The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events

The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events

    The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events Vivi V...

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    The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events Vivi Vajda, Antoine Bercovici PII: DOI: Reference:

S0921-8181(14)00147-7 doi: 10.1016/j.gloplacha.2014.07.014 GLOBAL 2154

To appear in:

Global and Planetary Change

Received date: Revised date: Accepted date:

9 March 2013 21 July 2014 30 July 2014

Please cite this article as: Vajda, Vivi, Bercovici, Antoine, The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events, Global and Planetary Change (2014), doi: 10.1016/j.gloplacha.2014.07.014

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The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: a template for other extinction events

Vivi Vajda a,*, Antoine Bercovici a

Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden.

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*Corresponding author. Tel.: + 46 46 222 4635

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E.mail address: [email protected]; (V. Vajda)

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Abstract

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Changes in pollen and spore assemblages across the Cretaceous–Paleogene (K–Pg) boundary

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elucidate the vegetation response to a global environmental crisis triggered by an asteroid impact in Mexico 66 Ma. The Cretaceous–Paleogene boundary clay, associated with the Chicxulub asteroid impact event, constitutes a unique, global marker bed enabling

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comparison of the world-wide palynological signal spanning the mass extinction event. The data from both hemispheres are consistent, revealing diverse latest Cretaceous assemblages

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of pollen and spores that were affected by a major diversity loss as a consequence of the K– Pg event. Here we combine new results with past studies to provide an integrated global

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perspective of the terrestrial vegetation record across the K–Pg boundary. We further apply the K–Pg event as a template to asses the causal mechanism behind other major events in

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Earths history. The end-Permian, end-Triassic, and the K–Pg mass-extinctions were

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responses to different causal processes that resulted in essentially similar succession of decline and recovery phases, although expressed at different temporal scales. The events

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share a characteristic pattern of a bloom of opportunistic "crisis" tax followed by a pulse in pioneer communities, and finally a recovery in diversity including evolution of new taxa. Based on their similar extinction and recovery patterns and the fact that Last and First Appearance Datums associated with the extinctions are separated in time, we recommend using the K–Pg event as a model and to use relative abundance data for the stratigraphic definition of mass-extinction events and the placement of associated chronostratigraphic boundaries.

ACCEPTED MANUSCRIPT Biostratigraphy; Palynology; asteroid; CAMP, Siberian traps, end-Triassic; end-Permian, P–

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T boundary, Tr–J boundary

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1. Introduction

As a consequence of an asteroid impact, 66 million years ago, the biosphere experienced a global extinction event so large that it defines the boundary between the Mesozoic and the

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Cenozoic eras (Alvarez et al., 1980; Alvarez, 1983; Renne et al., 2013). It is now over 30

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years since the hypothesis of an asteroid impact forming the Cretaceous–Paleogene (K–Pg) boundary was advocated by Walter and Luis Alvarez (Alvarez et al., 1980), based on

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anomalously high levels of iridium in the clay layer that forms the boundary between the Maastrichtian and the Danian- the so called boundary clay. The iridium anomaly was first

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detected in marine deposits from the boundary clay in Denmark (Stevns Klint; Fig. 1A–B) and Italy (Gubbio); Alvarez et al., 1980). Iridium has since been detected in association with

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the K–Pg boundary clay at other marine sites worldwide (Bohor et al., 1987a, b; Lerbekmo et

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al., 1999; Eggert et al., 2009; Schulte et al., 2010), and also at some sites that represented land during the end-Cretaceous event (Fig. 1C–F; Sweet et al., 1999; Sweet and Braman, 2001; Vajda et al., 2001, 2004; Nichols and Johnson, 2008 and references therein; Ferrow et al., 2011). Other anomalies corroborating an impact event have been detected since those first observations, including high levels of the elements; nickel, chromium and iron, and the presence of quartz grains with planar deformation features (PDFs) (Alvarez et al., 1995; Schulte et al., 2010). During the final phase of the Cretaceous, the typical marine food web was founded on protists such as coccolithophorids and foraminifera, which occurred in such vast numbers that their tests provided the main components of the extensive Upper Cretaceous chalk deposits.

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These protists mainly inhabited the photic zone, where most (but not all) were directly dependent on sun-light for photosynthesis. Other protists included dinoflagellates and

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radiolarians, the latter forming cherts in deeper marine environments. The Cretaceous oceans also hosted a diverse range of marine invertebrates; the most spectacular probably being the

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ammonites: cephalopods that reached large sizes and varied growth forms during the latest Cretaceous. The top predators in the oceans were the vertebrates, such as sharks, mosasaurs and plesiosaurs (e.g. Gallager et al., 2012). In non-marine environments, vertebrate faunas

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were dominated by non-avian dinosaurs, together with crocodilians, turtles, snakes and non-

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eutherian mammals (Buffetaut et al., 1999; Russell and Manabe, 2002; O’Leary et al., 2013). The late Maastrichtian saw the initiation of a period of major volcanic activity, the

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Deccan flood basalt volcanism (Traps) in what is today western India. During the ~1-millionyear-long main phase of eruption, ~80% of the Deccan activity occurred during two distinct

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intervals (Chenet et al., 2009 and references therein): one during the late Maastrichtian starting 400 kyrs before the K–Pg boundary (Ravizza and Peucker-Ehrenbrink, 2003;

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Robinson et al., 2009) and during the early Danian (Cripps et al., 2005). Some investigators

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have repeatedly advocated that Deccan volcanic activity was the primary cause behind the K– Pg extinction (Keller, 2008; Keller et al., 2009) based on the argument that the extinction of species was gradual during the Maastrichtian (Clemens et al., 1981). Although the mass extinction seems to have been preceded by ~4°C warming as a possible result of the Deccan volcanic activity (Wilf et al., 2003; Self et al., 2006), no major diversity loss within the Maastrichtian terrestrial biota has been noted. Instead, the North American fossil record documents an increase in plant diversity associated to the end-Cretaceous warming event (Wilf et al., 2003). Recent radiometric dating (Renne et al., 2013) shows that the asteroid impact was clearly coincident with abrupt global ecosystem disruption and the extinction of several key groups, such as the non-avian dinosaurs, pterosaurs, marine reptiles (Buffetaut,

ACCEPTED MANUSCRIPT 1984, 1990; Jiang et al., 2001; Fastovsky and Sheehan, 2005; Fastovsky and Weishampel, 2005; Gallager et al., 2012), the ammonites and marine protists (Smit, 2012). Major losses

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with widespread changes in terrestrial and marine biotic representation were also recorded within groups that did not go extinct such as the foraminifera, coccolithophorids, turtles,

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crocodilians and mammals (Longrich et al., 2011, 2012; Smit, 2012 and references therein; O’Leary et al., 2013 and references therein).

Thus far, about 350 K–Pg boundary sections have been described globally, among which

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105 are from terrestrial deposits (Fig. 2) based on the compilation by Nichols and Johnson,

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(2008). Different methods can be applied for identifying the K–Pg boundary in these sections, the most reliable being the identification of the geochemical and mineralogical

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signals outlined above that relate directly to the Chicxulub impact. This geochemically anomalous marker bed can be used as a unique and essentially instantaneous timeline for

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global correlation. Biostratigraphy can also be useful in pinpointing the boundary, especially where complemented by radiometric dating and paleomagnetic evidence where suitable

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material is available.

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A synthesis of the global data clearly shows that the terrestrial K–Pg boundary is currently best known from North America (Northern Great Plains of the United States and central Canada) for the Northern Hemisphere, and from New Zealand for the Southern Hemisphere (Fig. 2). Here we focus on the global record of terrestrial pollen and spore assemblages across the K–Pg boundary with supporting data from the macrofloral record. Studying the response of standing vegetation at the time of the impact integrated with geochemical and sedimentological analysis, and calibrated with high resolution radiometric dates (Renne et al., 2013), is key to understanding the extinction and recovery process. As primary producers, land plants underpin all terrestrial food chains and are a source of sustenance and shelter for

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numerous terrestrial faunal groups. The pollen and spore record is unique in that it does not represent individual organisms but rather reproductive cells produced in high quantities by

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land plants. Being produced in abundance, durable and microscopic, they have the advantage of being preserved in great quantities in terrestrial sediments (with 104 to 106 specimens per

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gram of sediments being typical), thus providing large datasets with high statistic fidelity. Moreover, terrestrial spores and pollen are transported in abundance into marine settings thus permitting correlation of terrestrial and oceanic events (Saito et al., 1986; Brinkhuis and

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Schiøler, 1996; Vajda and Raine, 2003; Molina et al., 2006; Willumsen and Vajda, 2010).

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Our study integrates published palynological data from terrestrial settings across the K– Pg boundary at a global scale, and provides new data on diversity trends related to the

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extinction event. This complements the comprehensive review of vegetation patterns across the K–Pg boundary compiled by Nichols and Johnson (2008). We also compare and contrast

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the event stratigraphy of the K–Pg with those of the Permian–Triassic (P–Tr) and the Triassic–Jurassic (Tr–J) boundaries to assess the criteria for stratigraphically defining major

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geological events in Earth’s history.

