o

139

Black chert samples from Cloud's locality 6 of 5/8/75 were collected from cherty angular limestone float in cliffside talus along the north face of a low ridge that rises a short distance south of the Belaya (or Khanda) River and is capped by Yudoma limestone. Stratigraphically it is about 55 m above the unconformably underlying upper Riphean Kandyk Suite. SYSTEMATICPALEONTOLOGY Domain Procaryota Chatton 1938 (emend. Cloud et al., 1975) Kingdom Monera Haeckel 1866 (emend. Hutchinson, 1967) Phylum Myxomonera Whittaker 1969 (emend. Cloud et al., 1975) Subphylum Cyanophyta Sachs 1874 (emend. Cloud et al., 1975) Class Hormogoneae Thuret 1875 Order Oscillatoriales Elenkin 1949 Family Oscillatoriaceae (S.F. Gray) Dumortier ex Kirchner 1898 Genus Eomycetopsis Schopf 1968 (?) Eomycetopsis? siberiensis n. sp. (Plate I, 1--8; Figs. 8, 9)

Diagnosis. Filaments slender, nonseptate (?), unbranched, straight, gently curved or strongly sinuous. They range from smooth~urfaced, light-brown, empty tubes to densely granular, dark-brown, solid cylinders. Lateral wall varies from a distinct thin wall of consistent thickness to an intermittent or poorly defined boundary. Specimens solitary or irregularly entangled in a woven mat-like cluster. Individuals generally of uniform diameter, ranging from 1.6 to 7.8 ~m and averaging 3.78 #m (standard deviation = 1.24 ~m; 375 specimens measured). Length may exceed 100 ~m, occasionally sharply fragmented. Characteristical hollow nature of filaments shown by circular openings observed in transversal sections.

Etymology. Refers to occurrence in Siberian Platform. Type locality. Lower Yudoma Suite, Cloud's locality 2 of 10/8/75. Type specimen. The great variety of surface texture and wall structure shown in this species is illustrated by Plate I, 1--8. The type specimen shown in Plate I, 1, is located 19.8 m m above and 28 m m to the right of a reference " X " on thin section C568(2) from the Cloud Collection, BCL, UCSB. Discussion. The Eomycetopsis? siberiensis filaments are the most prominent filamentous elements in the Yudomian microbiota. They occur at four different localities. About 400 well-preserved specimens were observed, measured, and described. These filaments are very consistent in gross morphology, except in surface texture. Based on the varieties of surface texture, two extremes can be described. Gradations between the two extremes exist, however, allowing both end members to be assigned to the same species.

140 (1) One extreme is characterized by smooth or very faintly granular surface texture and by a distinct thin b u t consistent lateral wall. Specimens are light brown and seem to be slender e m p t y tubes. Illustrations on Plate I, 1, 2, 5, 6, demonstrate their morphology and their mat-like habit. (2) The other extreme is characterized by a densely granular surface texture and a poorly defined wall which is delimited by discrete dark-brown to opaque granules. Specimens are commonly dark brown to opaque and look solid. Illustrations on Plate I, 3, 4, 8, show their morphology as well as their mat-like habit, similar to (1) above. Figs. 8, 9, show the histograms of size distribution of both smooth and densely granular extremes. Their size distribution is very similar. Gradational morphology between smooth and densely granular extremes is not only observed from the same fossil locality but also in the same thin sections. Some specimens even show smooth, sparsely granular and densely

granular

Smooth and slightly granular specimens of

Densely specimens of

Eomycetop$1$ .P sibe r/ens/s

Eomyceto#sis ? siberiensis

N = 238 MEAN

= 5.9

Standard deviation =

N = 137

1.02

MEAN = 5 . 5

80-

Standard deviation = 1 . 1 4

70-

6050.

~ 50 0 o

>,40~

g 40.

o r Q

Q m.

50-

0"

30,

0 m,,

20-

2O

I0

I0 2'5'4'51617~18

Widths of F i l a m e n t s

in/u.m

Widths of Filaments In /¢m

Fig.8 Comparative histograms of tubular diameters of smooth and slightly granular Eomycetopsis? siberiensis and densely granular E.? siberiensis.

141

130 120 Eomycefop$is ? s i b e r i e n s i $

II0 N = 375

MEAN = 3.78 Standard deviation = I .24

I00 90

/

80 70 6O

~J

Smooth and slightly granular specimens Densely granular specimens

u c

=* 50 O" G

" 40 30 20 o m

__

4'5'

Widths of F i l a m e n t s i n ~ m Fig.9. Composite histogram of tubular diameters of Eomycetopsis? siberiensis specimens.

granular textures in a single filament. Based on the above observation, smooth to granular filaments are interpreted as different preservation stages of Eomycetopsis? siberiensis. Most specimens of E.? siberiensis are tubular forms without any septation. Rarely, the dark-brown to opaque granules form a pattern on the surface which is similarto the preservation of multicellularstructures or septate structures (Plate I, 3, 4). However, the gradational morphology from smooth to sparsely granular to dense, roughly granular forms indicates that the patterns observed might be a result of preservation and diagenetic alteration. In the colony-like clusterillustratedin Plate I, 5, severalEomycetopsis? siberiensis filaments are observed to enclose thread-likesegments (Plate I, 7).

142

Diameters of the enclosing tubular E. ? siberiensis filaments uniformly measure 3.5 pm. Their enveloped thread-like segments are uniserial, densely granular and are 1.5 pm wide, having the morphologic characteristics of another Yudomian filamentous species Eomycetopsis? campylomitus. This association could be explained as unbranched filamentous cyanophytes composed of degraded uniserial trichomes enveloped by tubular sheaths and reveals the nature of E.? siberiensis as fossil filamentous cyanophytes. Regarding to the relationship of their internal trichome-like contents and the similar threadlike filaments, E.? campylomitus is discussed later.

Comparison. These Yudomian tubular filaments are assigned to the Family Oscillatoriaceae based on their unbranching, uniform filamentous morphology which is closely comparable to firm sheaths of the extant Phormidiumlike or Lyngbya-like osciUatoriacean cyanophytes. In general, specimens of Yudomian E.? siberiensis are also comparable to extant Leptothrix-like iron bacteria (Harder, 1919). However, these Yudomian filaments commonly form mat-like clusters, closely associated with abundant solitary or clustered spheroidal nannofossils, and some are observed to enclose internal contents showing "sheath + trichome"-like association. These features favor the interpretation of E. ? siberiensis as the firm sheaths of cyanophytes. Among fossil records, these Yudomian filaments are similar in morphology to Eomycetopsi8 Schopf (1968) from the Bitter Springs assemblage. However, unlike the reported Eomycetopsis Schopf, the septation is a questionable feature in the Yudomian filaments. And the diameter of Yudomian E.? siberiensis (1.6--7.8 pm) generally exceeds that of the two Bitter Springs species of Eomycetopsis Schopf (2--4.5 pm). Eomycetopsis filiformis Schopf (1968) was also reported from the Belcher Islands microbiota (Hofmann, 1976). The Belcher Island filaments were described as nonseptate, tubular filaments, 1.0--3.7 um across. Gradations between smooth and densely granular ornamentation were reported. But matlike clusters similar to those of the Yudomian E. ? 8iberiensis are not present in the Belcher Islands E. fiUiformis. Examination of the filaments of E.? siberiensis has demonstrated that the morphological and preservational characteristics of the Yudomian fossils are more varied than those of the Eomycetopsis filaments from the Bitter Springs and the Belcher Islands cherts. In view of the facts that the septations in the Bitter Springs Eornycetopsis are not entirely convincing (Hoffmann, 1976) and the taxonomic assignment of Eornycetopsis to the fungi is tenuous (Cloud, 1976a; Hofmann, 1976), it seems preferable to reexamine the taxonomic status and redefine the diagnosis of this genus to allow for inclusion of filaments with somewhat smaller and larger size range and the preservational variations such as observed in the Yudomian specimens.

