Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems following the Permian–Triassic extinction

Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems following the Permian–Triassic extinction

    Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems following...

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    Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems following the Permian-Triassic extinction Xiaoming Zhao, Jinnan Tong, Huazhou Yao, Zhijun Niu, Mao Luo, Yunfei Huang, Haijun Song PII: DOI: Reference:

S0031-0182(15)00193-5 doi: 10.1016/j.palaeo.2015.04.008 PALAEO 7245

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

21 December 2014 4 April 2015 6 April 2015

Please cite this article as: Zhao, Xiaoming, Tong, Jinnan, Yao, Huazhou, Niu, Zhijun, Luo, Mao, Huang, Yunfei, Song, Haijun, Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems following the Permian-Triassic extinction, Palaeogeography, Palaeoclimatology, Palaeoecology (2015), doi: 10.1016/j.palaeo.2015.04.008

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ACCEPTED MANUSCRIPT Early Triassic trace fossils from the Three Gorges area of South China: Implications for the recovery of benthic ecosystems

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following the Permian-Triassic extinction

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Xiaoming Zhao a, b, Jinnan Tong a, Huazhou Yao b, Zhijun Niu b, Mao Luo c, Yunfei Huang d, Haijun Song a* China University of Geosciences, State Laboratory of Biogeology and Environmental

Geology, Wuhan, Hubei, 430074, China

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Wuhan Institute of Geology and Mineral Resources, Wuhan, Hubei, 430205, China

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School of Earth and Environment, University of Western Australia, Perth 6009, Australia

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School of Geosciences, Yangtze University,Wuhan, Hubei, 430100, China

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*Corresponding author: [email protected]

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Abstract:

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Keywords: trace fossils; benthic ecosystem; biotic recovery; Early Triassic; Three Gorges area

The Lower Triassic Daye and Jialingjiang formations of the Three Gorges area (South China) record the recovery interval of benthic tracemaking invertebrates following the P-Tr mass extinction. A total of 17 ichnospecies in 14 ichnogenera are documented from Smithian and Spathian strata. Our trace fossil data, in combination with previously published studies, show that ichnodiversity in the Middle Yangtze region increased markedly in the early Spathian. Trace fossils in the Smithian are dominated by simple, small, horizontal burrows, including Didymaulichnus and Planolites, whereas Spathian trace fossils are diverse and abundant with moderate-high bioturbation levels

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ACCEPTED MANUSCRIPT and complex burrow networks, such as Thalassinoides. Both burrow sizes and penetration depths increased gradually from the early Spathian to the middle-late

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Spathian, implying a gradual recovery pattern for benthic ecosystems. Early Triassic

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ichnofossils are characterized by aspects of opportunistic behaviour (e.g., low-to-moderate ichnodiversity, low-to-moderate bioturbation, small burrow widths, and shallow tiering), suggesting stressed environmental conditions. The recovery tempo and

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pattern of ichnocoenoses in South China is likely structured by temporal and spatial

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changes of the refuge zone in the Early Triassic.

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1. Introduction

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The largest mass extinction in geological history happened near the Permian-Triassic (P-Tr)

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boundary and killed over 90% of marine species (Erwin, 1993; Song et al., 2013). Biotic recovery following this event has attracted much attention in recent years and remains the topic

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of much debate. It has long been considered that the P-Tr mass extinction was followed by a period of delayed biotic recovery that lasted until the Middle Triassic (Stanley, 1990). The Early Triassic is an interval characterized by continued low biodiversity (Erwin, 1993), blooms of opportunistic and disaster taxa (Bottjer et al., 2008), reduction in the size of both metazoans (Twitchett, 2007) and protozoans (Song et al., 2011a), and the absence of metazoan reefs (Flügel, 2002) and of calcareous algae (Flügel, 1985). Carbon and sulphur isotopic data illustrate a series of large fluctuations throughout the Early Triassic (Payne et al., 2004; Tong et al., 2007; Song et al., 2014a), reflecting stressed marine environments. Recent geochemical data show that the stressed environments of the Early Triassic are characterized by a long-term

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ACCEPTED MANUSCRIPT hot climate (Sun et al., 2012; Romano et al., 2013) as well as severe oceanic anoxia (Wignall et al., 2010; Song et al., 2012; Grasby et al., 2013). However, recent palaeontological data show

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that a fitful recovery of many marine organisms began in the Smithian and Spathian, including

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nektons, such as ammonoids and conodonts (Brayard et al., 2009; Stanley, 2009), benthic foraminifers and calcareous algae (Song et al., 2011b), and even metazoan reefs (Brayard et al., 2011). As a result, the mechanisms that caused biotic recovery in the Early Triassic are still a

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subject of debate.

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Trace fossils are good indicators of palaeoenvironmental conditions and the behaviours of benthic invertebrates. They have been used to reveal the tempo and pattern of ecologic

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recovery following the P-Tr mass extinction (e.g., Twitchett and Wignall, 1996; Wignall et al.,

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1998; Twitchett, 1999; Pruss and Bottjer, 2004; Twitchett and Barras, 2004; Zonneveld, 2004;

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Beatty et al., 2005; Twitchett, 2006; Beatty et al., 2008; Fraiser and Bottjer, 2009; Zonneveld et al., 2010a; Zonneveld et al., 2010b; Chen et al., 2011; Hofmann et al., 2011; Chen et al.,

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2012; Baucon et al., 2014). These ichnofossil data, derived from global localities, show that benthic invertebrate trace-makers at boreal palaeolatitudes, e.g., Spitsbergen (Wignall et al., 1998) and western Canada (Zonneveld, 2004; Beatty et al., 2005; Beatty et al., 2008; Zonneveld et al., 2010a; Zonneveld et al., 2010b), recovered before those at low and middle palaeolatitudes, e.g., the western United States (Pruss and Bottjer, 2004; Twitchett and Barras, 2004; Fraiser and Bottjer, 2009), northern Italy (Twitchett and Wignall, 1996; Twitchett, 1999; Twitchett and Barras, 2004), the Lower Yangtze region of South China (Chen et al., 2011), and Western Australia (Chen et al., 2012). However, recent studies showed that a diverse and complex ichnofauna began to occur in the late Griesbachian in Italy (Hofmann et al., 2011).

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ACCEPTED MANUSCRIPT Thus, the environmental factors that structure the temporal and spatial patterns of Early Triassic ichnofossil assemblages are still unclear and additional quantitative analyses are

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required (Fraiser and Bottjer, 2009).

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In this study, abundant and diverse trace fossils from the Lower Triassic successions in the Three Gorges area were described and analysed. Five proxies, including ichnodiversity, forms and complexity, bioturbation index, burrow sizes, and tiering levels, were used to evaluate the

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tempo and pattern of benthic ecosystem recovery in the aftermath of the P-Tr mass extinction

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in the Middle Yangtze region of South China. Additionally, the latest geochemical data combined with a hypothesized recovery model (Song et al., 2014b) were used to explain the

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temporal and spatial pattern of Early Triassic ichnofossil assemblages.