2. The Late Cretaceous

2.1. Cretaceous climate and sea level

During the Early Cretaceous, the two supercontinents Gondwana and Laurasia were almost entirely separated by the Tethys Ocean though each supercontinent was also undergoing internal fragmentation at this time (Fig. 3A; McLoughlin, 2001; Torsvik et al., 2001). The tectonic activity was intense, leading to extensive continental rifting, the opening of new ocean basins, and formation of new mountain ranges (Miller et al., 2005). By the end

ACCEPTED MANUSCRIPT of the Cretaceous, most of the modern continents had separated from each other and the North and South Atlantic were shaped (Torsvik et al., 2001). Although cool phases have been

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reported for the Early Cretaceous, the Late Cretaceous was a greenhouse world with high atmospheric CO2 levels (Beerling et al., 2001; Royer, 2003; Wang et al., 2014) and this is

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evident both in the composition of the fossil biota containing thermophilic elements in polar regions, and in the sedimentary record, expressed by widespread occurrences of coal, evaporites and laterites (Craggs et al., 2012). Polar ice caps were absent, or small in the Late

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Cretaceous (Kominz et al., 2008), resulting in high sea levels relative to present conditions

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(50–100 m higher: Miller et al., 2005; or 75–100 m higher: Kominz et al., 2008). A considerable portion of the areas that today are land were covered by sea during the

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Cretaceous. This included large parts of Western Europe, eastern South America, central Australia and northern parts of Africa (Scotese, 2001). Central North America was covered

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by the Western Interior Seaway, a shallow sea rich in marine life (Hoganson and Murphy,

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2002; Murphy et al., 2002).

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2.2. Floristic provinciality in the latest Cretaceous

The Cretaceous is characterized by the appearance of a new plant group, the angiosperms (flowering plants), which by the mid-Cretaceous had re-shaped global terrestrial ecosystems. The oldest records of angiosperms are represented by pollen grains in Hauterivian and Barremian sediments from various, mostly low-paleolatitude areas, from north-western Europe (Hughes et al., 1991) to central and northern Africa (e.g. Doyle et al., 1977). The fossil record reveals a remarkable increase in angiosperm diversity in the mid-Cretaceous (Crane and Lidgard, 1989; Friis et al., 2001, 2006) and by the Late Cretaceous, flowering plants began to dominate the global vegetation supplanting ferns, conifer, Bennettitales and

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seed ferns that had dominated earlier Mesozoic floras (Heimhofer et al., 2007; Friis et al., 2011).

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Floristic provinciality reached its Mesozoic zenith during the Maastrichtian, probably as a consequence of habitat fragmentation caused by continental breakup and high sea-levels.

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The Late Cretaceous world has been divided into four major and two minor palynofloristic provinces (Zaklinskaya, 1981; Herngreen et al., 1996 and references therein, Fig. 3B), largely based on distinctive angiosperm pollen and fern spores of restricted geographic and

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stratigraphic distribution. We have updated the boundaries of these provinces based on new

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palynological data, e.g., revealing that India included three provinces during the Maastrichtian (Fig. 3B). We outline the characteristics of the four main provinces below.

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The Aquilapollenites Province had a northern circumpolar distribution extending to latitude 50° N but ranging to lower latitudes in North America and Asia (Fig. 3B;

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Zaklinskaya, 1981; Herngreen et al., 1996). This province is characterized by abundant and diverse representatives of the morphologically distinctive Aquilapollenites pollen group

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(triprojectate pollen, with three protruding pores). The affinity of Aquilapollenites-

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Triprojectacites complex have been discussed extensively in the literature and it has been put forward that these pollen are closely related to pollen produced by the modern Saantalales, more specifically to the Loranthaceae family (Funkhouser, 1961; Jarzen, 1977; Muller, 1984) Loranthaceae are woody plants of which the majority are hemi-parasites, with mistletoe habit (Friis et al., 2011). Aquilapollenites Group pollen have been reported from numerous Late Cretaceous northern high-latitude areas, including Greenland, North America, northern and eastern Asia (Herngreen et al., 1996), but they also occur in relatively high abundance in Maastrichtian assemblages from the Palmae Province of Bolivia (Pérez-Leytón, 1987; VajdaSantivanez, 1999) and Brazil (Regali et al., 1974; Herngreen, 1975). Aquilapollenites reached its maximum distribution and diversity during the Maastrichtian but most representatives

ACCEPTED MANUSCRIPT become extinct at the K–Pg boundary with rare occurrences up to the Oligocene; e.g. Farabee (1991). The Khatanga-Lena subprovince is part of the Aquilapollenites Province and is

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distinguished by the absence of the oculate pollen Wodehouseia (Zaklinskaya, 1977; Fig. 3B). The Normapolles Province extended from eastern North America, through Europe and to

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western Siberia, the Middle East and Himalayas, (e.g. Herngreen and Chlonova, 1981; Herngreen et al., 1996; Song, 1995; Fig. 3B). The Normapolles Group is characterized by triaperturate and oblate pollen and a systematic affinity to the Fagales has been proposed

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(Friis et al., 2006). This group reached its maximum geographic distribution during the latest

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Cretaceous and constituted an important and diverse element of many Late Cretaceous and early Cenozoic floras of the Northern Hemisphere (Friis et al., 2006).

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The Palmae Province, characterized by the presence of palm pollen, extended through northern South America, central Africa and parts of India, zones that represented low-latitude

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regions during the Late Cretaceous (Fig. 3B, Pan et al., 2006). Palm pollen constitutes up to 50% of the pollen grains within assemblages from these areas (Herngreen and Chlonova,

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1981; Herngreen et al., 1996) and probably reflects warm, ever-wet conditions. Palms are

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also represented by fossil flowers and fruits in the Maastrichtian Deccan Intertrappean Beds of India (Friis et al., 2011). The Proteacidites/Nothofagidites Province was represented at high latitudes of the Southern Hemisphere including Australia, New Zealand, Antarctica, and the southern part of South America (Srivastava, 1978, 1981; Herngreen et al., 1996). This province is typified by the abundant presence of Nothofagiidites (Nothofagus) and Proteacidites pollen (Herngreen et al., 1996). Palynological assemblages from South Africa and Madagascar lack Nothofagiidites pollen, hence these areas do not strictly belong to this province (Nichols and Johnson, 2008). Each of the major floristic provinces is bounded by zones of intermixed floras (Fig. 3B).

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2.3. The latest Cretaceous palynological record in North America

The North American fossil record is probably the most suitable for documenting the

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evolution of the latest Cretaceous terrestrial ecosystems leading up to the K–Pg boundary. Excellent exposures of upper Maastrichtian to lower Danian strata spanning the K–Pg boundary yield a diverse range of abundant fossils (vertebrates, invertebrates, plant

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macrofossils and pollen) that can be studied with high stratigraphical resolution. Thus, the

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vast majority of high-resolution K–Pg palynological studies have been within the Aquilapollenites province of central and northern North America.

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The best existing fossil record has been described from the Williston Basin in North Dakota USA (Johnson et al., 2002) which extends further north into Canada (Saskatchewan

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and Alberta) up to the Canadian Northwest Territories (Sweet and Braman, 2001). The palynology of the upper Maastrichtian successions is well-constrained; assemblages

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correspond to the Wodehouseia spinata Assemblage Zone, the first occurrence of this species

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marking the base of the upper Maastrichtian (Nichols, 1994; Bercovici et al., 2012a; Braman and Sweet, 2012). The typical Wodehouseia spinata assemblage of North America features diverse and abundant fern spores and angiosperm pollen, among which are several species of Aquilapollenites (Nichols and Johnson, 2008). Most palynological studies have focused on the identification of the K–Pg boundary. Consequently, most high-resolution palynological studies have been limited to a few meters of stratigraphic interval spanning the boundary. Nevertheless, several lower resolution studies of the palynological record covering the entire Wodehouseia spinata Assemblage Zone have been conducted, and the results from these show that there is no major variation within the composition of the miospore assemblages (Nichols and Johnson, 2008).

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2.4. Fossil macrofloras before the end of the Cretaceous

Assemblages from the topmost Cretaceous in the Hell Creek Formation have yielded the

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richest known Cretaceous macrofloral assemblages in the world (Johnson, 2002). Multiple sites showing an exceptionally high abundance and diversity of leaf fossils have been described from various terrestrial depositional settings, such as flood plains, ponds, crevasse

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splays, levees and within channels (Fastovsky, 1987; Johnson, 2002). The fossil plant record

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leading up to the K–Pg boundary has been extensively sampled and documented in North Dakota (Johnson, 1989; Johnson et al., 1989). Diverse assemblages of rainforest-type plants,

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largely dominated by angiosperms, are associated with several ferns, some conifers (Taxodiaceae, Cupressaceae, Araucariaceae), cycads and ginkgoes (Johnson, 2002; Axsmith

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et al., 2004; Axsmith and Jacobs, 2005). The environment was clearly forested but the mix of deciduous and evergreen elements with palm trees and conifers, and extremely high diversity

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of angiosperms with lobed leaves in the Hell Creek forest communities do not have any close

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modern analogue (Johnson, 2002; Raynolds and Johnson, 2003; Vajda et al., 2013a). Floral changes are clearly visible within the stratigraphic succession of the Hell Creek Formation. Johnson and Hickey (1990) divided the Hell Creek macroflora into three assemblages subsequently revised by Johnson (2002); and named HCI (a and b), HCII (a and b) and HCIII (Fig. 4). These Hell Creek assemblages are largely dominated by angiosperms with a few rare ferns, one cycad, one ginkgo, and conifers representing the Taxodiaceae, Cupressaceae, and Araucariaceae (Nichols, 2002). A macroflora dominated by small leaved herbaceous and shrubby vegetation characterize the lower Hell Creek whilst the topmost macroflora is significantly more diverse (Fig. 4). The variations in floral composition reflect

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paleoclimatic changes with a significant mean annual temperature increase at the end of the Cretaceous (Johnson and Hickey, 1990; Wilf et al., 2003).