143

Occurrence. Lower Yudoma Suite, Cloud's localities 1 and 2 of 10/8/75, 3 of 11/8/75 and 6 of 5/8/75. Eomycetopsis? campylomitus n. sp. (Plate I, 9--11; Fig. 10) Diagnosis. Thread-like specimens, slender, filamentous, flexible, slightly curved to strongly sinuous. Nonseptated, unbranched. Surface slightly granular or highly granular; light- to dark-brown. Wall thin, distinct to poorly defined. Commonly a single filament includes a slightly granular section with a distinct wall and a highly granular portion without a delimiting wall. Filaments may be .~olitary or occur as many entangled threads forming elongate, belt-like clusters (Plate I, 11). Clusters are commonly surrounded by a b r o w n amorphous matrix suggesting a colonial association. Individual specimens are up to 250/~m long (incomplete specimen) and 0.8--2.0/~m wide, averaging 1.5/~m (standard deviation = 0.38/~m; 40 specimens measured). The width may be variable in a single specimen, giving way to discordant local construction. Eomycetop$i$ .P compylomifu$

15. ~ / ¢>, u

~o

t

N = 40 Stondorddeviation MEAN == 0.38 1.45

O"

u. 5'

Widths

of Flloments in /.Lm

Fig.10. Histogram of tubular diameters of Eomycetopsis? campylomitus.

Etymology. Refers to its curved thread-like morphology (Greek campylos = curved; mitos = thread). Type locality. Lower Yudoma Suite, Cloud's locality 6 of 5/8/75. Type specimen. The type specimen is the filament shown in Plate I, 9. It is located 8.9 mm above and 23.7 mm to the right of the reference "× " o n thin section C571(9) from the Cloud Collection, BCL, UCSB. Discussion. The Eomycetopsis? campylomitus filaments are distinguished from other Yudomian filamentous nannofossils by extremely small size and the elongate, belt-like clusters (Plate I, 11).

144

The constrictions observed in many specimens of E. ? campylomitus indicate shrinkage of the organism during post-mortem degradation or diagenetic processes. The presence of many irregularly spaced, dark-brown to opaque spots on the surface of some E.? campyIomitus filaments (Plate I, 10) "is perhaps another diagenetic feature as they have no obvious biological bearing. These gregarious filaments seem highly degraded, it is uncertain whether they represent the original growth habit. Its notably smaller size is the main criterion used to separate E. ? campylomitus (0.8--2.0/~m wide, mainly measured from specimens in two beltlike clusters, see Fig.10) from E.? siberiensis (1.6--7.8 pm wide). The possibility that E.? campylomitus specimens are highly degraded E.? siberiensis should not be excluded. interestingly, some slender segments, like E.? campylornitus in morphology, are sheathed by E.? siberiensis filaments (Plate I, 7). It is observed that these forms -- (1) the E.? siberiensis e m p t y tubular specimens, (2) the E.? campylomitus bare thread-like specimens, and (3) the sheathed E.? campylomitus-like segments with the outer E.? siberiensis sheaths -- are closely associated in the same thin section within a volume of 239.4 m m 3. This observation could be explained as being due to the thread-like specimens of E.? campylomitus representing degraded trichomes of ancient cyanophytes. However, it is questionable to associate the bare E.? campylomitus and tubular e m p t y E.? siberiensis genetically in same species as trichomes and e m p t y sheaths, because the nonseptate nature implies that whatever E.? campylomitus is, it lacks the critical features of a well-preserved algal trichome. A systematic search of 16 thin sections revealed only three specimens posessing a "sheath + trichome"-like association. In spite these sheathed internal segments being similar morphologically to E. ? campylornitus, the bare slender specimens of that species seem more likely to be shrunken remains of degraded sheaths.

Occurrence. Lower Yudoma Suite, Cloud's localities 6 of 5/8/75, 3 of 11/8/ 75 and 2 of 10/8/75. Genus Euryaulidion n. gen.

Type species. Euryaulidion cylindratum n. sp. Diagnosis. Broad tubes, cylindrical to somewhat flattened, straight or slightly sinuous, not septated(?), not branched, not enveloped. Commonly solitary. Straight, displaying broken segments showing transverse sections with circular openings, or seemingly pliable, with obtuse or gradually attenuated ends. Surface of tubes smooth or granular, fight- to dark-brown. Wall distinct, 0.5--2.3 pm thick, or not well defined. Some specimens with dark-brown, filamentous inner tubes that are long or segmented (e.g., Plate II, 2) and about 3.9--10

145

~m across. Length of these tubular specimens commonly around 200 pm, occasionally up to 400 ~m (incomplete specimen). Measured width 14.1-20.2 urn, averaging 17.2 um (standard deviation = 2.0 urn; 22 specimens measured). Width commonly shows gradual tapering. Etymology. Refers to its broad tubular morphology (Greek eury = wide, broad; aulidion = tube, pipe). Discussion. The broad tubes of Euryaulidion are designated as biological remains and are differentiated from crystal trails or crystal forms by the following characteristics: absence of glassy luster; absence of slender surface striations; absence of terminal crystals; light- to dark-brown pigmentation, similar to most organic remains; association with abundant spheroidal and filamentous procaryotic nannofossils; simple morphology and small size, fitting the low state of organization and the morphological characteristic of associated procaryotes. The genus Euryaulidion includes two forms of tubular structures: apparently pliable, slightly sinuous forms and sharply offset (brittle?}, nearly straight forms. These two morphologic characteristics are presumed to be the results of different environmental conditions at the time of deposition as morphological characteristics needed to achieve brittle or pliable states of preservation are not clear, it is possible that the sharply offset forms were deposited suddenly while the original organisms were still turgid. On the other hand, the pliable forms may have been deposited after death when organisms were dry, coherent, and more readily bent without breakage. Empty tubes of Euryaulidion are found in thin sections of black chert from Cloud's localities 3 of 11/8/75 and 2 of 10/8/75. Both offset and sinuous forms can be found close together in a single thin section. Size ranges of both are the same, 14.1--20.2 ~m (Fig.ll). Comparison. The genus Euryaulidion includes all broad tubular forms from the Yudomian microbiota. The size range of these tubes (Fig.ll) suggests a diversity of organisms rather than the normal distribution of a single entity. However, the small number of specimens and the simplicity of their morphology makes further taxonomic subdivision inadvisable. In gross morphology, the members of Euryaulidion share some distinct traits, such as tubular, unbranched filaments without heterocysts. These traits resemble those of the cyanophyte Family Oscillatoriaceae. Sinuous tubes and empty straight tubes of Euryaulidion resemble empty sheaths of the oscillatoriaceans. Tubes with inner-tube structures (Plate II, 2) resemble filaments (i.e., trichome + sheath) of oscillatoriaceans. The long, consistent filament-like or segmented cell-like, inner-tube structures are recognized by dark-brown pigmentation which is different from the light-brown pigmentation of outer tubes (Plate II, 2). Among modern genera, the oscillatoriacean Lyngbya Agardh, characterized by unbranched filaments with firm, easily ruptured sheaths is similar to Euryaulidion.