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2. Studied sections and stratigraphic setting The South and North Xiakou sections are situated 3 km west of Jianyangping, Xiakou

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Town, Xingshan County of Hubei Province and approximately 80 km away from Yichang City (Fig. 1). These two sections are located along both banks of the Xiangxi River, a branch of the Yangtze River. The present study made use of a previously established site along the south bank (starting at 110°48′13″ E, 31°06′52″ N, referred to as South Xiakou section) as well as a newly surveyed section on the north bank of the Xiangxi River (starting at 110°48′17″ E, 31°06′53″ N, referred to as North Xiakou section). The trace fossil assemblages of both sites have been systematically studied in this study. Lower Triassic successions widely crop out across the entire Yangtze Platform (Tong and Yin, 2002). The studied area belonged to the middle part of the upper Yangtze platform located

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ACCEPTED MANUSCRIPT at the eastern Tethys during the Early Triassic (Feng et al., 1997). The P-Tr boundary and Lower Triassic rocks at the Three Gorges area are well exposed (Fig. 2A). Lower Triassic

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sequences deposited in the Three Gorges region of South China constitute the Daye and

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Jialingjiang formations (Figs. 3, 4), which lie conformably above the Upper Permian Dalong Formation and are in turn overlain by the Middle Triassic Badong Formation (Wang and Xia, 2004; Zhao et al., 2005; Zhao et al., 2010; Zhao et al., 2013).

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The Daye Formation in this area is subdivided into four members. Member I is composed

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of laminated black shale interbedded with grey muddy limestone and is dominated by horizontal lamination (Fig. 2A). It is approximately 51.3 m thick at the South Xiakou section

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and 46.6 m thick at the North Xiakou section. This member yields an abundance of thin-shelled

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fossils, including the bivalves Claraia spp., Eumorphotis spp., and Posidonia spp., as well as

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the ammonoids Ophiceras, Lytophiceras, Prionolobus, and Gyronites (Li et al., 2009). Trace fossils are absent. Thus, the black shale- and muddy limestone-dominated sequences

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preserving epifaunal bivalves and ammonoids indicate an offshore setting. Member II consists mainly of light-grey medium to thickly bedded micritic limestone and vermicular limestone. The Early Triassic vermicular limestone has been proposed to be produced by chemical or microbial coacervation, or the diagenetic differentiation under low-energy environmental conditions (see Zhao et al., 2008). It is approximately 50.9 m thick at the South Xiakou section and 43.4 m thick at the North Xiakou section. This member yields the bivalve Posidonia spp. and the ammonoid Flemingites (Li et al., 2009). Trace fossils are very rare. Only small, horizontal burrows Planolites montanus were found at the base of Member II. The occurrence of Planolites has been documented as an indicator of restricted

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ACCEPTED MANUSCRIPT oxygen conditions (Martin, 2004). Accordingly, the limestone-dominated sequence, together with the epifaunal Posidonia-Flemingites assemblage and horizontal bedding and small-scale

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cross-bedding structures (Fig. 2E), suggests an offshore transitional setting.

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Member III is dominated by light-grey micritic limestone and vermicular limestone with minor components of yellow-grey calcareous mudstone (Fig. 2B). It is approximately 381.4 m thick at the South Xiakou section and approximately 105 m thick at the North Xiakou section.

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Trace fossils are abundant and diverse, including horizontal burrows, vertical burrows,

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branched burrows, and complex burrow networks. The limestone-dominated succession combined with planar lamination suggest an offshore setting.

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Member IV is characterized by thinly- to medium bedded micritic limestone, oolitic

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limestone, and vermicular limestone. The thickness of Member IV is approximately 36.7 m at

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the North Xiakou section. The occurrence of oolitic limestone and cross-bedding structures (Fig. 2F) indicates a highly agitated marine-water environment. Trace fossils are abundant and

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diverse in micritic limestone and vermicular limestone, including horizontal burrows, vertical burrows, branched burrows, and complex burrow networks. As a result, the succession with oolitic limestone and diverse trace fossils suggests a paralic setting. The Jialingjiang Formation is subdivided into four members. Member I is dominated by light-grey thin or medium-thick stratified muddy dolomite and minor thin- to medium micritic limestone (Fig. 2C). Evaporite solution breccias occur occasionally in some dolomite beds. The thickness of this member is approximately 17.3 m. Trace fossils are absent. The dolomite-dominated succession possibly suggests a lagoon setting. Member II is composed of thin-medium micritic limestone, vermicular limestone, and

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ACCEPTED MANUSCRIPT calcarenitic limestone (Fig. 2D). The thickness of this member is approximately 170.5 m. Erosional surfaces and flat pebble conglomerates were found in some wackestone/packstone

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beds, indicating storm-wave dominated conditions. Trace fossils are abundant and diverse in

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micritic limestone and vermicular limestone, including horizontal burrows, vertical burrows, and complex burrow networks. The sharp occurrence of normally graded unit on the vermicular limestone suggests bioturbation under fair-weather conditions. This evidence

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base, i.e., in an offshore transitional zone.

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indicates that Member II was formed between the fair-weather wave base and the storm wave

Member III is characterized by greyish-white micrite dolomite. The thickness of this

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member is approximately 95 m. Some oolitic beds were found in the dolostone, suggesting a

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lagoon-to-shoreface setting. Member IV is approximately 70 m thick. This member is

lagoon setting.

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dominated by light-grey thickly bedded dolomite with evaporite solution breccia, suggesting a

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The Griesbachian is characterized by conodont Hindeodus parvus, Isarcicella isarcica, Neoclarkina krystyni zones, ammonoid Ophiceras-Lytophiceras zone, and bivalve Claraia stachei-C. griesbachi assemblage (Wang and Xia, 2004; Li et al., 2009; Zhao et al., 2013). The Dienerian contains the Sweetospathodus kummeli and Neospathodus dieneri conodonts-zones, Prionolobus-Gyronites ammonoid zone, and the Claraia concentrica-C. hubeiensis and Eumorphitis multiformis-E. inaequicostata bivalve assemblages (Li et al., 2009; Zhao et al., 2013). The Smithian is defined by the Novispathodus eowaageni conodont zone, Flemingites-Euflemingites ammonoid zone, and the Posidonia circularis-P. cf. wengensis bivalve assemblage (Li et al., 2009; Zhao et al., 2013). A carbon isotopic curve has been

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ACCEPTED MANUSCRIPT produced for the South Xiakou section (Tong et al., 2007), which has been considered a good proxy for the Early Triassic stratigraphic correlation because Early Triassic carbon isotope

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profiles derived from various depositional settings in South China yield comparable excursion

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patterns (e.g., Payne et al., 2004; Tong et al., 2007).