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Crossing the K–Pg boundary, the diverse angiosperm-rich macroflora is replaced by a low diversity, angiosperm-dominated flora. In the lowermost Paleocene, local occurrence of

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specific mire vegetation is thought to represent survival of Cretaceous swamp vegetation (Johnson, 2002).

New Zealand Upper Cretaceous and Paleogene non-marine sediments host well-

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preserved macrofloras incorporating leaves, flowers and seeds. These floras were first studied

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by von Ettingshausen (1887, 1891) who focused mainly on Late Cretaceous assemblages from the Otago and Canterbury areas. Later, extensive work by Couper (1960) documented

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the palynological and the macrofossil record of the Maastrichtian, and the biogeography of the New Zealand vegetation was presented in Mildenhall (1980). More recent works include,

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e.g., those of Pole (1992, 1995, 2008), Pole and Douglas (1999), Kennedy (1993, 2003) and Kennedy et al. (2002). Generally, the Cretaceous floras of New Zealand comprise

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Araucariaceae, Taxodiaceae, Podocarpaceae and dicotyledonous angiosperms (Pole and

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Douglas, 1999; Kennedy, 2003). The most striking transition in the macroflora is the significant decrease of Araucariaceae across the K–Pg boundary (Pole, 2008). High floral diversity in the upper Hell Creek Formation is correlated to an episode of warming that occurred during the last 400 kyrs of the Cretaceous. Leaf margin analysis (LMAT) based on North American leaf assemblages (Wilf et al., 2003; Fig. 4), shows an estimated ~4°C warming on land at around 66.5 Ma, i.e. 500 kyrs prior to the K–Pg event. This warming event is also reflected in variations within the marine record and lasted to 100 kyrs before the K–Pg event (Thibault and Gardin, 2010). Wilf et al. (2003) suggested that the temperature increase was driven by pCO2 rise from Deccan volcanic activity (Fig. 4).

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The Hell Creek and contemporaneous formations in North America (i.e. the Scollard, Willow Creek, North Horn, Frenchman, Lance Creek, Denver, and McRae formations) yield

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large quantities of latest Cretaceous vertebrate fossils, including fish, amphibians (frogs and salamanders), turtles, lizards, crocodilians, champsosaurs, dinosaurs including birds, pterosaurs and early mammals. Though renowned for its megaherbivores, such as Triceratops

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spp. and Edmontosaurus annectens, together with large carnivores, such as Tyrannosaurus

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rex (Pearson et al., 2001, 2002; Sampson and Loewen, 2005; Lyson and Longrich, 2011), the Hell Creek non-avian dinosaur fauna is of relatively low diversity (Vavrek and Larsson,

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2010; Mitchell et al., 2012; Brusatte et al., 2012). Despite previous claims of a latest Cretaceous gap, dinosaur fossils have been recovered up to the very top of the Hell Creek

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Formation. Extensive sampling of the vertebrate content of the Hell Creek Formation has demonstrated that there is no significant decline in vertebrate diversity during the last 1.3

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Myrs of the Cretaceous (Fig. 4; Sheehan and Fastovsky, 1992; Pearson et al., 2002; Sheehan

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et al., 2000; Fastovsky et al., 2004). This is in agreement with studies of marine reptiles from New Jersey, USA, showing a sudden extinction of Mosasaurs (Gallager et al., 2012).

3. The global K–Pg boundary interval

3.1. The boundary layer

Deposition associated with the Chicxulub impact is characterized by ballistically and debris-flow transported material from the target rock in Yucatan. This sequence thins with distance from the impact site (Bohor et al., 1984; Bohor and Izett, 1986; Ocampo et al., 2006;

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Schulte et al., 2010). Thus, the thickest deposits occur in Belize and southern Mexico proximal to the crater, where the successions reach up to 45 m in, e.g. Belize (Ocampo et al.,

New Zealand (Moody Creek Mine, Vajda et al., 2001).

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1996). The thinnest deposits constitute only a few millimeters in antipodal regions such as

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In Europe, mainly marine K–Pg boundary successions are preserved and no complete terrestrial K–Pg boundary section (with a diagnostic boundary clay including shocked quartz and a geochemical anomaly) has yet been identified there.

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In North American terrestrial successions, the K–Pg boundary is commonly evidenced by

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a several centimeter thick boundary claystone that can typically be subdivided into 2–3 units (Sweet et al., 1999; Sweet and Braman, 2001). At Mud Buttes (North Dakota, USA) the K–

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Pg boundary claystone contains a 1 cm thick white kaolinitic "boundary" clay representing low-angle ejecta (Fig. 1 C, D). This is overlain by an orange layer of millimeter-sized glass

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spherules altered to clays, representing the high-angle ejecta (Sweet et al., 1999; Sweet and Braman, 2001; Ocampo et al., 2006; Nichols and Johnson, 2008). Above this occurs sub-

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millimetric quartz grains with planar deformation features typical of shock metamorphism

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(Izett, 1990; Nichols and Johnson, 2008), together with nickel-bearing spinels derived from the impactor itself vaporizing on atmospheric entry (Robin et al., 1992; Rocchia et al., 1996). The iridium anomaly varies considerably from site to site and reaches maximum concentrations of around 56 ppb in North American boundary claystone (Izett, 1990) and 71 ppb in New Zealand where the boundary is within a coal seam (Vajda et al., 2001). The iridium anomaly co-occurs with high concentrations of other elements such as Fe, Cr, Ni and Au presumably derived from the asteroid (Schulte et al., 2010). The proximal ejecta deposits in northern Belize and southern Quintana Roo, Mexico, are characterized by two distinct stratigraphical units; the lower spheroid bed and the upper diamictite bed, both part of the Albion Formation (Pope et al., 1993; Ocampo et al., 1996; Wigforss-Lange et al., 2007;

ACCEPTED MANUSCRIPT Vajda et al., 2014). These deposits are directly linked to the Chicxulub impact where the spheroid bed represents the initial vapor-plume deposits, and the overlying diamictite bed

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constitutes a debris flow attributed to the collapse of the ejecta curtain (Ocampo et al., 1996) the direct link to the Chicxulub impact is further corroborated by geochemical analysis of

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these ejecta deposits (Wigforss-Lange et al., 2007). Radiometric dating of shocked zircons found in the boundary claystone in Haiti (Schulte et al., 2010), Spain and Italy (Kamo et al., 2011) reveal a direct link to the basement rocks of the Yucatan Peninsula. Additionally, Ar/39Ar dates of 66.032 ± 0.072 Ma obtained from impact glass (tektites) from Beloc, Haiti,

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40

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derived from the Chicxulub impact, match 40Ar/39Ar dates obtained for local volcanic ashes bracketing the K–Pg boundary layer in northern USA (Renne et al., 2013). This synchronicity

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reaffirms the Chicxulub impact as the cause of the extinction at the K–Pg boundary (Renne et

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al., 2013).

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3.2. Palynostratigraphy of the K–Pg boundary interval

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Where the geochemical and mineralogical markers of the asteroid impact are not present or ill-defined, high resolution palynostratigraphy is used as the main method for locating the K–Pg boundary. Flowering plants and especially dicotyledonous angiosperms were most affected by the K–Pg extinction event, as evidenced in both the macrofloral and palynofloral records of North America and in New Zealand (Nichols, 1990; Pole, 2008; Vajda et al., 2001, 2004; Nichols and Johnson, 2008). In the Aquilapollenites Province of North America (Fig. 3B), a series of taxa become extinct at the K–Pg boundary and are commonly referred to as "K-species" (Hotton, 1988, 2002) or "K-taxa" (Nichols, 2002), K signifying Cretaceous, (i.e. Cretaceous key-taxa), not to be confused with K-strategy taxa of ecological studies (MacArthur and Wilson, 1967). The stratigraphically important key species include most

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species of Aquilapollenites, and several other morphologically distinctive angiosperm pollen taxa (Cranwellia edmontonensis, Erdtmanipollis cretaceus, Leptopecopites pocockii,

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Marsypiletes cretaceus, Tricolpites microreticulatus, Striatellipollis striatellus, Styxpollenites calamitas; Nichols, 2007; Bercovici et al., 2012a). The Paleocene assemblages of North

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America are characterized by a suite of Momipites and/or Caryapollenites (Nichols and Johnson, 2008 and references therein).

Within the Proteacidites/Nothofagidites province, more specifically in New Zealand K–

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Pg successions, a similar scenario is evident where typical end-Cretaceous key-species,

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mainly angiosperms, disappear at the boundary, including: Tricolpites lilliei, Quadraplanus brossus, Nothofagidites kaitangata, a range of Proteaceae taxa (Wanntorp et al., 2011) and

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the water-fern Grapnelispora (Vajda and Raine, 2003). The Paleocene key-taxa are represented by Nothofagidites waipawaensis and Tricolpites phillipsii (Raine 1984; Vajda

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and Raine, 2003).