146

Euryoulidion cylindrotum

N-- 22 MEAN == 2.0 17.2 Stondard deviQtion

i l~t

18 20 Widths of Filoments in /J.m Fig.11. Histogram of tubular diameters of Euryaulidion cylindratum.

The only broad, filamentous nannofossfl comparable in size to Euryaulidion is the Bitter Springs Siphonophycus Schopf {1968). The smooth surface and the lack of capitate terminals in Euryaulidion differentiate it from Siphonophycus.

Euryaulidion cylindratum n. sp. (Plate II, 1--3; F i g . l l ) Diagnosis. As for genus. Etymology. Lower Yudoma Suite, Cloud's locality 2 of 10/8/75. Type specimen. The type specimen shown in Plate II, 1, is located 10.2 mm above and 30.0 mm to the right of a reference "× " o n thin section C568(4) from the Cloud Collection, BCL, UCSB. Occurrence. Lower Yudoma Suite, Cloud's localities 2 of 10/8/75 and 3 of 11/8/75.

Class Coccogonophyceae Kirchner 1898 (as Coccogoneae emend. Bourrelly 1970) Order Chroococcales Wettstein 1924 Family Entophysalidaceae Geitler 1925 Genus Eoentophysalis Hofmann 1976 Eoentophysalis yudomatica n. sp. (Plate II, 4--8; Figs.12, 13)

Diagnosis. Cell-likeunits, commonly spheroidal, occasionally eUipsoidal, discoidal, or subpolyhedral. Dimensions of cell-likespheroids (diameter of innermost wall) 4.7--20 #m, averaging 11 p m (standard deviation = 2.98 pro; 180 specimens measured). Long axis of ellipsoids,discoids, and subpolyhedrons, 8--26.2 pro, averaging 14.4 p m (standard deviation = 3.8 pro; 114 specimens measured); short axis 4--20.2 pm, averaging 10.3 p m (standard

147 deviation = 2.8 ~m; 114 specimens measured). Wail commonly thin, <0.8 ~ m in thickness. Surface of cell-likeunits smooth, yellowish brown, or highly granular and reddish-brown, seldom opaque. Individual cell-likeunit commonly with thin, well
Etymology. Refers to occurrence of the species in Yudomian rocks. Type locality. Lower Yudoma Suite, Cloud's locality 2 of 10/8/75. Type specimens. The cell cluster shown in Plate II, 4, is designated as the holotype of the species. It is located 13.8 mm above and 26.6 m m to the right of the reference "×" on thin Section C568(4) from the Cloud Collection, BCL, UCSB. Plate II, 4--6, shows typical colony-like clusters in thin sections. Discussion. The cell-like units of Eoentophysalis yudomatica represent the most prominent spheroidal nannofossils in the Yudomian microbiota. They are abundant in non~stromatolitic chert from three different levels of the lower Yudoma Suite. E. yudomatica differs from other Yudomian spheroidal nannofossils in possessing distinct individual multiple envelopes and in comprising tabular or piano-roundish colony-like clusters. These clusters are embedded in amorphous cohesive red brown matrix which may reflect the original colonies encompassed by mucilage. Morphologically variable elements observed in thin sections and illustrated in Fig.12 and Plate II, 4--8, include: (1) Surface texture gradually varies from smooth to highly granular or opaque (Fig.12, A). (2) Wall varies from an intermittent or poorly defined outline to distinct and thin and to halo-like thick peripheral zone (Fig.12, B). (3) Multiple envelopes vary from narrow, tightly encompassed forms to broadly separated forms (Fig.12, C). Separation between the innermost wall-like structure and its next adjacent envelope measures from nearly zero to 2.5 ~m. The distance between the innermost wall-like structure and outermost envelopes measures 1.5--4.0 ~m.

148

(4) Forms of colony-like clusters vary from tabular or piano-roundish to irregular shapes. (5) Variable arrangements of individual envelopes of cell-like units and c o m m o n envelopes of two to three cell-like units are observed (Fig.12, D, E). Considering the demonstrable shrinkage and distortion observed in decaying recent microorganisms and the importance of other degradational features (Awramik et al., 1972; Golubic and Hofmann, 1976), the morphological variations observed in E. yudomatica are mainly attributed to the effects of post-mortem changes in a single taxon.

A O@ S

' .......... "

O0

C

F@@@@ Fig.12. Variations in morphology, observed in Eoentophyaaliz yudomatica. A. Surface texture varies from smooth to granular or opaque. B. Wall varies from poorly defined outline to distinct, thin wall or to halo-like thick peripheral zone. C. Multiple envelopes vary from narrow, tightly encompassed form to broadly separated forms. D and E. Variable arrangements of individual envelopes of cell-like units and c o m m o n envelopes of two to three cell-like units are observed.'

Because of probable post-depositional changes, original cell sizes are uncertain. The innermost wall-like structure of E. yudomatica might not represent the original cell wall. However, for this study the inner wall-like structure and the surrounding multiple envelopes are interpreted as original mucilaginous sheaths preserving the approximate cell morphology, and the inner surface of the innermost wall is used as the best estimate for original cell size. Of 180 spheroidal E. yudomatica measured, 61% have diameters between 10--12 urn. Histograms showing size variation measured from the innermost wall of the spheroids and ellipsoids are presented in Fig.13.

149 Spheroidol ¢lli-ilko units of Eoenfophysoh$ yudomotico 40 N-leO MEAN-,I Stondord . 298 deviotion • 70-

-

~

~

Io

/ 1 L~

50,

~ 40,

N• 4 MEAN = , 0 6 r-J ~ " " " [ I Stanaora = 2 8 I L1 deviati°n "

I [

~30. ce = 20.

l

60.