3. Materials and methods

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In this study, trace fossils in the Lower Triassic succession at Xiakou have been

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investigated bed-by-bed. A total of 17 ichnospecies in 14 ichnogenera were identified based on field observations and descriptions of specimens collected from the Daye and Jialingjiang

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formations (Figs. 5–8). In addition, taxonomic revision of the Lower Triassic trace fossils

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previously reported from the Middle Yangtze region (Yang et al., 1992; Ma et al., 2008) was

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undertaken based upon the original descriptions and illustrations (Table 1). Several semi-quantitative methods have been proposed to document variations in the extent

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of bioturbation on vertical profile and bedding planes, such as the ichnofabric index (ii, Droser and Bottjer, 1986) and bedding plane bioturbation index (BPBI). Here, BPBI was measured following the method of Miller and Smail (1997): BPBI1 = no bioturbation, BPBI2 = 0–10% disruption, BPBI3 = 10–40% disruption, BPBI4 = 40–60% disruption, and BPBI5 = 60–100% disruption. Furthermore, the forms and complexity of trace fossils as well as the sizes of burrows have proven to be good indicators of palaeoenvironmental conditions, especially for the oxygen content of sediments (e.g., Twitchett, 1999; Pruss and Bottjer, 2004) and were consequently used in this study. Burrow diameters were measured on bedding planes and in vertical exposures. A total of 43 bedding planes were measured to determine the size

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ACCEPTED MANUSCRIPT distribution of trace fossils, including one bedding plane in Member II of the Daye Formation; 24 and 3 bedding planes in Members III and IV of the Daye Formation, respectively; and 15

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bedding planes in Member II of the Jialingjiang Formation. Tiering levels, describing the

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distribution of benthic organisms within space (Ausich and Bottjer, 1982; Bottjer and Ausich, 1986), were also assessed in this study.

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4. Characteristics of Early Triassic trace fossils at the studied sections

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4.1. Arenicolites isp.

The trace fossil Arenicolites, a common ichnotaxon in the Lower Triassic of Three Gorges

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region, occurs in Members III and IV of the Daye Formation and the base of Member II of the

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Jialingjiang Formation (Figs. 3, 4). Arenicolites isp. is preserved as vertical to slightly oblique

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U-tubes without spreite. The penetration depth of burrows is difficult to measure. On bedding planes, this trace fossil occurs as pairs of holes (Fig. 5A, B). The burrow diameters of tubes

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range from 1 to 8 mm and the spacing of limbs from 3 to 10 mm (Fig. 9A, B). These traces are found associated with Planolites montanus and Palaeophycus tubularis. Arenicolites is generally classified as a dwelling trace, which could be made by various types of animals, e.g., polychaete worms (Swinbanks, 1981) and amphipod crustaceans (Bromley, 1996). The several types of Arenicolites preserved in the Lower Jurassic Neill Klinter Formation suggest these burrows could have acted both as permanent domiciles and as temporary shelters (Dam, 1990).

4.2. Chondrites filiformis Heer, 1877 Chondrites is a common ichnotaxon in the Phanerozoic and is known from a wide variety

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ACCEPTED MANUSCRIPT of marine sedimentary rocks, including mudstone, sandstone, and limestone (Simpson, 1956; Bromley and Ekdale, 1984). Here, Chondrites filiformis was found in wackestone in Member

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III of the Daye Formation (Figs. 3, 4). Chondrites filiformis, a highly branched burrow system,

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is preserved parallel or slightly oblique to the bedding plane (Fig. 5C). The branching angles range between 30° and 60°. The tunnel diameter of Chondrites filiformis ranges from 1 to 3 mm, and the vertical depth is less than 4 mm. The tunnels are filled with clay material that is

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darker than the host rock. These traces are found to be associated with Didymaulichnus lyelli,

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Paleodictyon isp., and Planolites montanus. Chondrites is generally interpreted as a fodinichnion probably produced by endobenthic deposit-feeding animals that probably lived

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in oxygen-poor conditions (Bromley and Ekdale, 1984; Kotake, 1991).

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4.3. Circulichnus montanus (Vyalov, 1971) This ichnogenus of circular to ovate traces from the Triassic of Russia was originally

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termed Circulichnis by Vyalov (Vyalov, 1971), but the correct orthography is actually Circulichnus, as discussed by Keighley and Pickerill (Keighley and Pickerill, 1997). Circulichnus montanus occurs in the middle part of Member III of the Daye Formation and at the base of Member II of the Jialingjiang Formation (Figs. 3, 4). This trace fossil is a circular to slightly ellipsoidal trace. The burrow diameter ranges from 3 to 5 mm. Circle or ellipse diameter ranges from 15 to 27 mm. These traces are preserved on the bedding plane in positive relief (Fig. 5D, E). Circulichnus is widely distributed in various types of marine sedimentary rocks and is a monospecific facies-crossing ichnotaxon that has been recorded not only in marine deposits but also in nonmarine facies (Fillion and Pickerill, 1984; Keighley and

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ACCEPTED MANUSCRIPT Pickerill, 1997). The origin of Circulichnus remains obscure.

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4.4. Cochlichnus kochi (Ludwig, 1869)

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The ichnospecies was originally put in Belorhaphe by Ludwig (Ludwig, 1896), but was referred to as Cochlichnus kochi by Calver (Calver, 1968). Cochlichnus kochi occurs in the top limestone of Member III of the Daye Formation (Fig. 3). This trace fossil is a sinusoidal,

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tunnel-like trace with uniform diameter (Fig. 5F). The tunnel diameters range from 0.5 to 2

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mm. The sinusoidal wavelength ranges from 5 to 30 mm. These traces were found to be associated with Planolites montanus, and both have been found occupying the same bedding

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surface of wackestone. Cochlichnus is known from a wide variety of marine environments, and

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it is also common in non-marine deposits (e.g., Maples and Archer, 1989; Goldring et al.,

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2005). This ichnogenus has been regarded either as a grazing trace, a locomotion trace, or a feeding structure (Chen et al., 2012), and its proposed trace-makers include worms or

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worm-like animals, nematodes, and dipterous insect larvae (Metz, 1998).

4.5. Didymaulichnus lyelli (Rouault, 1850) This ichnospecies was originally described as Fraena lyelli by Rouault (1850), but was referred to as Didymaulichnus lyelli by Young (1972). Didymaulichnus lyelli is a common ichnotaxon in the Lower Triassic of the Three Gorges region and has been found in Member III of the Daye Formation and Member II of the Jialingjiang Formation (Figs. 3, 4). This trace fossil occurs as a curved, bilobate structure with marginal smooth lobes and a central furrow (Fig. 6A–C). The bilobate width ranges from 2–12 mm, and the furrow width from 1–4 mm

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ACCEPTED MANUSCRIPT (Fig. 9D, E). Didymaulichnus has been interpreted as a trail left by gastropods (Glaessner, 1969), trilobites (Crimes and Herdman, 1970), or polycladid flatworms (Zonneveld et al.,

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2010a).