Changes in the palynological record occur at the K–Pg boundary in China (Nichols,

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2003; Nichols et al., 2006). Data from the Songliao Basin (Li et al., 2011) are consistent with

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the North American signal in revealing an extinction of several taxa at the K–Pg, including the angiosperm pollen Aquilapollenites, Cranwellia, Wodehouseia and some Normapolles. Other studies include K–Pg successions at Jiaying in the Heilongjiang province, NE China (Sun et al., 2003, 2011), the Nanxiong site in the Guangdong Province, Southern China (Zhao et al., 2002) and the Dangyang site of the Hubei Province, Central China (Li et al., 2014). The K–Pg boundaries in these localities have been well studied and new progress has been made in recent years. Within the Palmae Province, the disappearance of the following key-taxa typifies the K– Pg boundary; Aquilapollenites magnus, Crassitricolporites brasiliensis, Buttinia andreevi, Proteacidites dehaani and Gabonisporis vigourouxii (Vajda-Santivanez, 1999). Subsequent

ACCEPTED MANUSCRIPT Paleocene assemblages include Mauritiidites franciscoi, and are dominated by periporate pollen (Muller et al., 1987; Vajda-Santivanez, 1999). Groups such as Bombacaeae,

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Proxapertites and Mauritiidites dominate upper Paleocene assemblages from the Cerrejón

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Formation, Colombia (Jaramillo et al., 2007).

3.3. Diversity trends

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The average extinction proportion of miospore taxa in North America is around 30%

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(Nichols, 2002; Nichols and Johnson, 2008) and this is synchronous with a significant drop in gross palynological diversity (Fig. 5). An additional 20–30% of the miospore taxa underwent

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a statistically significant decline in abundance (Hotton, 2002). The extinction magnitude was apparently lower within New Zealand assemblages where 15% of pollen and spores species

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disappeared (Vajda and Raine, 2003) but this might be an underestimate due to lower taxonomic differentiation of austral spore-pollen groups. For example, many New Zealand

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angiosperm pollen types have been grouped within the broadly defined Tricolpites and

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Proteacidites genera, hence artificially reducing apparent diversity compared to other floristic provinces. There is also a discrepancy between the diversity within the palynoflora and the macroflora seen at e.g. Murderers Creek, New Zealand (Pole and Vajda, 2009) and at Pakawau (Kennedy, 2003). Part of the reason is explained by differential preservation, e.g. Lauraceae produce pollen grains prone to degradation which leads to higher preservation of macro remains. Fern spores, at the other hand, are more resistant to degradation compared to parent plant. North American Late Cretaceous pollen assemblages have been studied intensely taxonomically, which is probably the reason behind the higher diversity numbers of angiosperm pollen, compared to those of New Zealand. More recent taxonomic work on the

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Proteacidites group from uppermost Maastrichtian deposits of Campbell Island (New Zealand) has shown that the proteacean pollen in the Nothofagus Province is represented by

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significantly higher diversity than previously documented (Wanntorp et al., 2011). This could also explain the discrepancy in the diversity patterns when comparing the fossil macro- and

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palynofloras, showing much higher diversity for dicotyledonous angiosperms in the macrofossil record (Kennedy, 2003; Pole and Vajda, 2009).

Some of the K-taxa within the North American assemblages persist in low numbers up to

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a few meters above the K–Pg boundary (Hotton, 2002; Bercovici et al., 2009, 2012b) and this

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pattern of short-term survival/reworking is strikingly similar to the pattern described from Southern Hemisphere (New Zealand) K–Pg boundary sections (Vajda and Raine, 2003;

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Ferrow et al., 2011). This observation warrants a discussion on the stratigraphical utility of Last Appearance Datums (LAD) of Cretaceous marker species to identify the position of the

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K–Pg boundary.

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3.4. Palynofloristic signal in the aftermath of the asteroid impact

The palynological record provides the best insights into ecosystem recovery in the immediate aftermath of the impact event from the first centimeters of sediments above the impact claystone. Initial recovery of the plant cover on the devastated landscape appears to have taken place during a short time interval, possibly over the course of a few centuries to millennia (Vajda and McLoughlin, 2007) – a temporal scale now confirmed by radiometric dating of ash beds within North American strata hosting palynological evidence of the recovery succession (Renne et al., 2013). In New Zealand, evidence of proliferation of fungi (a fungal-spike) was identified from a few millimeter thick layer of fungal-spore-rich coal indicating cessation of photosynthesis,

ACCEPTED MANUSCRIPT where saprotrophs thrived on the abundant decomposing organic matter during a phase where sunlight was suppressed (Vajda and McLoughlin, 2004; Vajda, 2012). This is matched by

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increased microbial activity recorded from the basal K–Pg boundary clay at Stevns Klint (Sepúlveda et al., 2009), further supporting cessation of photosynthesis for a short period.

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The subsequent resurgence of the light-dependent life is initially represented by an increase in spore abundance, or "spore spike". This high abundance of spores, over a restricted stratigraphic interval corresponds to the re-establishment of the terrestrial

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herbaceous pioneer flora over the devastated landscape. The spore spike has been reported

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from multiple localities throughout North America (Fig. 6, 7; Fleming and Nichols, 1988, 1990; Sweet et al., 1999; Sweet and Braman, 2001; Nichols and Johnson, 2008; Bercovici et

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al., 2012b), from multiple sections (both marine and terrestrial) from New Zealand (Vajda et al., 2001; 2004; Vajda and Raine, 2003; Ferrow et al., 2011) and from a Japanese marine K–

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Pg boundary succession (Saito et al., 1986). In these areas the spores mostly represent Cyathidites and Laevigatosporites species and derive from ferns. The so called fern-spike is

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matched by a corresponding drop in the relative abundance of angiosperm pollen in sections

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from both hemispheres (Vajda et al., 2001; Sweet et al., 1999; Sweet and Braman, 2001; Nichols and Johnson, 2008). Although the end-Cretaceous successions in Europe mainly represent marine depositional settings, the ecological collapse on land is reflected in the marine sediments at several sites in the Netherlands by a spore spike within the basal part of the boundary clay where the spore spike is represented in this case by bryophyte spores (Herngreen et al., 1998; Brinkhuis and Schiøler, 1996). Though representing a different plant group, bryophytes are ecologically similar to ferns in being opportunistic primary colonists of disturbed sites. More recent biomarker analyses at Caravaca revealed massive and sudden increase in the supply of terrestrial organic matter represented by the presence of long-chain n-fatty acids (C20–C30)

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in the basal Danian (Mizukami et al., 2013), reflecting enhanced run-off following the vegetation die-back. A study on sedimentary leaf wax n-alkanes across the K–Pg boundary at

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Loma Capiro, Central Cuba further supports vegetation turn over followed by recovery succession during warmer conditions during the Paleocene (Yamamoto et al., 2010).

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Following the initial resurgence of spore-producing plants, an extended recovery succession of angiosperms initially represented by the pollen Ulmoideipites and Kurtzipites is seen in the North American successions (Sweet and Braman, 1992). However, the apparent

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rise in abundance of these angiosperm taxa is a function of the severe decrease in other

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angiosperms (i.e. this is an auto-correlation phenomenon) and the spike in these taxa is only evident when angiosperms are considered individually.

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A monospecific angiosperm pollen anomaly, or ―angiosperm-spike‖, represented by high relative abundances of Triorites minor occurs directly above the fern spike in New Zealand

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successions (Vajda et al., 2004). In contrast to the Canadian ―angiosperm-spike‖, this is a genuine angiosperm abundance anomaly where Triorites minor reaches up to 30% of total

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miospores. This pollen type originated during the Cretaceous and its resurgence in the early

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Paleocene is probably related to the parent plant being an opportunistic herbaceous earlysuccessional colonist.

The comparable interval in North America is typified by a succession with increased relative abundance of conifer pollen (taxodiaceae and pinaceae). These conifers represented climax vegetation, and were high pollen producers (Sweet and Braman, 1999, 2001; Bercovici et al., 2009). Long-term changes in the Southern Hemisphere floras are characterized by substantially lower relative abundances of Araucariaceae pollen in the Paleocene compared to the Maastrichtian. Araucariaceae appears to have become supplanted from its habitat by podocarpacean conifers, mainly Lagarostobos franklinii (Huon Pine) represented by the

ACCEPTED MANUSCRIPT pollen Phyllocladidites mawsonii. This has previously been noted in the macrofloral record from New Zealand, where Araucariaceae macrofossils are virtually ubiquitous in the

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Cretaceous assemblages but are absent or rare in Paleocene deposits (Pole, 2008). This trend can further be traced into marine environments in European successions (e.g.,

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Caravaca, Spain) where conifer biomarkers drop in concentration at the boundary, only to recover 10 kyrs after the K–Pg event (Mizukami et al., 2013).

The same broad pattern occurs in Patagonia where the Cheirolepidiaceae (represented by

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Classopollis pollen), which played a subordinate role during the late Maastrichtian,

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proliferated in the Paleocene (Barreda et al., 2012). In that area they appear to have outcompeted the Podocarpaceae in the basal Danian. A general pattern of post-catastrophe

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recovery is thus evident in both hemispheres. Initial vegetation collapse is followed by a brief proliferation of saprotrophs (observed at Moody Creek Mine site in New Zealand; Vajda and

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McLoughlin, 2004) followed by a resurgence of pioneer free-sporing plants, in turn succeeded by seed plants, with regional variations in the dominant taxa being controlled by

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the composition of the pre-existing vegetation and local environmental conditions.

3.5. The K–Pg macroflora turnover

In North America, extinction among plants based on macrofloral evidence is striking, with 78% of the macrofloral species from the Hell Creek disappearing at the K–Pg boundary (Johnson, 2002; Wilf and Johnson, 2004). The extinction rates are remarkably high and concentrated at the K–Pg boundary itself, with extinction levels variously calculated between 57 and 66% (Wilf and Johnson, 2004). The recovery flora, characterized by a low diversity assemblage (Johnson, 2002; Bercovici et al., 2008), is dominated by angiosperms (Wolfe, 1985).