Elliploidol cell-lihe units of Eoenh~hylo/i$ yudomo#ico

_1

Short Diometorsof Ellipsoidol Cell-like U n i t s i n F m

I/

~3o ,r

/ /

2O

1

-

/

~

1

I

: 3o-t g

/

N - ll4 MEAN = 14.7

S,ondo,,'... deviotion

"'~

I0

,,, ,., ~;'Y,:;' ~2,,I',?;i~ "°l

'o/ H "~s' 'l~' ',.' ~ . ~ Long Oiometers of Ellipsoidol Cell- like Units in/J.m

Fig.13. Histograms of diameters of spheroidal and ellipsoidal cell-like units of Eoentophysalis y udoma tica.

Comparison. Both individual and colonial morphologies of E. yudomatica suggest b l u e , t e e n algal (cyanophyceae) affinities. Regarding family assignment, E. yudomatica is morphologically most consistent with assignment to the Family Entophysalidaceae. In fact, multiple envelopes of E. yudomatica resemble the lamellated vesicular sheaths observed in living Gloeocapsa Elenkin of the Family Chroococcaceae. But colonies of Gloeocapsa Elenkin contain 2--8 individual cells, seldom more. According to Desikachary (1959), the genus Entophysalis Kiitzing generally resembles Gloeocapsa Elenkin, but differs in the arrangement of the cells in distinct rows, which is clear especially in early stages of growth, but the linear arrangement is nearly lost in the later stages. Although the lineal' arrangement is not c o m m o n in the Yudomian fossils, when all morphologic characteristics are taken into account, considering the cell clusters as colonies, E. yudomatica is most closely compared to living entophysalidaceans. The colonies of E. yudomatica illustrated in Plate IV, 1--8, resemble a partial growth cycle of Entophysalis Kiitzing (see Desikachary, 1959, plate 19, 7; Geitler, 1932, figs.146b and 147a). Among other reported ancient nannofossils we see similar brown, lamellated envelopes in the % 1,900 Ma old Belcher Island microorganism Eoentophysalis belcherensis Hofmann (1976). Based on comparable gross

150

morphology, Yudomian specimens are assigned to the genus Eoentophysalis which was erected by Hofmann in 1976. The inferred divisional stages of E. belcherensis (see Hofmann, 1976, fig.5) resemble the undivided cell-like units in pairs, triads and planar tetrads surrounded by c o m m o n envelopes as seen in E. yudomatica. Also, structures resembling the "capsulata" form of E. belcherensis (encapsulated pigmented spheroids and ellipsoids, illustrated in Hofmann, 1976, fig. 5) are similar to the multi-enveloped E. yudomatica. Characteristic differences observed between these two fossil species are: (1) The Yudomian E. yudomatica spheroids and ellipsoids (4.7--20.2 gm) are apparently larger than the Belcher E. belcherensis (2.5--9 um across). (2) Specimens of E. yudomatica display different colonial habits. They do not form extensive layers nor stratiform laminae but form tabular or piano-roundish colonies, up to 180 um across. (3) "Capsulopunctate" forms (encapsulated pigmented spheroids and ellipsoids with dark inclusions) and "punctate" forms (individuals with dark inclusions without prominent capsula) such as observed in E. belcherensis are not found in E. yudomatica. Some colonial uniceUs, reported by Schopf et al. (1977) from the upper Riphean Minyar Formation of Bashkiria, resemble the Yudomian E. yudornatica in posessing sheath~enclosed cell-like units of similar size range (Schopf et al., 1977, fig.2, F-I). The Minyar colonial uniceUs, however, have not as yet been described in detail. Family Chroococcaceae N~geli 1849 Genus Tetraphycus Oehler 1978 Tetraphycus conjunctum n. sp. (Plate III, 1--5; Fig.14)

25

N= 20 Standard ~, (J

15

:57

MEAN

= 4.96

deviotlon

= 0.78

Tetrophycu$ conjuncture

C I)

=r iO ¸ b.

5 °

Diameters

5! of C e l l - l i k e U n i t s

in/cLrn

Fig.14. Histogram of diameters of Tetraphycus conjuncture.

151

Diagnosis. Cell-like units spheroidal, ellipsoidal or hemispheroidal. Surface smooth, light- to dark-brown. Wall thin, distinct, dark-brown. Cell-like units undivided, joined in groups of two to five, commonly in planar tetrads, occasionally in dyads, overlapped triads and chained triads; rarely in packets of five. Usually a c o m m o n envelope and adherent matrix for each group appears to be absent. Rarely, planar tetrads are surrounded by single or double-layered c o m m o n envelopes, but such envelopes do not surround individual cell-like units. Group of two to five cell-like units may be solitary or in planar clusters (one or two layers). C o m m o n envelope or encompassing amorphous matrix is not present in these clusters. Diameter of spheroids 3.1-6.2/~m, averaging 4.9 ~m (standard deviation = 0.78 ~m; 37 specimens measured). Clusters usually in groups of 18--45 spheroids and ellipsoids, ranging from 20--50 ~m across.

Etymology. Refers to its undivided habit (Latin conjuncture = joined). Type locality. Lower Yudoma Suite, Cloud's locality 3 of 11/8/75. Type specimen. The type is the cluster shown in Plate II, 1. It is located 13 m m above and 20.4 mm to the right of the reference " × " on thin section C670(1) from the Cloud Collection, BCL, UCSB. Discussion. In gross morphology, shape of cell-like units, wall texture, smooth surface and the c o m m o n arrangement of cell-like units in planar tetrads and diads, Yudomian specimens of Tetraphycus conjuncture resemble members of the type species Tetraphycus gregalis Oehler (1978). However, the Yudomian T. conjuncture differs from previously reported four species of Tetraphycus (Oelder, 1978), in its lack of adherent amorphous matrix, its occasional appearance of overlapped and chained triads and packets of five cell-like units, and in its diametral range of 3.1--6.2 ~m which is larger than the four described species of Tetraphycus (0.4--5 ~m) from the Australian Balhirini Dolomite (Oehler, 1978). These distinctions warrant the erection of a new species. The specimens here assigned to T. conjuncture display a distinct cyanophytic attribute: the characteristic habit of two to five cell-like units in groups. These undivided dyads, overlapped and chained triads, planar tetrads and packets of five (Plate II, 1--5) may indicate the possible sequence of cell division patterns in chroococcacean cells replicating by division in three dimensions. The habit of two to five units forming distinct groups, is considered diagnostic in locating modern analogues to these nannofosslls. A close resemblance to T. conjuncture is found among species of Gloeocapse Elenkin. Gloeocapsa crepidinum Thuret possesses cells in groups of two to four, rarely more, and has cells 4--8 ~m in diameter without a sheath (refer to Desikachary, 1959, p. 117 and plate 27,4--5; Geitler, 1932, p. 191).