4.6. Diplocraterion parallelum Torell, 1870

Diplocraterion parallelum was found in Members III and IV of the Daye Formation (Fig.

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3). Diplocraterion parallelum consists of vertical U-shaped burrows with parallel tubes and a

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unidirectional spreite. This trace fossil is seen in two dimensions on upper bedding planes as dumbbell shapes including two circles as horizontal sections (Fig. 6D). The limbs range from

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5–6 mm in diameter and from 8–10 mm apart. These traces were found to be associated with

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Arenicolites isp. and Planolites montanus. Diplocraterion is generally regarded as a dwelling

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burrow of suspension feeders or benthic predators such as polychaete annelid, enteropneusts, echiurans, and sipunculans (Fürsich, 1974; Ekdale and Lewis, 1991; Zonneveld et al., 2010a).

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4.7. Diplocraterion isp.

Diplocraterion isp. occurs in the upper part wackestone of Member II of the Jialingjiang Formation (Fig. 3). A more complete vertical section of Diplocraterion with two approximate parallel limbs is observed in Fig. 6E. Spreite laminae in the cross section view are clear. The limbs from 4–5 mm in diameter and from 17–24 mm apart. This trace fossil occurs associated with Palaeophycus tubularis and Planolites montanus.

4.8. Gordia molassica (Heer, 1865) Gordia molassica occurs on the bedding plane in Member III of the Daye Formation (Figs.

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ACCEPTED MANUSCRIPT 3, 4). This trace fossil consists of overlapping loops and meandering trails (Fig. 6F). The width ranges from 2–10 mm (Fig. 9C) and remains consistent in each trail. Loop distances are less

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than 40 mm in diameter. This ichnogenus has been regarded either as a feeding burrow or trail

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produced by polychaetes (Książkiewicz et al., 1977), or a locomotion trace produced by worms or gastropods (Yang, 1984; Aceñolaza and Buatois, 1993; McCann, 1993).

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4.9. Mammillichnis? isp.

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Mammillichnis? isp. occurs in the middle part of Member III of the Daye Formation (Fig. 3). These traces are preserved as hemispherical mounds (Fig. 7A, B). The mounds consist of a

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1–3 mm high hemicircular apex. The diameter of mounds ranges from 2–4 mm. The classic

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Mammillichnis consists of a subhemispherical tubercle being surrounded by a tyre-like ring

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(see Chamberlain, 1971, pl.30, figs. 6, 7). However, these traits are not clear in our samples. Mammillichnis? isp. was found to be associated with Planolites montanus. Chamberlain (1971)

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suggested three possible explanations for the origin of Mammillichnis including resting or hiding trace of an animal, body fossils of juveniles or eggs deposited in the sediment, or excurrent end of tubular burrow where animal worked sediment for food or packed it with fecal pellets.

4.10. Oldhamia radiata Forbes, 1849 Only one Oldhamia radiata sample was collected from the lower part of Member III of the Daye Formation (Fig. 4). This trace fossil is preserved on the upper surface of wackestone as radiating daisy-shaped rays (Fig. 7C). In total of 13 separate rays were identified on the

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ACCEPTED MANUSCRIPT bedding plane. Rays vary in length from <4 mm to 10 mm long. The width is approximately 0.5 mm and remains consistent in each ray. This trace fossil is associated with Palaeophycus

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tubularis and both have been found occupying the same bedding surface of wackestone.

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Oldhamia is generally interpreted as a fodinichnion probably produced by worm-like organisms (MacNaughton, 2007).

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4.11. Oldhamia isp.

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Oldhamia isp. occurs in the lower part of Member III of the Daye Formation (Fig. 4). This trace fossil consists of irregular radiating burrows parallel to bedding (Fig. 7D). Individual rays

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are irregularly curved to nearly straight and vary in length <3 mm to 10 mm long. The width is

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approximately 0.5 mm and remains consistent in each ray. The diameter of the whole radiating

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structure is approximately 30 mm. This trace fossil is associated with Didymaulichnus lyelli

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and Gordia molassica.

4.12. Palaeophycus heberti (de Saporta, 1872) Palaeophycus heberti occurs in the middle part of Member III of the Daye Formation (Fig. 4). This trace fossil is preserved as slightly curved, smooth burrows parallel to bedding (Fig. 7E). The filling is structureless and similar to host rock. The cross-section is circular or oval and the diameter ranges from 1–2 mm. These traces are associated with Planolites montanus and both have been found occupying the same bedding surface of wackestone. Palaeophycus is generally interpreted as repichnia or domichnia probably produced by carnivorous or omnivorous invertebrates (Pemberton and Frey, 1982; Keighley and Pickerill, 1995) and

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ACCEPTED MANUSCRIPT arthropods (Zonneveld et al., 2010a).

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4.13. Palaeophycus tubularis Hall,1847

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The trace fossil Palaeophycus tubularis, a common ichnotaxon in the Lower Triassic of the Three Gorges region, occurs in Members III and IV of the Daye Formation and the lower part of Member II of the Jialingjiang Formation (Figs. 3, 4). Palaeophycus tubularis, as a straight or

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slightly curved, unbranched burrow system with circular or oval cross-section, is preserved

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parallel or slightly oblique to the bedding plane (Fig. 7F, G). The burrows range from 1–12 mm in width and from 6–80 mm in length (Fig. 10). The filling is structureless and similar to

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host rock. These traces were found to be associated with Didymaulichnus lyelli and Planolites

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montanus.

4.14. Paleodictyon isp.

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Two specimens of Paleodictyon isp. were found in the middle Member III of the Daye Formation (Figs. 3, 4). The bigger one occupies an area of approximately 6 × 8 cm2, but the specimen is certainly larger than that (Fig. 8A). On the same bedding plane, Chondrites filiformis and Planolites montanus are present. The present specimens are not well preserved, but this preservational state is sufficient to reconstruct the original shape. Paleodictyon isp. is preserved as honeycomblike network of five- to six-sided meshes. The mesh diameter ranges from 8–12 mm. The string width varies between 3 and 8 mm (Fig. 9F). Analysis of modern Paleodictyon suggests two alternative interpretations: (1) a burrow consistent with interpretation of the ancient form as a trace fossil, (2) a compressed form of a hexactinellid

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ACCEPTED MANUSCRIPT sponge (Rona et al., 2009). Therefore, further work is needed to uncover the nature of

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Paleodictyon.