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Surviving taxa are not those that were dominant during the Cretaceous. They represent species that were most common in Cretaceous mire facies (Johnson, 2002), hence there may

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be important local environmental or taphonomic biases in the data. The first occurrence of a typical Paleocene taxon is Paranymphaea crassifolia, which is recorded a few meters above

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the K–Pg boundary (Johnson, 2002; Bercovici et al., 2008). The distribution of major Paleogene vegetation types was also discussed by Macrofloristic diversity remained low in some North American ecosystems for several million years following the end-Cretaceous

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event and did not reach end-Cretaceous values until the Eocene (Johnson and Ellis, 2002;

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Barclay et al., 2003; Barclay and Johnson, 2004; Peppe, 2010). However, the vegetation in other areas recovered much faster and exceeded Cretaceous diversity levels, only 1 Ma after

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the K–Pg event. Sites showing this pattern, such as the Castlerock flora in Colorado (Johnson and Ellis, 2002; Ellis et al., 2003), may have experienced accelerated vegetation recovery due

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to their occurrence in shelter zones such as intermontane basins. Some studies have also suggested that species able to produce polyploid forms were

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better adapted to dramatic ecosystem change (Fawcett et al., 2009). As these are mainly

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represented amongst the angiosperms, advantages such as altered gene expression would have benefited this group in the longer-term recovery phase (Fawcett et al., 2009). A fluvial sequence near Cave Stream (north of Castle Hill Village, central Canterbury), New Zealand, is a classical site. A comparison between the palynoflora and the macroflora across the K–Pg boundary showed a drop of c. 50% of Cretaceous macrofloral taxa at the boundary, the major change being the disappearance of the Araucariaceae both within the macro- and palynoflora (Pole and Vajda, 2009). Paleocene plant macro-assemblages contain new conifer genera and new angiosperm taxa including Dryandra comptoniaefolia. The high abundance of Podocarpaceae (Pole and Vajda, 2009) suggests that these filled the niches vacated by Araucariaceae. This process has been extensively discussed in Jablonski (2008) showing that

ACCEPTED MANUSCRIPT the post-extinction environments where formed by the survivors, and where the ―winners‖ defined the long-term evolution of individual clades.

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Studies of a Late Cretaceous flora from Pakawau in northeastern south island New Zealand, revealed 58 angiosperm leaf-forms together with podocarp and araucarian leaves

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and some ferns (Kennedy, 1993; Kennedy et al., 2002; Kennedy, 2003). The angiosperm leaves dominate both in diversity and abundance and all but one of the 58 leaf-types were dicotyledonous including representatives of the Nothofagaceae, Lauraceae and Proteaceae

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(Kennedy, 2003). The depositional setting for the Cretaceous Pakawau site has been

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interpreted as dominantly lacustrine with mire development at the lake margins, but there are also intervals interpreted as vegetated floodplains (Kennedy, 2003). The Paleocene assemblages are also dominated by angiosperms with only minor

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components of conifers and rare ferns. The Paleocene floras from Ian’s Tip and Pillar Point

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Track incorporate 23 and 28 dicotyledonous forms respectively, including Banksiaeformis and Dryandra (now considered a section within Banksia; Mast and Thiele, 2007). Compared

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to the Cretaceous assemblage, the Paleocene flora shows a 50% drop in angiosperm taxa.

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Quantitative palaeoclimate estimates using both Leaf Margin Analysis (LMA) and the Climate Leaf Analysis Multivariate Program (CLAMP) on New Zealand leaf assemblages consistently indicate cool-temperate conditions for the Paleocene of New Zealand (Kennedy, 2003). In the New Zealand record, a 7 ± 0.8°C decrease in mean annual temperature over the K– Pg boundary is evident based on Leaf Margin Analysis (LMA). Based on data compiled by Kennedy (2003), the mean annual temperature estimates from top-Maastrichtian leaf assemblages at Pakawau are 14.8 ± 0.8°C for LMA, and 13.0 ± 1.2°C for CLAMP. The mean annual temperature estimates for Paleocene leaf assemblages from three sites estimate temperatures at 7.5 ± 0.8°C for LMA and 10 ± 1.2°C for CLAMP (Kennedy, 2003).

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This is consistent with data presented in Wilf et al. (2004), based on LMAT on macroflora across the boundary in North Dakota, showing evidence for a dramatic cooling

earliest Paleocene correlated to the Chicxulub impact (Fig. 4).

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directly following the K–Pg event (Vellekoop et al., 2014). The data shows a 5°C drop in the

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The plant record also provides insights into insect diversity across the K–Pg boundary transition) via the range of insect-induced damage types on leaves (Labandeira et al., 2002a). While only five undetermined insect body fossils have been found throughout the Hell Creek

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and Fort Union formations, the enormous leaf fossil database provides a unique opportunity

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for the examination of the response of insect herbivores to the K–Pg event. The insect damage data for leaf assemblages from North Dakota reveal a stable range and quantity of

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insect damage types throughout the end of the Cretaceous (Fig. 4, Labandeira et al., 2002b). However, the data shows that insects suffered significant extinction at the K–Pg boundary, as

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recorded by the disappearance of most specialized types of insect damage on leaves (Labandeira et al., 2002b; Wilf et al., 2006; Fig. 4). This is further supported by molecular

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phylogenetic analyses indicating massive extinctions of bees linked to the K–Pg extinction

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event (Rehan et al., 2013).

Only two specimens of insect body fossils have been described from Upper Cretaceous deposits of New Zealand, one from Canterbury (Harris and Raine, 2002) and another from Hawkes Bay (Craw and Watt, 1987). However, arthropod mesofossils such as scale-insect shields have been encountered in abundance at some Upper Cretaceous sites in Australia, the Chatham Islands and New Zealand (Tosolini and Pole, 2010). Most scale-insects in modern ecosystems spend the majority of their life cycle as sedentary organisms on leaf surfaces under a waxy shield and extract sap directly from the plant's vascular system. Interestingly, at the New Zealand Cave Stream site, (the only site where these fossils have been studied across the K–Pg boundary) the scale insects only occur below the K–Pg, and disappear at the

ACCEPTED MANUSCRIPT boundary (Tosolini and Pole, 2010) suggesting that this specialist feeding guild was severely affected by the ecological crisis. The occurrence of leaves with insect damage has also been

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reported from the Cretaceous of New Zealand (e.g. Kennedy, 2003) although no quantitative studies of their abundance or diversity have been performed to our knowledge.

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Raine (1984) and Sweet and Braman (2001) noted that pollen that disappeared at the K– Pg boundary are mostly represented by large angiosperm grains with thick and complex surface structure (exine). These taxa might be attributed to zoophilous; more specifically

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entomophilous (insect-pollinated) taxa, providing possible indirect support for the extinction

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of certain insects.

The diversity of insect damage types on leaves was suppressed for an extended period in

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North America regaining pre–Pg levels only 10 Myrs later (Wilf et al., 2006). This possibly indicates that insect communities had a longer recovery time compared to plant communities.

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At the Menat site in France, insect damage on Paleocene leaves deposited 5 Ma after the K– Pg event show ―normal‖ pre-impact levels of insect damage abundance (Wappler et al., 2009)

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tentatively indicating that insect recovery was faster at this site compared to the US-sites.

3.6. K–Pg fauna turnover in North America

Although several major Mesozoic groups such as the non-avian dinosaurs and pterosaurs went extinct at the K–Pg boundary, ecosystem disruption also led to the disappearance of numerous species within groups that are still represented today. Within reptiles, a level of 83% species extinction has been documented together with a dramatic decrease in morphological diversity (Longrich et al., 2012). However, turtles seem to have been relatively unaffected by the event and persisted into the Paleogene without significant diversity loss (Lyson et al., 2011). Birds are the only group of dinosaurs that survived the K–

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Pg mass extinction event. However, birds also suffered significant extinction at that time, since none of the "archaic" birds known from North America survived into the Paleocene

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(Longrich et al., 2011). Mammals were also affected (Fox, 1989, 1997; Wilson, 2005; Meredith et al., 2011; O'Leary et al., 2013) and in the Western Interior of North America,

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communities dominated by marsupials and multituberculates in the Cretaceous were replaced by communities of diverse eutherians in the Paleocene (Archibald and Bryant, 1990; Eberle and Lillegraven, 1998; Hunter, 1999; Hunter and Archibald, 2002; O'Leary et al., 2013).

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Multituberculates seem to have recovered to some extent after the extinction, probably due to

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their opportunistic lifestyle (Wilson et al., 2012). Others groups, including sharks and rays, disappear regionally from the fossil record in North Dakota at the K–Pg boundary (Archibald

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and Bryant, 1990; Sheehan and Fastovsky, 1992; Pearson et al., 2002).

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4. The K–Pg event as a tool for assessing other mass extinction events

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4.1. The definition of the K–Pg boundary

Extinction of genera and species is a continuing process but the evolution of life on Earth has been disrupted several times by major, abrupt, global extinctions. Five large Phanerozoic extinction events have so far been identified in the paleontological record, but the causes of most catastrophes remain hotly disputed. The detailed micropaleontlogical datasets from global K–Pg boundary sites generated over decades, now provide excellent templates for assessing the various hypotheses explaining the patterns and causes behind other major global extinctions. The recognition of mass extinction events requires an understanding of the background rates of biological turnover and needs specific criteria for recognizing stratigraphic boundaries. The K–Pg

ACCEPTED MANUSCRIPT boundary is unique in that a global timeline is provided by the boundary clay (Fig. 1). Biological extinction and recovery signals both in the marine and terrestrial realms can be

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tied to this timeline. By comparing and contrasting the paleontological signals with those of other mass extinction events, causal mechanisms can be deduced when relating to the fast

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impact triggered extinction at the K–Pg.