152

It was noted that most cell-like spheroids and ellipsoids of T. conjunctum bear only one distinct thin wall. Gloeocapsa-type envelopes are absent and the smooth surface texture of T. conjunctum does not show any sign of degraded cell-membranes or collapsed inner sheath structures. However, two planar tetrad groups of T. conjunctum were observed to be surrounded by single- or double-layered common envelopes (Plate III, 4). Actually, many diagenetic studies of extant coccoid cyanophytes indicate that the cellular structures deform more than their extracellular envelopes, and the final product of the degradation is an empty envelope (Knoll and Barghoorn, 1975; Golubic and Barghoorn, 1977). Thus, it is not unlikely that the delimiting thin walls of the T. conjunctum nannofossils are the preserved sheaths of ancient cyanophytes, similar to modern Gloeocapsa crepidinum Thuret. Illustrations of Plate III, 2, 3, show two planar tetrads without common envelopes. These two tetrads are superposed and must be photographed to two distinct focus levels in the thin section. Interestingly, the morphology of these two tetrads resembles a three~limensional, cubical arrangement of eight cells as in the cyanophyte genus Eucapsis Clements et Shantz. Yet, modern Eucapsis usually develops 32--128 cells arranged in cubical patterns in sets of fours. Clusters of fossil T. conjuncturn are apparently not in cubical patterns.

Occurrence. Lower Yudoma Suite, Cloud's locality 3 of 11/8/75. Genus Myxococcoides Schopf 1968 Myxococcoides staphylidion n. sp. (Plate III, 6--9; Fig.15) Myxococcoide$ stophylidion 80. N :

70

286

MEAN = 5.5

Standard deviation = 2 . Z 3

60 50

o

30 20

141

'6' '8' '1~

Diameters of Ceil- like Units in/L/.m

Fig.15. Histogram of diameters of

Myxococcoides staphylidion.

153

Diagnosis. Cell-likeunits, spheroidal to ellipsoidal,occasionally polyhedral in crowded clusters. Surface smooth, unornamented, light-brown. Wall thin, solid or broken, rarely obscure, diffuse, thickness usually less than 0.8 ~m. Individual units not encompassed by any envelope. Rarely solitary,usually 10--70 individuals clumped in three
Etymology. With reference to its clustered habit (Greek staphy = cluster of grapes).

Type locality. Lower Yudoma Suite, Cloud's locality 2 of 10/8/75.

Type specimen. Illustrations6--9 of Plate III show typical clusters in thin section. The one shown in illustration 7 is cited as the type specimen. It is located 11.9 m m above and 37.8 m m to the right of the reference "X" on thin section C568(3) from the Cloud Collection, BCL, UCSB. Discussion. These specimens are distinguished from other Yudomian spheroidal nannofossils by the cell-like units clumped in three-dimensional or twodimensional colony-like clusters and by the absence of an individual envelope. The clusters of these specimens are commonly embedded in amorphous, organic matrix as well as a few globular clusters that are surrounded by halolike common envelope, suggesting an original growth habit for these clusters.

Comparison. The spheroidal and ellipsoidal nannofossils here included in the Myxococcoides staphylidion are referred to the Family Chroococcaceae on the bases of size, individual morphology, and colonial habit. Among modern Chroococcaceae, M. staphylidion seems most similar to the members of Microcystis Kiitzing (synonym: Anacystis Menegh). The nannofossfls of M staphylidion resemble extant members of Microcystis in their absence of individual envelopes and the well-packed arrangement of individual cell-like units into three~timensional subspheroidal colonies or two
154

M. staphylidion unit~ells measured 1.6--12.1 ~m in diameter while M. reticulata measured 12.5--14.7 ~m in diameter. Specimens described as M. cracens by Oehler (1978) from the Balbirini Dolomite of Australia, also resemble specimens of M. staphylidion and only differ in diametral range. Australian M. cracens measured 3.0--6.8 ~m in diameter. Another fossil to be compared with M. staphylidion is Palaeoanacystis vulgaris Schopf, one of the more abundant members of the Bitter Springs assemblage (Schopf, 1968). M. Staphylidion differs from P. vulgaris in the much less numerous component ceil-like units in each colony (P. vulgaris is characterized by more than 300 cells in each colony) and in its greater diamet. ral range (P. vulgaris measured 4.3--7.3 ~m in diameter). In fact, Palaeoanacystis and Myxococcoides possess similar morphologic characteristics and both resemble colonies of extant Mycrocystis-like chroococcacean cyanophytes. Differences between Palaeoanacystis and Myxococcoides are reported in the diametral range and the number of cells in each colony (see Schopf 1968, p.673, table 4). It is possible that members of Myxoccoides represent fragmented parts of members of Palaeoanacystis. Occurrence. Lower Yudoma Suite, Cloud's localities 1 and 2 of 10/8/75, 3 of 11/8/75 and 6 of 5/8/75. Genus Brachypleganon n. gen.

Type species. Brachypleganon khandanum n. sp. Diagnosis. Consisting of ellipsoidal to short cylindrical, wiener-like rods that are straight or curved in a lunate, semicircular, or sigmoid shape, the ends closed, rounded, nonattenuated. A few transverse sections show circular crosssections (Plate II, 11). Rods not septated, nor branched, nor enveloped. Most are single, isolated, and randomly distributed. Locally they are loosely aggregated, unattahced, and irregularly oriented in aggregations. These aggregations, however, have neither definite shape, nor common envelope or amorphous matrix. Three types of surfaces and wall structures observed: yellowish brown, smooth surface with thin, distinct wall; dark-brown di,ffuse or highly granular surface with poorly defined wall. Diameters of rods 0.8--2.7/~m, linear distances between two ends 6.0--20.3 ~m. Etymology. Refers to the short rod-like morphology (Greek brachy = short; pleganon = rod). Discussion. Short rods of the genus Brachypleganon are abundant in nonstromatolitic cherts of lower Yudomian limestones. The Brachypleganon rods are easily distinguished from other Yudomian nannofossils by their wienerlike shapes. Of the 136 rods measured, 76% have widths of 1.6--2.3 ~m.

155

Measured lengths of rods are the linear distances between two round ends. Because the curvature of the rod was small for the majority of specimens measured, measurements of linear distances are close to true lengths. The gradual change in wall structure and surface texture of these rods can be clearly observed in thin sections under the light microscope. The gradual variation between two end members -- smooth, yellowish brown rods with thin, distinct wall and densely granular, dark-brown rods without clearly delimited wall -- is interpreted as the result of post-mortem degradation and diagenesis. A few rods were found having several cilium-like structures are not considered to be true cilia of organisms, however; they more closely resemble crystal fibers of secondary origin.