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4.15. Planolites montanus Richter, 1937

Planolites montanus, one of the most common ichnotaxon in the Lower Triassic of Three Gorges region, occurs in Members II, III, and IV of the Daye Formation and Member II of the

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Jialingjiang Formation (Figs. 3, 4). These traces are preserved as straight or gently curved,

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unbranched burrows parallel or slightly oblique to the bedding plane (Fig. 8B, C). Burrow diameter ranges from less than 1 mm to 12 mm (Fig. 11). Their length range from 5 mm to

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over 150 mm but is typically 20–50 mm. The burrows are filled with clayed material that is

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darker than the host rock. Planolites montanus is the dominant ichnospecies in the Lower

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Triassic assemblage and has been found to be associated with Arenicolites isp., Chondrites filiformis, Cochlichnus kochi, Diplocraterion parallelum, Diplocraterion isp., Mammillichnis?

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isp., Palaeophycus heberti, Palaeophycus tubularis, Paleodictyon isp., and Thalassinoides isp. Planolites is generally interpreted as a backfilled structure probably produced by deposit-feeding activities of worm-like organisms (Pemberton and Frey, 1982).

4.16. Skolithos isp. Skolithos isp. occurs in the middle part of Member II of the Jialingjiang Formation (Fig. 3). These traces are preserved as simple straight, unlined tubes in the cross section (Fig. 8F, G). The tubes have cylindrical to sub-cylindrical cross sections, 3–4 mm in diameter, and are not branched. The penetration depth is approximately 45 mm. The filling is structureless and

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ACCEPTED MANUSCRIPT similar to host rock. Skolithos isp. was found associated with Planolites montanus. Skolithos is generally regarded as domichnia probably produced by many creatures such as insects,

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arachnids, and arthropods (Ratcliffe and Fagerstrom, 1980; Netto et al., 2007). Skolithos is a

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common and easily recognized ichnotaxon in the Precambrian and Phanerozoic and is known from various depositional environments from the deep-sea to non-marine (Droser, 1991). Skolithos has been found in the Griesbachian and Dienerian of Canada (Zonneveld et al.,

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2010a), but the tube diameter and penetration depth of those traces is apparently smaller than

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our specimen.

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4.17. Thalassinoides isp.

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Thalassinoides isp. occurs as positive reliefs on the upper surface of vermicular limestone

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bed from the top of Member III of the Jialingjiang Formation (Fig. 3). The trace fossil Thalassinoides isp. consists of both Y-shaped and irregular branching burrows (Fig. 8D, E).

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Burrow diameter ranges from less than 3 mm to 11 mm. The filling is structureless and similar to host rock. These traces were found associated with Palaeophycus tubularis and Planolites montanus. Thalassinoides is a common ichnotaxon of the Phanerozoic and is known from a wide variety of marine sedimentary rocks. This ichnogenus is generally regarded as the work of deposit feeders or as domichnia produced by many creatures such as sea anemones, crustaceans, or enteropneust acron worms (Bromley and Frey, 1974; Bromley and Ekdale, 1984; Bromley, 1996; Zonneveld et al., 2010a).

5. Trace fossils as indicators of benthic biotic recovery after the P-Tr extinction

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ACCEPTED MANUSCRIPT 5.1. Trace fossil diversity Only one ichnospecies, Planolites montanus, was uncovered from Member II of the Daye

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Formation in the Three Gorges area. At present, no trace fossil has been found in this member

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in any other area of the Middle Yangtze region, including the Guangji and Huangshi areas (Table 1; Yang et al., 1992, Ma et al., 2008). The two ichnospecies, Didymaulichnus lyelli and Planolites montanus, are present in the lower part of Member III of the Daye Formation (Figs.

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3, 4).

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Trace fossils in the middle and upper parts of Member III of the Daye Formation are diverse and abundant at the studied sections, including 14 ichnospecies in 12 ichnogenera, viz.

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Arenicolites isp., Chondrites filiformis, Circulichnis montanus, Cochlichnus kochi,

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Didymaulichnus lyelli, Diplocraterion parallelum, Gordia molassica, Mammillichnis? isp.,

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Oldhamia radiata, Oldhamia isp., Paleodictyon isp., Palaeophycus heberti, Palaeophycus tubularis, and Planolites montanus. Among these ichnospecies, Planolites montanus has also

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been found in the Guangji area (Yang et al., 1992). Yang et al. (1992) also found Megagrapton isp., Phycodes isp., and Thalassinoides isp. from Member III of the Daye Formation in the Guangji area.

The Member IV ichnocoenosis consists of four ichnospecies in four ichnogenera at the studied section, including Arenicolites isp., Diplocraterion parallelum, Palaeophycus tubularis, and Planolites montanus. At present, no trace fossil has been found in this member in any other area of the Middle Yangtze region, including the Guangji and Huangshi areas (Table 1; Yang et al., 1992; Ma et al., 2008). No trace fossil was uncovered in the dolomite of Member I of the Jialingjiang Formation at

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ACCEPTED MANUSCRIPT the studied section. The Member II ichnocoenosis is diverse, including eight ichnospecies in eight ichnogenera, viz. Arenicolites isp., Circulichnis montanus, Didymaulichnus lyelli,

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Diplocraterion isp., Palaeophycus tubularis, Planolites montanus, Skolithos isp., and

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Thalassinoides isp. Moreover, Palaeophycus tubularis and Planolites montanus have been reported from Member II of the Jialingjiang Formation elsewhere in the Middle Yangtze region (Table 1; Yang et al., 1992; Ma et al., 2008). In addition, Yang et al. (1992) and Ma et al.

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(2008) reported Helminthopsis abeli, Paleodictyon isp., Palaeophycus cf. heberti,

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Palaeophycus isp., Phycodes palmatus, Planolites beverleyensis, Rhizocorallium cf. jenense, and Scalarituba cf. missouriensis in Member II from Guangji and Huangshi areas.

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Trace fossil assemblages of the Middle Yangtze region show an increase in trace fossil

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diversity from the absence of trace fossils in Member I of the Daye Formation, to a

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comparatively low diversity with two ichnospecies in Member II and the lower part of Member III of the Daye Formation, and finally to a considerably high diversity in the upper Daye

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Formation and the Jialingjiang Formation.