At the official GSSP at El Kef in Tunisia, the base of the Danian stage is set to coincide with the base of the boundary claystone (Molina et al., 2006). This level is also characterized

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by the presence of the iridium anomaly, recognized to have been generated by the impact

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process. Thus biostratigraphy plays a secondary role at sites where the K–Pg boundary clay is present. In fact, the International Commission of Stratigraphy (ICS) does not even mention

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the first occurrence of Paleocene fossils (the First Appearance Datum (FAD) of the P0 foraminiferal zone in the marine sections) as a valid criterion for defining the K–Pg

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boundary, but mention the association of the extinction of several fossil taxa.

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4.2. The definition of the Permian–Triassic boundary

The end-Permian Event (EPE) spans ~200 kyrs, peaking at ~ 252.28 Ma as revealed by U-Pb dating of the marine Global Stratotype Section and Point (GSSP) at Meishan, China (Yin et al., 2001; Shen et al., 2011), where the base of the Triassic is set at the First Appearance Datum of the conodont species Hindeodus parvus (Ogg et al., 2008; Peng and Shi, 2009). Over 90% of biota at species level got extinct at a global scale with extensive evidence suggesting the Siberian Traps as the main cause (e.g. Mundil et al., 2004; Schneebeli-Hermann et al., 2013; and references therein). Photochemical model calculations have shown a decrease in O2 that might have further contributed to the extinction (Kaiho and Koga, 2013). Ongoing debate surrounds the synchroneity of the extinction in the terrestrial

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and marine realms (Peng and Shi, 2009; Shen et al., 2011) and the duration of the event, which has been estimated to have spanned 40 kyrs (Twitchett et al. 2001), 200 kyrs (Shen et

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al., 2011) or 700 kyrs (Huang et al., 2011). No standard reference section has been defined for the non-marine Permo–Triassic

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transition, although the southern Karoo Basin, South Africa, with its relatively complete continental section and well-constrained record of vertebrate fossils (Ward et al., 2005; Coney et al., 2007) is a potential candidate. The Permian–Triassic Boundary (PTB) in the

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Karoo Basin is placed biostratigraphically at the Last Appearance Datum of Dicynodon

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lacerticeps (Smith and Ward, 2001; Ward et al., 2005).

Similar successions have been proposed as non-marine global reference sections in

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Guizhou and Yunnan, South China (Peng et al., 2005). Furthermore, these sections feature several bentonite beds beneficial for correlation and accurate timing of the extinction event

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(Yu et al., 2007; Shen et al., 2011). In South China, the EPE is associated with the disappearance of coal-bearing and organic-rich sedimentary facies (Yu, 2008) and also

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characterized by a drop in abundance of plants and pollen, co-occurring with a major facies

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change. Vegetation turnover within end-Permian successions in South China included a major drop in abundance of the Cathaysian cordaitalean and gigantopterid flora and the earliest Triassic is instead dominated by lycopsids (Annalepis) and peltasperms (Yu et al., 2010; Bercovici et al., 2014). The global vegetation pattern is typified by an extinction of vegetation predating the defined Permian–Triassic boundary, and diversity of some key groups declined throughout the Late Permian. Further, the pattern of floristic turnover may have been diachronous across latitudes (McLoughlin et al., 1997; Coney et al., 2007; Lindström and McLoughlin, 2007; Retallack et al,. 2007; Yu et al., 2007). Additionally, the timing of the extinction of the terrestrial flora is still not well constrained and the signals appear to differ between the

ACCEPTED MANUSCRIPT Northern and Southern Hemispheres (McLoughlin et al., 1997; McElwain and Punyasena, 2007; Peng and Shi, 2009; Shi et al., 2010).

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The sediments spanning the PTB in Central Europe are developed in two major facies domains; the chiefly non-marine facies of the Germanic realm and the mainly marine facies

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of the Alpine realm (Kürschner and Herngreen, 2010; Bourquin et al., 2011). Miospore assemblages of latest Permian age from the Zechstein deposits (Germany) are characterized by diverse gymnosperm spore assemblages comprising alete pollen, both taeniate and non-

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taeniate bisaccate pollen, co-occuring with diverse spore assemblages (Kürschner and

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Herngreen, 2010). The EPE palynological record, in Central Europe is typified by a significant turnover evident as a dramatic decrease in gymnosperm pollen resulting in an

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increase in relative abundance of spores (Visscher and Brugman, 1988). The basal Triassic in this area is set at the base of the Lundbladispora obsoleta–Protohaploxypinus pantii Zone

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characterized by typical Early Triassic spore taxa such as Aratrisporites, Densoisporites playfordii, and Kraeuselisporites produced by lycopsids and invoked to mirror a herbaceous

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pioneer vegetation occupying well-drained floodplain and coastal environments (Retallack,

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1977; van der Zwan and Spaar, 1992). Comparative studies across Gondwana reveal that the close of the Permian was characterized by the extinction of glossopterid and cordaitalean gymnosperms, and these were replaced by peltasperms, scale-leafed conifers and lycophytes during the earliest Triassic (McLoughlin et al., 1997; Hill et al., 1999). On the other hand, there are also scattered reports of some typical Gondwanan Permian elements (e.g. Glossopteris) surviving into the earliest Triassic (McLoughlin, 2011). A similar scenario has been described from the Northern Hemisphere where gymnosperm-dominated Permian floras were replaced by a pioneer succession of mainly lycopsids and ferns in the earliest Triassic (Fig. 8; Looy et al., 1999; Hochuli et al., 2010; Hermann et al., 2011). In some areas (e.g. Norway) palynological

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data reveals rapid recovery of gymnosperms following the spore-producing vegetation (Hochuli et al., 2010).

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Palynological sampling of beds in close vicinity to the PTB world-wide has additionally revealed anomalous percentages of Reduviasporonites exemplifying a so-called ―fungal-

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spike‖ (Eshet et al., 1995; Visscher et al. 1996, 2011, or alternatively interpreted as algae by Foster et al., 2002) signifying mass extinction both in land and in the sea and with a sharp increase in ―disaster species‖ (Visscher et al., 2011; Algeo et al., 2012 and references

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therein). Importantly, in the context of this paper, is that the ―fungal-spike‖ signaling altered

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ecological conditions, predated the PTB as defined by the FAD of Hindeodus parvus (Fig. 8) although in terrestrial successions in Southern China fungi/Reduviasporonites have been

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identified within the boundary zone (Bercovici et al. 2014). The inhospitable ecological conditions of Early Triassic shallow marine environments

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are also expressed by a high abundance of cyanobacterial activity manifest by the presence of microbialites, such as wrinkle structures and stromatolites on most continents (Kershaw et al.,

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2012). This bloom of cyanobacteria is possibly linked to extreme environments, in which

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grazing organisms where not present, resulting in uncontrolled growth of extremophiles. Interestingly, the microbial mats also seem to have served as refuges for some organisms including ostracods (Forel and Crasquin, 2011; Forel, 2012; Forel et al., 2013).

4.3. The definition of the Triassic–Jurassic boundary

During the end-Triassic Event (ETE) the number of living families was reduced by around 35% (Raup and Sepkoski, 1982) and the extinctions punctuated the great wave of Late Triassic radiations amongst terrestrial plants and vertebrates and led to the rise of radically different floras and faunas during the Jurassic. In the terrestrial realm, the basal

ACCEPTED MANUSCRIPT archosaurs and a large percentage of the amphibians disappeared, providing ecospace opportunities for the radiation of dinosaurs (Benton, 1995; Olsen et al., 2002). Ammonoids

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and bivalves were decimated and the conodonts were finally pushed to extinction (Benton, 1995). Although floras were less affected compared to the faunas, clear changes occurred in

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the vegetation and the global carbon cycle across the Triassic–Jurassic boundary (TJB) (Hesselbo et al., 2002, 2007).

The TJB is dated to 201.3 ± 0.2 Ma, based on the First Appearance Datum of the

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ammonite Psiloceras spelae tirolicum in the marine realm, coinciding with the first

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occurrence of the gymnosperm pollen taxon Cerebropollenites thiergarthii in Northern Hemisphere terrestrial settings (Kürschner et al., 2007; Hillebrandt et al., 2008; Ogg et al.,

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2008). The Kuhjoch Section, Karwendel Mountains, Northern Calcareous Alps, Austria is the type section for the TJB where the boundary occurs 5.80 m above the base of the

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Tiefengraben Member of the Kendelbach Formation (Ogg et al., 2008). The mass-extinction preceded the Triassic–Jurassic boundary by approximately 100,000

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years and has been invoked as to have been caused by the massive outpourings of basalts and

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associated intrusions of the Central Atlantic Magmatic Province (CAMP) by many authors (e.g. McElwain et al., 1999; Hesselbo et al., 2002; Bonis et al., 2010; Steinthorsdottir et al., 2011; Mander et al., 2013 and references therein). U-Pb dates from the CAMP, exclusively based on samples from continental sequences of Morocco and Eastern North America, and further tied to palynological signals, show that the initiation of the extinctions in the tropics and subtropics coincided with oldest CAMP flows. Nevertheless, the pattern of extinctions was significantly different in temperate northern and southern latitudes, although correlation is hampered by these differences and as yet untested by indepedant geochrononological dating methods.