Comparison. R o d morphology is c o m m o n in bacteria and in several genera of cyanophytes in the Family Chroococcaceae. Because bacterial classification is based largely on biochemical traits, the Brachypleganon rods are more reasonably compared to the Chroococcaceae on the bases of their typically small size, simple morphology, a low state of organization, and their close association with other Yudomian chroococcacean nannofossils. Among modern cyanophytes, the Brachypleganon rods have morphologic characteristics similar to the genera Rhabdoderma Schmidle and Lauterborn, and Gloeothece N~igeli. According to Desikachary (1959), it is difficult to distinguish Rhabdoderma from Gloeothece. At the species level, Brachypleganon rods bear a strong resemblance to Rhabdoderma lineare Schmidle and Lauterborn, R. gorskii Woloszynska, R. sigmoidea Carter, and Gloeothece lineare N~geli. The Brachypleganon specimens display a wider size variation than any living genus. Table II shows the comparison in size range. The age, size variation, lack of a colonial sheath (or envelope), and lack of

TABLE II Comparison of size ranges of Brachypleganon khandanum and similar species Organisms

Width

Length

(~m)

(~m)

Living species: Gloeothece linearis N~geli Rhabdoderma gor~kii Woloszynska R. Uneare Schmidle et Lauterbont R. sigmoidea Carter

1.5--2.5 1.5--2.0 2.0 2.0--3.0

10 10 6 4

Fossil species: Eosynechococcus moorei Hofmann E. medius Hofmann E. grandis Hofmann Brachypleganon khandanum n. gen., n. sp.

1.2--3.5 3.0--4.0 5.0--7.0 0.8--2.7

3 -- 9 5.5-- 7.5 11.5--19 6 --20.3

--18 --13 --10 --13

156 information on cell division prevent the confident assignment of these fossilrods to any of the living taxa mentioned above. One might consider that the mucilaginous sheath (or envelope) of the colony was probably destroyed during fossilization and diagenetic degradation processes. The other possibility is that these fossil-rods display different growth habits without any colonial mucilage, or even without colonial habit. Actually, it seems probable that the aggregate fo.rms (e.g., Plate II, 12) represent the true growth habit of these rod~shaped nannofossils. Rhabdoderma-like rods have not previously been reported from the fossil record. Compared to the oblong or rod~haped pre-Cambrian nannofossfl Eosynechococcus Hofmann from the Belcher Island, the Yudomian wienerlike Brachypleganonspecimens are more slender, more sigmoid, and lack sausage-like aggregates or closely clumped colonies (refer to plate 2 in Hofmann, 1976). Specimens of Eosynechococcus are generally about twice as long as wide, while the Brachypleganon rods are typically more than 4 times as long as wide, and average 7 times as long (128 specimens measured). Moreover, no dark inner inclusions like those of Eosynechococcus were observed in the Brachypleganon rods.

Occurrence. Lower Yudoma Suite, Cloud's locality 3 of 11/8/75. Brachypleganon khandanum n. sp. (Plate II, 9--12; Fig.16) Diagnosis. As described for the genus. Short rods 0.8--2.7 ~m in diameter, averaging 1.7 ~m {standard deviation = 0.47/~m; 136 specimens measured); 6.0--20.3 ~m long, measurement of linear distance between two ends, averaging 12.4 ~m (standard deviation = 3.91 urn; 128 specimens measured). These rods are typically more than 4 times as long as wide, averaging about 7 times as long (128 specimens measured).

Etymology. Named for the Khanda River, the native Mongoloid name for the Belaya River of Soviet usage.

Type locality. Lower Yudoma Suite, Cloud's locality 3 of 11/8/75. Type specimen. A rod illustrated in Plate II, 12, is designated as the type specimen of the species. It is located 1.9 mm above and 18.4 mm to the right of the reference " × " on thin section C670(2) from the Cloud Collection, BCL, UCSB. Plate II, 12 shows typical rods in loose aggregations. Illustrations 9 and 10 of Plate II show rods with cilium-like crystal fibers. Occurrence. Lower Yudoma Suite, Cloud's locality 3 of 11/8/75. Genus Aphetospora n. gen.

Type species. Aphetospora euthenia n. sp.

157 Brochyp/egonon khondonum 60' N = 136 MEAN = 1.76 Standard deviation - 0 . 4 7

55 50 45 40

25-

55

20-

F g ,°-

50 25

N =127 MEAN =12.2 Standard deviation = 3.92

[

g,o-

J

~'5-

oI0 5.

fl'

8 I0 12 14 16 18 20 Lengths of wiener-shaped un its in/J.m

! !i !

0.8 1.6 2.4

Widths of wiener-shaped units in ~ m

Fig.16. Histograms of diameters and lengths of Brachypleganon khandanum.

Diagnosis. Spheroidal to ellipsoidalcell-likeunits. Surface smooth, lightbrown or containing numerous submicron~ized opaque spots randomly scattered on the surface.Wall distinct,solid and thin, less than 1 ~ m in thickness, red-brown, or not well defined, preserved as interrupted surface. Habit commonly solitary,less frequently connecting or overlapping in uniseriate aggregates two to four cell-likeunits long, or a few loosely and irregularly attached together. No distinctclusteringin colony-like arrangement observed. No individual envelopes or encompassing amorphous matrix observed. Dimensions of spheroids and of long axis of ellipsoids2.3--19.5 ~m, averaging 7.8 ~ m (standard deviation = 3.8 #m; 260 specimens measured).

Etymology. With reference to the spheroidal morphology and solitary or loosely aggregated habit (Greek aphetos = free, loose). Discussion. Aphetospora is proposed as a form genus to include all solitary and loosely aggregated spheroids and ellipsoids in the lower Yudomian assemblage. The spheroids and ellipsoids so lumped together are characterized by a lack of distinct colonial organization and cannot be confidently assigned to any Yudomian taxon consisting of spheroids and ellipsoids arranged in distinc tive clusters of colonial form.

158

Considering the wide size distribution shown in the histogram of Fig. 17, it is likely that the genus Aphetospora includes biologically diverse taxa. Some cell-like objects here referred to Aphetospora may represent loose cells of colonial taxa. Because of the simple procaryotic morphology, however, it is difficult to assign them to other taxa where they are not closely associated with colonial populations. Both smooth specimens with apparently solid walls (Plate III, 12) and granular specimens with interrupted walls (Plate III, 10,11) are included in the same genus Aphetospora, that variation being interpreted as a product of diagenetic or degradational processes. Aphefosporo

euHPenio

5045-

N = 260 MEAN = 7 . 7

40-

Standord deviotion = 3 . 8

3530-

r

25-

I0 5-

3

4

Diometers

6

8

I0

of C e l l - l i k e

12

14

Units

16

18

in /u.m

Fig.17. Histogram of diameters of Aphetospora euthenia.