5.2. Bedding plane bioturbation index No bioturbation is present in Member I of the Daye Formation at both the North and South Xiakou sections (Figs. 3, 4), thus indicating a BPBI of 1. No bioturbation is present in Member II of the Daye Formation at North Xiakou section (Fig. 4). However, at the South Xiakou section, for the one bedding plane containing Planolites montanus in Member II of the Daye Formation, the coverage is approximately 5% indicating a BPBI of 2. In Member III of the Daye Formation, approximately 30% of the stratum is comparatively well bioturbated, with a

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ACCEPTED MANUSCRIPT BPBI of 2–4 (Figs. 3, 4). About half of the strata of Member IV of the Daye Formation was covered by trace fossils, including Arenicolites isp., Diplocraterion parallelum, Palaeophycus

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tubularis, and Planolites montanus. Coverage of the bedding plane by signs of bioturbation

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rangs from 5%–15%, indicating a BPBI of 2–3 (Fig. 3). No bioturbation is present in Member I of the Jialingjiang Formation, thus indicating a BPBI of 1. Approximately 35% of the strata are bioturbated in Member II of the Jialingjiang Formation (Fig. 3). Several bedding planes

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containing Circulichnis montanus and Palaeophycus tubularis have coverage of over 50%,

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suggesting a BPBI of 4–5 for these beds. No bioturbation was found from Member III of the Jialingjiang Formation in the Three Gorges area. However, one bedding plane containing

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Asteriacites isp. has coverage approximately 50%, indicating a BPBI of 4 for Member III of

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the Jialingjiang Formation in Guangji area (Yang et al., 1992, pl. 1, fig. 8).

5.3. Trace fossil form and complexity

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The trace fossil assemblage preserved in the Lower Triassic Daye and Jialingjiang formations shows a wide variety in trace fossil forms and complexity. Only one simple, horizontal ichnospecies, Planolites montanus, is present in Member II of the Daye Formation (Fig. 12A). The trace fossil assemblage in the lower part of Member III of the Daye Formation is dominated by simple, horizontal burrows, such as Didymaulichnus lyelli and Planolites montanus. Trace fossil behavioural diversity is comparatively high in the middle and upper parts of Member III of the Daye Formation, including simple, horizontal burrows, subhorizontal branched burrows, simple vertical burrows, subvertical branched burrows, and complex burrow networks (Fig. 12B). The trace fossil assemblage of Member IV of the Daye

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ACCEPTED MANUSCRIPT Formation is dominated by simple horizontal burrows and vertical burrows (Fig. 12C). The trace fossil assemblage in Member II of the Jialingjiang Formation is dominated by simple

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horizontal burrows, vertical burrows, and complex burrow networks (Fig. 12D).

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The trace fossil assemblages of the Middle Yangtze region show an increase in trace fossil complexity, from simple, horizontal Didymaulichnus and Planolites burrows of Member II and the lower part of Member III of the Daye Formation to complex burrow networks in the upper

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Daye Formation and the Jialingjiang Formation. Furthermore, the presence of Thalassinoides,

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which is typically indicative of normal marine environments, represents an increase in behavioural complexity that typically indicates advanced recovery after the P-Tr crisis

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(Twitchett and Wignall, 1996; Pruss and Bottjer, 2004; Twitchett and Barras, 2004; Twitchett,

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2006; Chen et al., 2011; Hofmann et al., 2011).

5.4. Trace fossil size

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Burrow diameters were measured to determine the size distribution of each of Arenicolites isp., Gordia molassica, Didymaulichnus lyelli, Palaeophycus tubularis, Paleodictyon isp., Planolites montanus, and Thalassinoides isp. (Figs. 9–11). Among these ichnospecies, both Palaeophycus tubularis and Planolites montanus occur in multiple horizons (Figs. 10, 11), allowing comparisons of the burrow size of the same ichnotaxon throughout the Early Triassic. The smallest Palaeophycus tubularis occurs in the lower Member III of the Daye Formation; the mean burrow diameter for 246 specimens in five horizons is 0.9 mm, and the maximum burrow diameter is 2.0 mm. Burrow sizes of the middle Member III of the Daye Formation are small, with mean and maximum burrow diameters of 1.8 mm and 2.6 mm,

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ACCEPTED MANUSCRIPT respectively (N = 28). A significant increase in burrow size of Palaeophycus tubularis happened in Member IV of the Daye Formation; the mean and maximum burrow diameters

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reach 3.9 mm and 13.0 mm, respectively (N = 52). The burrow sizes of Palaeophycus tubularis

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in the Jialingjiang Formation are similar to those found in Member IV of the Daye Formation (Fig. 10).

The smallest Planolites montanus occurs in Member II of the Daye Formation; the mean

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and maximum burrow diameters are 0.9 mm and 1.1 mm, respectively (N = 10). The burrow

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sizes of Member III of the Daye Formation become larger, with mean burrow diameters of 1.1 mm, 1.2 mm, and 1.9 mm in the lower, middle, and upper parts, respectively (Fig. 11). This

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size increase persists throughout Member IV of the Daye Formation and Member II of the

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Jialingjiang Formation; the mean and maximum burrow diameters increase from 2.7 mm and

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5.5 mm in Member IV of the Daye Formation to 4.1 mm and 12.0 mm in the middle Member II of the Jialingjiang Formation (Fig. 11).

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The mean burrow diameters for all trace fossils measured in the Three Gorges area show a persistent growth trend from Member II of the Daye Formation to the upper Member II of the Jialingjiang Formation (Fig. 13). The maximum burrow diameters show a similar trend, but reach a maximum in the middle and upper Member III of the Daye Formation. The increase of trace fossil size is reflected in both the growth of small ichnotaxa, e.g., Arenicolites isp., Didymaulichnus lyelli, Palaeophycus tubularis, and Planolites montanus. (Figs. 9–11), and the reappearance of some large ichnotaxa, e.g., Thalassinoides isp. (Fig. 9G). The burrow diameters of Thalassinoides isp. rang from less than 4 mm to over 11 mm, and the mean burrow diameter is 7.1 mm (N = 31), which is almost twice of that found in the Nanlinghu

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ACCEPTED MANUSCRIPT Formation of the Lower Yangtze region (Chen et al., 2011), but slightly greater than that found

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in the Virgin Limestone Member of the western United States (Pruss and Bottjer, 2004).

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5.5. Infaunal tiering

Tiering, the vertical stratification of marine benthic fauna above and below the substrate level, is a useful measure of ecological recovery (Ausich and Bottjer, 1982; Twitchett, 1999).