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Although the CAMP is put forward as the main contributior to the extinction event, an impact of an extraterrestrial body has been invoked as possible conributor to the event (Olsen

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et al., 2002; Tanner and Kyte, 2005). The main candidate is the Rochechouart impact in France, dated at 201 ±3 Ma by 40Ar/39Ar which renderes an age coeval with the oldest CAMP

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lavas at the base of the ETE (Schmieder et al., 2010). Anomalous concentrations of iridium from the USA (Olsen et al., 2002) and Canadian Tr–J boundary successions (Tanner and Kyte, 2005) and the presence of shocked quartz in Tr–J boundary layers in Italy (Bice et al.,

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1992) have been identified but more investigations are required in order to tie these impact

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markers to the Rochechouart crater (Olsen et al., 2002; Tanner and Kyte, 2005). Several studies have documented dramatic changes in the composition of the Late

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Triassic and Early Jurassic atmosphere, showing a two to fourfold increase in CO2, plausibly associated greenhouse warming of 3–4°C, based on plant stomatal data from Sweden,

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Greenland, UK (McElwain et al., 1999; Steinthorsdottir et al., 2011) and Australia (Steinthorsdottir and Vajda, 2013). Although stratigraphical resolution of these studies are

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not the best being based on museum speciens with low stratigraphical control or, deriving

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from one single well-dated bed as in Steinthorsdottir and Vajda (2013), the results reveal that the end-Triassic and Early Jurassic atmospheric CO2 was significantly higher than present values.

Eventhough floras were less affected compared to the faunas, clear changes occurred in the vegetation during the ETE. In the Northern Hemisphere, the ETE is characterized by a macrofloral turnover evidenced by extinction of the peltasperm Lepidopteris and its replacement by the dipteridacean fern Thaumatopteris, and a flora rich in ginkgoaleans, conifers and (McElwain et al., 2007; Pott and McLoughlin, 2009; Mander et al., 2013; Vajda et al., 2013b). In the pollen record, the ETE is met with major and dramatic changes. For example, Rhaetian deposits at several European localities are typified by anomalous

ACCEPTED MANUSCRIPT abundances of the enigmatic gymnosperm pollen Ricciisporites tuberculatus followed by a spore-spike (Götz, 2009; Larsson, 2009; van de Schootbrugge et al., 2009; Mander et al.,

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2013; Vajda et al., 2013b and references therein). The Early Jurassic recovery floras include high percentages of Classopollis (Cheirolepidiaceae) recorded e.g. from Sweden (Lund,

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1977; Guy-Ohlson, 1981; Vajda, 2001; Vajda et al., 2013b), Greenland (Pedersen and Lund, 1980) and Britain (Mander et al., 2013) and elsewhere. During the Early Jurassic the Cheirolepidiaceae probably took over the niche that was previously occupied by the seed-fern

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groups. The high abundance of Classopollis is in Southern Hemisphere successions

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interpreted to mark the earliest Jurassic together with important lycophyte key-species within the genus Retitriletes, including Retitriletes rosewoodensis and R. austroclavatidites (de

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Jersey and Raine, 1990; Zhang and Grant-Mackie, 2001; Akikuni et al., 2010). China was located at higher latitudes during the ETE compared to present and the ETE

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has been identified by palynology in fluvial and lacustrine deposits in the southwestern Junggar Basin of Xinjiang Uygur Autonomous Region, northwestern based on FAD of

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lycophyte key-taxa (Ashraf et al., 2010; Sha et al., 2011). These several 1000 meters thick

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deposits spanning the ETE need more studies in order to assess the high resolution palynological signal related to the this extinction event. Recent progress on stratigraphy and biodiversity studies on successions spanning the end-Triassic and the Tr–J boundary in NW China and in the Sichuan Basin in Southern China has emerged over the last years (Wang et al. 2010a, b; Li, 2013). Also in Southern Hemisphere records, significant vegetation changes occurred, where the Triassic seed-fern dominated floras (mainly corystosperms) were replaced by more complex associations of conifers, Bennettitales and new seed-fern groups during the Early Jurassic (Hill et al., 1999; Turner et al. 2009; de Jersey and McKellar, 2013 and references therein). These changes seen in the macroflora is however not easily detectable in the Southern

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Hemisphere palynological record, possibly due to the fact that the new incoming seed fern groups produced pollen grains with similar morphologies as the ones going extinct – usually

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grouped within the genus Alisporites. The prominent and easily detectable Ricciisporites are lacking in Southern Hemisphere records. Instead the Triassic–Jurassic boundary is noticeable

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by the abundant occurrence of Classopollis and FAD of some species within the genus Retitilietes. In New Zealand palynological assemblages, a turnover of species is evidenced at one TJB where characteristic Triassic taxa such as Alisporites spp. and Densoisporites

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psilatus declined dramatically (de Jersey and Raine, 1990) but the turnover seems more

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gradual in coeval Australian successions (de Jersey and McKellar, 2013). Interestingly, prominent rise in the abundance of Classopollis is an Early Jurassic marker, comparable to

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that of the Northern Hemisphere. However, there is an ongoing discussion whether this change may really be used as a marker for the boundary, or not. The general opinion is rather

across paleolatitudes.

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that Classopollis pollen is a climate indicator and thus its appearance could be diachronous

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Linking marine and terrestrial signals through the ETE has also proved to be difficult and

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debate continues as to whether the extinction on land precedes that in the oceans. Highresolution studies from New Zealand have shown that the palynofloral turnover precedes the extinction in the marine realm (Akikuni et al., 2010). In contrast, correlations based on palynological assemblages from TJB sections in Greenland and the marine TJB succession at Audrey’s Bay in the UK, (two areas that were geographically much closer during the end Triassic) show evidence for synchronous disruption in the marine and terrestrial realms (Mander et al., 2013). Clearly, further investigations are required to develop a global integrated picture of the environmental changes across the TJB.

5. Summary and Conclusions

ACCEPTED MANUSCRIPT We have outlined paleontological signatures of three major extinction events that profoundly affected the evolution of the Phanerozoic biota: the end-Permian, end-Triassic and

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Cretaceous–Paleogene events. We have focused on terrestrial environments, more precisely the palynological and, to some extent, the paleobotanical records.

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The end-Permian Event was the largest Phanerozoic extinction event and although exact numbers are difficult to deduce, the extinction affected over 90% of the biota at species level globally and the initial numbers put forward by Raup and Sepkoski (1982) are still generally

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valid. Recent high-resolution radiometric dating of Chinese PTB successions (including

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Meishan) reveal that the major extinction interval occurred at ca. 252.3 Ma and that the anomalous environmental conditions persisted for less than 200 000 years (Shen et al., 2011).

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The extinction can be summarized by the abrupt decline of the dominant woody gymnosperm vegetation (cordaitaleans and glossopterids) and its replacement by spore-producing plants

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(mainly lycophytes) before the typical Mesozoic woody vegetation evolved. The postextinction Lower Triassic interval is represented by a coal- , chert- , and a coral-gap, i.e.,

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deposits of these types do not occur in the geological record from this time interval, possibly

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implying that organisms producing these deposits were absent. Instead, ecosystems from this interval seem to have been dominated by extremophiles, such as cyanobacteria forming microbialites (Kershaw et al., 2012). The end-Triassic event is the least well-studied of the three global crises outlined herein, with major data sets from Southern Hemisphere lacking. The boundary between the Triassic and Jurassic occurs in strata dated at 201.3 Ma; the period boundary post-dates the major phase of the extinctions by approximately 100 000 years and is argued to have been caused by the outpouring of greenhouse gases produced by the Central Atlantic Magmatic Province (CAMP). Vegetation was affected to a minor degree compared to the faunas, although major turnovers within palynofloras include an interval dominated by the disaster taxon

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Ricciisporites tuberculatus followed by a spore-spike in the earliest Triassic. Although this is true for Northern Hemisphere settings, a similar signal has yet not been identified in Southern

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Hemisphere settings, where the the vegetation turnover instead is manifest by a shift from an Alisporites (corystosperm)-dominated assemblage to a Classopollis (cheirolepidiacean)-

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dominated one.

The best studied mass extinction is the K–Pg boundary event having the benefit of a an independent reference datum expressed by an iridium-enriched, globally distributed ejecta

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layer. The event occurred 66 Ma and ca. 75% of biota at species level perished. Our review

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has shown that the faunas were more severely affected by the Chicxulub impact, with greater losses in abundance and diversity, compared to the vegetation. This is explained by the short

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phase of anomalous post-impact conditions, e.g., with darkness, fires and reduced temperatures (Kring, 2007). A time-span of a few years would have been sufficient for large

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animals to starve under such conditions. However, plants were able to recover owing to factors such as long-term germinule survival and the potential for asexual reproduction, much

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the same as the dinoflagellates in the marine environments. Although the extinction in the

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end-Cretaceous palynological record is only moderate, a striking mass-kill of vegetation is evidenced by high-resolution sampling, which shows a complete diversity loss across the boundary followed by a fungal/microbe-dominated interval, in turn followed by recovery of ferns, then gymnosperms, and finally angiosperms. The development of the entire pioneer succession probably spanned an interval of 100–1000 years, at most, and a true extinction is detectable chiefly amongst angiosperms. When analyzing at centimeter-scale resolution, data from both hemispheres reveal that the K–Pg crisis is most clearly detectable as a major change in the abundance of plant groups and that defining the boundary based on presenceabsence data (FADs and LADs) is not reliable. High-resolution palynological studies of both marine and terrestrial successions show that first appearances of Paleogene taxa typically

ACCEPTED MANUSCRIPT occur several centimeters to meters above the boundary clay. Therefore, we strongly recommend abundance data be used as criteria for defining the K–Pg boundary at sites where

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the boundary clay is not present. With the large body of data now available, we can apply the paleontological record

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spanning the K–Pg mass-extinction interval as a template for interpreting other extinction events. A model for biotic responses to extremely abrupt events (e.g., asteroid impact) can be derived from the K–Pg boundary record, and can be contrasted against the biotic signals and

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timing for other events with the aim of assessing their underlying causal mechanisms.