Comparison. Referred to the relatively simple morphology and the small size of the spheroidal and ellipsoidalnannofossiis, the genus Aphetospora is characteristically procaryotic and chroococcacean. A m o n g fossilrecords, the morphology and size range of Aphetospora spheroids and ellipsoids is similar to that of an older form-genus, Huroniospora Barghoorn, from cherts of the Gunflint Iron Formation. However, the minute aperture sometimes exhibited at the more constricted end of Huroniospora has not been observed in Aphetospora. Some members of Aphetospora occasionally connect in uniseriate aggregates or loosely attached groups whereas Huroniospora consists exclusively of unattached bodies. These specimens of Aphetospora connected in uniseriate aggregates two to three cell-likeunits long (Plate III,12) are comparable to chained specimens

159

of Myxoeoccoides inornata Schopf, described from cherts of the Bitter Springs Formation (Schopf, 1968), and the Belcher Islands Group (Hofmann, 1976). The Myxococcoides inornata specimens from the Bitter Springs and the Belcher Islands assemblages are distinguished from chained specimens of Aphetospora, however, by their occasional colonial habit and the appearance of amorphous organic matrix encompassing the chained specimens.

Occurrence. Lower Yudoma Suite, Cloud's localities I and 2 of 10/8/75, 3 of 11/8/75 and 6 of 5/8/75. Aphetospora euthenia n. sp. (Plate II, 10--12; Fig.17) Diagnosis. As for the genus. Etymology. Refers to its abundant presence (Greek euthenia = abundance). Type locality. Cloud's locality 1 of 10/8/75. Type specimen. The type specimen is shown in Plate II, 10. It is located 14.5 m m above and 36.9 m m to the right of the reference "×" on thin section C659(1) from the Cloud Collection, BCL, UCSB. Occurrence. Lower Y u d o m a Suite, Cloud's localitiesI and 2 of 10/8/75, 3 of 11/8/75 and 6 of 5/8/75. Genus Eozygion Schopf and Blacic 1971 Eozygion sp. (Plate IV, 1,2)

Description. Spheroidal to hemispheroidal cell-likeunits occurring in pairs within a prominent, non4amellated envelope. Appressed surfaces of pairs flattened. Surface smooth or granular, dark-brown. Individual units 10.1-21.8 ~ m long and 10.1--18.2 ~ m wide, averaging 17.5 ~ m long and 13.6 ~ m wide (eight cell-likeunits measured in four envelope~nclosed pairs).Prominent, non4amellated envelope, about 0.8 ~ m thick.

Discussion. The enveloped pair of spheroids and hemispheroids in Plate IV, 1, is comparable to the type specimen of Eozygion grande from the Bitter Springs cherts (Schopf and Blacic, 1971, plate 111, figs.2a--2c). They differ only in the larger size of the Yudomian form. One other Yudomian Eozygion is composed of two different sized spheroids, closely connected to a large granular and enveloping sphere (Plate IV, 2). Occurrence. Lower Y u d o m a Suite, Cloud's locality 1 of 10/8/75. Spheroids in triads (Plate IV, 8--11)

160

Description. Spheroids and ellipsoids,occurring in undivided, subtriangular triplets.Surface smooth or slightlygranular,light-brown, with distinctthin wall or diffuse thick wall 0.5--4 ~ m in thickness. N o envelope or matrix observed. Spheroids and ellipsoids1--1.6 times as long as wide (averaging 1.1 tinies),8--17.2 ~ m in the long diameter with average dimensions of 12.5 ~ m long and 11.0 ~ m wide (18 spheroids and ellipsoidsmeasured in 6 triads). The three cell-likeunits in each subtriangulartripletmay be equal in size or consist of one large unit and two smaller ones.

Discussion. These triads are well preserved in Yudomian cherts from two fossil localities. Their size and shape suggest that some of them may represent elements of the Yudomian Aphetospora euthenia. However, their distinct geometrical arrangement and isolated appearance precludes a confident assignment to genus.

Occurrence. Lower Yudoma Suite, Cloud's localities 1 of 10/8/75 and 3 of 1118175.

Diplococcus-shaped microstructures (Plate IV, 12--15) Description. Spheroidal to hemispheroidal cell-like units, 1--1.7 times as long as wide (averaging 1.2 times), 8--21.8 ~m long and 6--21.8 ~m wide with an average length of 12.5 pm and an average width of 10.2/~m (19 specimens measured). These cell-like units occur in Diplococcus-like pairs with adjacent surfaces flattened. Surface smooth or slightly granular, light-brown. Wall thin or thick, red-brown, 0.8--2.3 ~m thick. Mostly without envelopes or matrix; occasionally a common envelope and yellow amorphous matrix is present. Diplococcus-shaped pairs occur singly or in planar colonies.

Discussion. These Diplococcus-shaped microstructures resemble chroococcacean cellsat division.Cell-likeunits, occurring in undivided pairs,have been reported from the Bitter Springs assemblage as Sphaerophycus Schopf (1968) and from the Belcher Islands cherts as some elements of EoentophysalisHofmann (1976). However, the Yudomian Diplococcus-shaped units differ from Sphaerophycus (2.1--3.6 ~m) and Eoentophysalis (2.5--9 pro) in their larger size,their hemispheroidal shape and the absence of any inner contents. They m a y represent elements of the Yudomian Aphetospora at a different stage of growth.

Occurrence. Lower Y u d o m a Suite, Cloud's localitiesI and 2 of 10/8/75, and 3 of 11/8/75.

Spheroids in linear arrangement Family Chroococcaceae N~igeli1849 (?) (Plate IV, 16, 17)

161

Description. Spheroids, continuously overlapped or connected in a single linear direction forming a uniseriate aggregate. Surface smooth, light-brown, no envelope, no amorphous matrix. Wall thin, or thick, red-brown, about 1 ~ m in thickness. Spheroids 3.9 ~m--8.5 ~ m in diameter, averaging 6.2 ~ m (12 specimens measured).

Discussion. Two uniseriate aggregates of spheroids are observed from one fossil locality, 3 of 11/8/75. One is composed of six spheroids, 7.8--8.5 ~m in diameter and averaging 8.0/am. The first five spheroids of this uniseriate aggregate overlap at regular intervals. The last spheroid drops from the linear arrangement (Plate IV, 16). The other uniseriate aggregate contains four spheroids in linear arrangement and a pair of the same kind of spheroid (Plate IV, 17). These spheroids measure 3.9--5.5 ~m in diameter and average 4.5/am. The linear arrangements of these cell-like spheroids are geometrically unique and are unusual among ancient nannofossils records. Based on their simple spheroidal morphology, their close association with loosely aggregated spheroids of Aphetospora and the comparable size, these linear aggregations are probably ancient organisms of chroococcacean affinity. On the other hand, these chains of spheroids might represent part of the compressed sheathless multi-cellular trichome, like some members in the nostocacean cyanophytes, or pseudohorrnocysts (chains of round cells), as in Westiellopsis Janet, a stigonematacean cyanophyte.