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Accordingly, tiering of Early Triassic trace fossil assemblages has previously been described in

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the Lower Yangtze region of South China (Chen et al., 2011), the western United States (Pruss and Bottjer, 2004; Fraiser and Bottjer, 2009), western Canada (Zonneveld et al., 2010a), and

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Western Australia (Chen et al., 2012). In this study, infaunal tiering levels were assessed based

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on the measurements of penetration depth of trace fossils from vertical exposures. Trace fossils

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in Member II and lower Member III of the Daye Formation usually have simple, horizontal burrows (Planolites montanus), and infaunal tiering is generally very shallow and primarily in

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the 0 to -3 mm tier. In the middle and upper parts of Member III of the Daye Formation, vertical burrows of Arenicolites isp. extend to a depth of 9–14 mm into the sediment, indicating a higher level of tiering. In Member IV of the Daye Formation and Member II of the Jialingjiang Formation, vertical burrows of Skolithos isp. penetrate to a depth of up to 42–74 mm (Figs. 8H, I, 12). Tiering levels above and below the substrate have not remained constant throughout the Phanerozoic, but have fluctuated between +100 cm and -100 cm (Ausich and Bottjer, 1982; Bottjer and Ausich, 1986). Recently, it has been demonstrated that the P-Tr mass extinction event was followed by a dramatic reduction in tiering levels (Twitchett, 1999; Ausich and

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ACCEPTED MANUSCRIPT Bottjer, 2001). Twitchett (1999, 2006) identified four distinct stages in tiering levels during the recovery interval from the P-Tr mass extinction: during stage 1, epifaunal and infaunal tiering

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was reduced to a bare minimum; during stage 2, infaunal tiering began to recover; during stage

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3, epifaunal tiering began to recover; and during stage 4, pre-extinction tiering levels were once again present. In the Three Gorges area, the penetration depths from Member I to the lower part of Member III of the Daye Formation are very shallow, and the deepest tier is less

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than 3 mm, indicating stage 1 of ecological recovery. The middle and upper parts of Members

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III and IV of the Daye Formation have moderate penetration depths and thus belong to stage 2 of ecological recovery. The deepest tier in Member II of the Jialingjiang Formation is up to 74

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mm, which is almost the level of Late Permian trace fossils found in Shangsi section in South

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China (Wignall et al., 1995), indicating stage 4 of recovery.

5.6. Benthic recovery from an ichnological perspective

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Previous studies have shown that trace fossils underwent a dramatic reduction in diversity, complexity, size, and tiering level during the P-Tr mass extinction, followed by a slow recovery in the Early Triassic (Twitchett and Wignall, 1996; Twitchett, 1999; Pruss and Bottjer, 2004; Twitchett and Barras, 2004; Fraiser and Bottjer, 2009; Chen et al., 2011). Relative to populations in normal environmental settings, biotic assemblages that occurred in the immediate aftermath of the P-Tr mass extinction are characterized by low taxonomic diversity (Song et al., 2011b), high abundance (Rodland and Bottjer, 2001; Song et al., in review), and reduced body size of both metazoans (Twitchett, 2007) and protozoans (Song et al., 2011a). Environmental stresses following the P-Tr extinction result in similar characteristics in fossil

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ACCEPTED MANUSCRIPT assemblages that occur during extinction and survival intervals. In the studied areas, trace fossils were very rare in the Griesbachian-Smithian intervals. No trace fossil was found in the

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Griesbachian and Dienerian strata in the studied areas. Only two ichnogenera were uncovered

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from the Smithian strata. Trace fossil data from the Middle Yangtze region show that recovery of trace makers did not begin until the Spathian. Spathian trace fossils in the Three Gorges, Huangshi, and Guangji areas are very diverse and abundant, comprising up to 21 ichnogenera

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(Table 1). Almost all of these ichnofossil-based proxies, including BPBI, complexity, burrow

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size, and tiering level, were at low levels in the Smithian. In contrast, these proxies in the Spathian were clearly at high levels and gradually increased from the early Spathian to the

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middle-late Spathian.

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The recovery pattern of benthic ecosystems, as indicated by trace fossils in the Middle

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Yangtze region, is similar to that found in Lower Yangtze region (Chen et al., 2011). Trace fossil assemblages from Italy and the western United States also show a similar pattern, with

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the recovery of complex forms such as Rhizocorallium and Thalassinoides delayed until the Spathian (Twitchett and Wignall, 1996; Twitchett, 1999; Pruss and Bottjer, 2004; Twitchett and Barras, 2004). Recently, Fraiser and Bottjer (2009) reported a relatively diverse trace fossil assemblage from the Sinbad Limestone Member of the western United States. This ichnofossil assemblage is characterised by low-to-moderate ichnodiversity, low-to-moderate bioturbation, small burrow widths, shallow tiering and non-specialised behaviour. Fraiser and Bottjer (2009) proposed that the Smithian trace fossil fauna reflects a phenomenon of opportunistic behaviour of benthic trace-maker populations rather than full recovery after the P-Tr mass extinction. Early Triassic trace fossil assemblages from South China are also characterized by

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ACCEPTED MANUSCRIPT low-to-moderate ichnodiversity and bioturbation, small burrows, and shallow tiering, suggesting the opportunistic behaviour in benthic trace-makers. Compared to the extremely

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diverse trace fossil assemblages of the Middle Triassic (Zonneveld, 2001), the diversity of

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Spathian trace fossils in South China is moderate. Opportunistic behaviour is also present in Lower Triassic protozoan foraminifers. Song et al. (2011b) found that two significant phases of foraminiferal diversifications following the P-Tr mass extinction happened in the Smithian and

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Spathian, respectively, with a brief pause near the Smithian-Spathian boundary. The latest

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studies show that many foraminiferal species, which principally drive repopulation, are opportunistic taxa with low taxonomic diversity and high abundance (Song et al., in review).

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It is interesting that a rapid recovery of ichnodiversity occurred in the immediate aftermath

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of the P-Tr mass extinction (Griesbachian) in the high-palaeolatitude regions, such as

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Spitsbergen (Wignall et al., 1998) and western Canada (Beatty et al., 2005; Beatty et al., 2008; Zonneveld et al., 2010a). Recent studies show that moderate diverse trace fossils occur in the

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Griesbachian and Dienerian successions of Italy and Persian Gulf (Knaust, 2010; Hofmann et al., 2011). The advanced recovery of trace-makers has been attributable to localities devoid of anoxia in the shallow marine environments (see Hofmann et al., 2011). The recovery pattern of benthic ecosystems, as indicated by trace fossils in South China, might be explained by the “refuge zone model” proposed by Song et al. (2014b). Recent studies based on conodont-based geochemical data from the same geological sections in South China (i.e., the Meishan and Guandao sections) show that global warming and oceanic anoxia occurred at the end-Permian and lasted throughout almost the entire Early Triassic although several fluctuations existed for both redox and SST (sea surface temperature) curves (Joachimski et al., 2012; Song et al., 2012;

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ACCEPTED MANUSCRIPT Sun et al., 2012). Moreover, the redox and SST curves changed synchronously in the Early Triassic (Song et al., 2014b). The “refuge zone model” suggests that there was a narrow

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mid-water zone between lethally warm, shallow waters and anoxic deep waters. Some

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thermally tolerant organisms and hypoxia tolerant species could survive in the refuge zone during and after the P-Tr extinction event (see Song et al., 2014b). A two-phase expansion of the refuge zone in the Smithian and early-middle Spathian might have played a significant role

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in the diversification of ichnofossils in South China.