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Essential points in the evaluation of extinction patterns are 1, the establishment of a highresolution time frame; 2, comparison of the synchroneity and extent of marine versus

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terrestrial extinctions; 3, comparison of patterns between the Southern and Northern hemispheres.

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When comparing the biotic signals expressed by terrestrial floras for the three extinction events, major differences, but also similarities are identified. All three events show the

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following signal: 1, an initial diverse flora; 2, extinction/mass-kill; 3, short term bloom of

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opportunistic "crisis" taxa proliferating in the devastated environment; 4, pulse in pioneer communities (spore spike); 5, recovery in plant diversity, with radiation and appearance of new taxa that may reach dominance in the long term. Although the abrupt extinction and rapid recovery following the K–Pg event points to a single high-energy event, the earlier events have a more protracted signal. The EPE and ETE share a pattern of extinctions occur over a prolonged time, leading to successive pulses of diversity loss. The drawn-out signal of the EPE was likely caused by longer-term atmospheric changes driven by Siberian trap volcanism. The ETE shows a more ambiguous extinction pattern: CAMP volcanism is coeval with the onset of the event and consistent with some biotic and isotopic signals. However, an impact in the Northern Hemisphere of a

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medium-sized asteroid may have exacerbated the crisis towards the end of the ETE, resulting in a synergy of processes. Further, comparison of the signals between the two hemispheres

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reveals a synchronous and similar extinction pattern for the K–Pg and the EPE, whereas the ETE seems to have affected the Northern Hemisphere biota more than its southern

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counterpart.

Based on these observations, it is evident that the longer the extreme environmental conditions last, the greater is the extinction rate, and the extinction patterns between

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autotrophs and heterotrophs, and between terrestrial and marine faunas become more similar.

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Employing the last and first appearances of individual taxa to define major extinctionrelated boundaries is difficult because commonly there will be short-term survivors,

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reworked specimens, fossils repositioned by post-depositional bioturbation, or groups that survived the event in extremely low numbers and appear as lazarus taxa higher in the

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geological record. Such taxa can still be valuable subsidiary indices when applying the abundance approach. First appearance data are also inadequate for high-precision location of

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chronostratigraphic boundaries based on these events because a significant lag-time exists

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between the extinction event and the evolution of new taxa (Fig. 9). Despite having very different causes, the end-Permian, end-Triassic, and the K–Pg events are consequences of dramatic environmental upheavals that generated comparable extinction patterns, and similar phases of vegetation recovery but at different temporal scales. Based on the observed lagtime for the FADs after such biotic crises, we recommend using relative abundance data for the stratigraphic definition of mass extinction events and the placement of chronostratigraphic boundaries.

Acknowledgements

ACCEPTED MANUSCRIPT This research was jointly supported by the Swedish Research Council (VR), Lund University Carbon Cycle Centre (LUCCI) and the Royal Swedish Academy of Sciences. A.

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Bercovici is supported through the Swedish Research Council (VR) postdoctoral fellowship grant 2011-7176. Dean Pearson and the Pioneer Trails Regional Museum are thanked for

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long term collaboration, discussions and assistance in the field. Stephen McLoughlin is thanked for language review and constructive criticism of an early version of this paper. Mikael Pole and Yongdong Wang are thanked for comments that greatly improved this

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paper. Three anonymous reviewers are further thanked for providing constructive criticism.

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Figure captions

Fig. 1. (A) View over the marine K–Pg succession at Rødvig, Stevns, Denmark. The carbonates below the K–

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Pg boundary are mainly coccolith chalk whilst the carbonates above the boundary comprise re-precipitated CaCO3 followed by bryozoan limestone. (B) The so called ―fish-clay‖ or K–Pg boundary at Rødvig is a thin dark layer followed by the red iron- and sulphur-rich bed, succeeded in turn by light grey sediments.

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Photographs have been taken by the authors. (C) The K–Pg boundary in the badlands, of Marmarth, North Dakota USA. The K–Pg lies in close proximity of the Hell Creek – Fort Union Formation which is easily

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recognized by several features such as color change (especially from homogeneous darker gray to golden yellow and lighter banded units), break in slope, and the presence of modern vegetation just above the contact. (D) The K–Pg boundary layer at Mud Buttes. The sequence shows a 1-cm-thick pinkish clay layer (low angle ejecta),

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overlain by a 3-cm-thick orange layer containing altered spherules (high angle ejecta). The top part features shocked quartz and the maximum concentration of Iridium. (E) Moody Creek Mine K–Pg boundary locality at

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Greymouth Coal field, south island, New Zealand. (F) K–Pg boundary at Moody Creek Mine. The boundary

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itself is barely visible and lies within the top of the 12 cm dark coal.

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Fig. 2. Compilation of 105 terrestrial K–Pg boundary sections, based on data from Nichols and Johnson (2008). The figures in parenthesis represent the number of sections from each area (in bold) and significance score based on the robustness of the identification of the K–Pg boundary as outlined by Nichols and Johnson (2008). The mineralogical and geochemical evidence is emphasized because of its decisive importance regarding the placement of the K–Pg boundary. Palynological analysis - 1point; Sedimentological analysis and sampling performed at less than decimeter resolution - 3 points; Iridium anomaly present - 3 points; Boundary claystone present – 3 points; Shocked minerals present – 3 points; Paleomagnetic analysis performed – 1 point; Radiometric dating performed – 1 point; Cretaceous faunal assemblage recovered – 1 point; Paleocene faunal assemblage recovered – 1 point; Cretaceous macrofloral assemblage recovered – 1 point; Paleocene macrofloral assemblage recovered – 1 point; Fern-spore spike identified – 1 point).

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Fig. 3. (A) Paleogeographic map 66 Ma (Scotese, 2001). Location of the terrestrial K–Pg boundary section and the Global Boundary Stratotype Section and Point (GSSP) for the K–Pg in the shallow marine deposits of El Kef, Tunisia. (B) The main palynological provinces at the end of the Cretaceous (modified from Herngreen et

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al., 1996), with representative pollen morphologies.

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Fig. 4. Compilation of vertebrate, macroflora and insect damage diversity trends over the last 1.3 Myrs of the Cretaceous in the Marmarth area, North Dakota, USA. Mean annual paleo-temperatures are derived from leaf margin analysis, and further correlated to two global events: the Deccan volcanism and the Chicxulub impact.

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The Sunset Butte section is used as a baseline for the thickness of the Hell Creek Formation in North Dakota

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albeit paleontological data is compiled from a multitude of localities from within the Marmarth area.

Fig. 5. Compilation of the average composition of the palynoflora derived from five K–Pg boundary

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sections in North Dakota, USA (PTRM 170/156, 166, 168, 171; Bercovici et al., 2009, and PRTM 191; Lyson et al., 2011). The relative abundance of K-taxa and rarefied palynological diversity at 200

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specimens per sample are plotted on the right (with small dots representing individual samples, large dots representing averaged 20 cm stratigraphic bins and line representing sliding average).

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Stratigraphical alignment of the samples from the five sections is based on the point of major loss in K-taxa. The stratigraphic section represents a most common rock sequence spanning the K–Pg

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boundary in the Marmarth area.

Fig. 6. Selection of published global terrestrial and near-shore marine K–Pg boundary sites showing the occurrence of a fern spike immediately following the K–Pg boundary event, compilation based on studies referenced in the figure. Cumulative relative abundance of Cyathidites and Laevigatosporites is presented in those cases where the authors have separated the different fern spore components, otherwise the relative abundance of all fern spores as a group is presented.

Fig. 7. Schematic illustration over the pre-impact palynological assemblages followed by the post-impact, long term recovery succession globally.

ACCEPTED MANUSCRIPT Fig. 8. Schematic illustration comparing the three events discussed in this paper. The figures have been correlated along the respective period boundaries. This emphasizes how these boundaries relate to different

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phases of the extinction process

Fig. 9. (A) Theoretical section with relative abundance analysis of two taxa (A and B). The placement of three

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chronological events frames an interval of discussion for biostratigraphical correlation and event characterization. (B) Time constant equation used to model decay in relation to an abrupt change from equilibrium conditions. V0 is the original state of the system, and system response is an exponential decay

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driven by its internal time constant τ. (C) Plot of the above equation, showing the response of an elastic system (plain line) to the abrupt change from state A to state B (dashed line). At 5τ, the exponential decay is equal to

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the transition from state A to state B.

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1% of the original V0 state and can be approximated to the response time which the system requires to operate

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Graphical abstract

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We summarized the paleontological signal across the K-Pg boundary The palynological signal across the K-Pg was used as template for other events The end-Permian and end-Triassic events have similar vegetation signals as K-Pg By comparing the palynological signals of different events, causes can be deduced Relative abundance data should be used to define mass-extinction events