Occurrence. Lower Yudoma Suite, Cloud's locality, 3 of 11/8/75. Genus Caryosphaeroides Schopf 1968 (?) Caryosphaeroides? sp. (Plate IV, 3--7)

Description. Spheroidal and ellipsoidal to hemispheroidal cell-like units, commonly occuring in undivided pairs, occasionally solitary, or in triads or a few units together. Each unit contains dark central inclusions. The inclusions appear in two types: (1) multiple dark granules, densely clustered into subspheroidal bodies about 3 #m in diameter; (2) spheroidal to oblong bodies that are massive, brown, and 3.9--8.0/am in longest dimension with an average diameter of 5.0/am (15 bodies measured), usually one to three bodies adhering in center of cell-like units. Cell-like units 10.9,19.2/am long and 7.8--16.2/am wide with an average length of 13.8 ~m and an average width of 11.3 um (13 specimens measured). Surface smooth, light-brown, with distinct thin wall, without matrix, with obscure or no envelope.

Discussion. These cell-like units, shown in Plate IV, 3--7, resemble Caryosphaeroides of the Bitter Springs assemblage (Schopf, 1968, p.677, plate 85, figs. 1--6). The undivided pairs of specimens from the lower Yudoma Suite also resemble the Australian Caryosphaeroides shown in the paper of Schopf and Blacic (1971, p . l l l , fig.4).

162

The Yudomian specimens appear to differ from the Australian specimens in being larger and displaying a more variable morphology of dark inner inclusions. The structures of "inner wall" and "nuclear residue" described by Schopf (1968) are not discernable. In fact, after the observation and recognition of important degradational features in fossil cyanophytes, the assignment of Caryosphaeroides to the eucaryotic Family Chlorellaceae by Schopf (1968) based on the interpreted degraded "nucleus" must be,questioned. The characteristics of inner inclusions observed in these Yudomian specimens are also comparable to those of Melasrnatosphaera Hofmann described from the Belcher Islands assemblage (Hofmann, 1976) and Glenobotrydion Schopf described from the Bitter Springs assemblage (Schopf, 1968). Hofmann (1976) suggested that Melasmatosphaera, Glenobotrydion, and Caryosphaeroides may be morphological variants of a single biologic species of chroococcacean cyanophytes. Such an interpretation is consistent with observations here made. The colony-like cluster shown in Plate IV, 4,5, including a few single celllike units and a few undivided pairs of cell-like units, seems to represent phases of cell growth and fission. Among these significantly ornamented celllike units, undivided pairs are predominant.

Occurrence. Lower Yudoma Suite, Cloud's localities 1 and 2 of 10/8/75, and 3 of 11/8/75.

Incertae sedis Genus Eosphaera Barghoorn 1965 (?) Eosphaera? sp. (Plate IV, 18) N=8 C Q

~

I0

MEAN = 6.6

5

M/cfhysfridtum

sp.

U_

Diometer$ of Cell- like Units In fLm Fig.18. Histogram of diameters of Micrhystridium sp.

Description. A complex spheroidal structure consisting of a thick-walled, slightly granular red-brown spheroid having four finely granular spheroidal to ellipsoidal envelopes, encompasses the entire inner thick-walled spheroid and its peripheral tubercles. Dimension of inner spheroid, 13.1 ~m; dimension of outer ellipsoidal envelope, about 16 u m X 20 ~m; four tubercles 3.9 ~m × 3.9 ~m, 3.1/~m × 4.0/~m, 2.3 ~m × 4.8 ~m, 3.1 ~m X 7.0 #m.

163

Discussion. Only one specimen of this composite form has been found. Its compound nature, general morphology, and size suggest Eosphaera Barghoorn of the Gunflint microbiota. Although the Yudoma specimen is somewhat bigger than Barghoorn's Eosphaera, the features displayed are consistent with a provisional assignment to the Gunfiint taxon, considering vagaries of preservation and diagenesis. Confident taxonomic assignment, however, must await the discovery and study of additional material.

Occurrence. Lower Yudoma Suite, Cloud's locality 2 of 10/8/75. Branched tubes with ellipsoidal inclusions (Plate IV, 20) Diagnosis. Incomplete tubular microstructureswithout observed septae but branched and containing ellipsoidalinclusionsthat suggest a pluricellular structure.Wall thin and distinct.Surface granular.Solitaryor in sparse aggregates.Gently curved. Width not uniform, pinches and swells.Swollen areas up to 10.9 # m in diameter, constrictedparts about 3.9 ~m, averaging about 7.6 p m (5 specimens measured). These tubular structurescontain brown to opaque ellipsoidalinclusions,6.2--7.8# m wide and 11.7 ~ m long, with an average diameter 7.2 ~ m X 11.7 # m (4 ellipsoidsmeasured). These ellipsoidsare spaced about 2.5 ~ m apart in the tube.

Discussion. These broken tubular filaments are rare but distinctive features in the Yudomian cherts. The regularly spaced ellipsoidal inclusions may represent cells of a trichome. The distinct branching feature is shown in one specimen. The angle of branching measures 88 ° . These tubular microstructures might represent branching filamentous cyanophytes such as those of the Family Stlgonemataceae, or small eucaryotes, or fungal hyphae. They have no close counterparts among previously described nannofossils.

Occurrence. Lower Y u d o m a Suite,Cloud's locality2 of 10/8/75. Group Acritarcha Evitt 1963 Subgroup Acanthomorphitae Downie, Evitt and Sarjeant 1963 Genus Micrhystridium Deflandre 1937; emend. Downie and Sarjeant 1963; emend. Sarjeant 1967 Micrhystridium sp. (Plate IV, 19, 21--23; Fig.18)

Description. Distinct spheroidal bodies with sparse short spines. Light- to dark-brown, with a single thin, homogeneous wall of constant thickness. Surface simple, not divided into fields or plates by crests and without any surficial reticulum. The tiny, simple spines at the surface are thorn-like, sharply pointed, closed distally, radiating, and numerous. Specimens are solitary, rarely clustered in small groups. Diameter without spines 5.0--8.6 ~m, averaging 6.6 ~m (8 specimens measured). Length of the spines less than 1.5 ~m. Opening not observed.

164

Discussion. Based on the observed morphology in thin sections, these tiny, spiny spheroidal bodies are assigned to the form genus Micrhystridium, under the acritarch subgroup Acanthomorphitae. This type of acritarch is a rare element in the Yudomian microbiota. Only eight specimens have been found in a.systematic search of eight thin sections from two localities. The length and the density of spines vary among these Yudomian Micrhystridium. The specimens illustrated in Plate IV, 21 and 22, bear fewer, longer, more thorn-like spines, 1--1.5/~m long. The specimens in Hate IV, 19, have more numerous, shorter, more cilium-like spines, about 0.8--1.0 ~m long. These spiny acritarchs are closely associated with solitary, smooth spheroidal nannofossils but not filamentous forms. Along with other evidence, and in the absence of data to the contrary, they are taken as indicative of a Phanerozoic, probably Ediacarian age.

Occurrence. Lower Yudoma Suite, Cloud's localities 3 of 11/8/75 and 2 of 10/8/75. ACKNOWLEDGEMENTS

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