6. Conclusion

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The Lower Triassic Daye and Jialingjiang formations of the Three Gorges area record the

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recovery interval of marine organisms in the aftermath of the P-Tr mass extinction. These

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strata include abundant and diverse trace fossil assemblages. A total of 17 ichnospecies in 14 ichnogenera are documented from the Smithian and Spathian strata. These trace fossils are

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interpreted to represent activities of a diverse invertebrate infauna and epifauna, including anemones, crustaceans, molluscs, nematodes, and polychaetes. The trace fossil assemblages from Three Gorges area, combined with those found in the Huangshi and Guangji areas, show that ichnodiversity in the Middle Yangtze region did not markedly increase until the early Spathian, from two ichnogenera in the Smithian to 13 ichnogenera in the early Spathian. Trace fossils in the Smithian strata are dominated by simple, small, horizontal burrows, including Didymaulichnus and Planolites. In comparison, Spathian trace fossils are more diverse and abundant, with moderate to high bioturbation levels and complex burrow networks, such as Thalassinoides. It is notable that both burrow sizes and penetration depths gradually increased

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ACCEPTED MANUSCRIPT from the early Spathian to middle-late Spathian, implying a gradual recovery of the benthic ecosystem.

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Our trace fossil data, in combination with other published studies in South China, indicate

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that the Early Triassic ichnofossils are characterized by aspects of opportunistic behaviour, suggesting that stressed environmental conditions continued till the Spathian. The recovery tempo and pattern of ichnocoenoses in South China is likely influenced by the temporal and

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spatial changes of the refuge zone proposed by Song et al. (2014b).

Acknowledgments

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This study was supported by the 973 Program (2011CB808800), the National Natural

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Science Foundation of China (Nos. 41240016, 41272372, 41172312, 41302010), and the 111

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Project (B08030). It is a contribution to the IGCP-630 program “Permian-Triassic climatic and

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environmental extremes and biotic response”.

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Zonneveld, J.-P., Gingras, M.K., Beatty, T.W., 2010a. Diverse ichnofossil assemblages following the P-T mass extinction, Lower Triassic, Alberta and British Columbia, Canada: Evidence for shallow marine refugia on the northwestern coast of Pangaea. Palaios 25, 368–392. Sedimentology

and

ichnology

of

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Zonneveld, J.-P., MacNaughton, R.B., Utting, J., Beatty, T.W., Pemberton, S.G., Henderson, C.M., 2010b. the

Lower

Triassic

Montney

Formation

in

the

Pedigree-Ring/Border-Kahntah River area, northwestern Alberta and northeastern British Columbia. Bull. Can.

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Pet. Geol. 58, 115–140.

Figure captions

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Fig.1. A, Palaeogeographic map of the Early Triassic with approximate locations of the Lower Triassic successions containing ichnofossil assemblages (modified after Scotese, 2001). B,

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Inset map of the People’s Republic of China. C, Locality of the studied sections.

Fig. 2. Field photos of the Lower Triassic South and North Xiakou sections. A, the Permian-Triassic boundary successions exposed at the South Xiakou section. B, the Lower Triassic successions exposed at the North Xiakou section. C, the topmost Daye Formation and the lowest Jialingjiang Formation at the South Xiakou section. D, calcarenitic limestone in Member II of the Jialingjiang Formation. E, small-scale cross beddings in Member II of the Daye Formation. F, cross beddings in Member IV of the Daye Formation.

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ACCEPTED MANUSCRIPT Fig. 3. Stratigraphic distribution of trace fossils with bedding plane bioturbation index (BPBI, Miller and Smail, 1997) at the South Xiakou section. Conodont zones: 1, Clarkina yini and C.

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meishanensis Zones; 2, Hindeodus parvus Zone; 3, Isarcicella stachei and I. isarcica Zones; 4,

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Neoclarkina krystyni Zone ; 5, Neoclarkina discreta Zone; 6, Sweetospathodus kummeli Zone; 7, Neospathodus dieneri Zone; 8, Novispathodus eowaageni Zone; 9, Novispathodus pingdingshanensis Zone. Bivalve assemblages: ①, Claraia stachei-Claraia griesbachi

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Assemblage; ②, Claraia concentrica-Claraia hubeiensis Assemblage; ③, Eumorphotis

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multiformis-E. inaequicostata Assemblage; ④, Posidonia circularis-P. cf. wengensis Assemblage. Ammonoid zones: I, Ophiceras-Lytophiceras Zone; II, Prinolobus-Gyronites

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Zone; III, Flemingites Zone.

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Fig. 4. Stratigraphic distribution of trace fossils with bedding plane bioturbation index (BPBI,

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Miller and Smail, 1997) at the Lower Triassic North Xiakou section.

Fig. 5. Lower Triassic trace fossils at the South and North Xiakou sections (coin diameter is 2 cm). A, B, Arenicolites isp. on the upper bedding plane of the middle part of Member III of the Daye Formation, North Xiakou section (A) and the middle part of Member II of the Jialingjiang Formation, South Xiakou section (B), upper surface. C, Chondrites filiformis from the middle part of Member III of the Daye Formation, North Xiakou section, upper surface. D, E, Circulichnis montanus from the middle part of Member III of the Daye Formation, South Xiakou section (D) and the base of Member II of the Jialingjiang Formation, South Xiakou section (E), upper surface. F, Cochlichnus kochi on the upper bedding plane of the upper part

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ACCEPTED MANUSCRIPT of Member III of the Daye Formation, South Xiakou section, upper surface.

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Fig. 6. Lower Triassic trace fossils at the South and North Xiakou sections (coin diameter is 2

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cm). A–C, Didymaulichnus lyelli on the upper bedding plane of the upper part of Member III of the Daye Formation, North Xiakou section (A), the lower part of Member III of the Daye Formation, South Xiakou section (B), and the lower part of Member II of the Jialingjiang

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Formation, South Xiakou section (C), upper surface. D, Diplocraterion parallelum from the

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upper part of Member III of the Daye Formation, South Xiakou section, upper surface. E, Diplocraterion isp. from the middle part of Member II of the Jialingjiang Formation, South

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Xiakou section, cross section. Arrow 1: spreite laminae, arrow 2: limb. F, Gordia molassica on

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section, upper surface.

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the upper bedding plane of the upper part of Member III of the Daye Formation, North Xiakou

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Fig. 7. Lower Triassic trace fossils at the South and North Xiakou sections (coin diameter is 2 cm). A, B, Mammillichnis? isp. from the upper part of Member III of the Daye Formation, South Xiakou section, subface. C, Oldhamia radiata from the lower part of Member III of the Daye Formation, North Xiakou section, upper surface. D, Oldhamia isp. from the lower part of Member III of the Daye Formation, North Xiakou section, upper surface. E, Palaeophycus heberti on the upper bedding plane of the middle part of Member III of the Daye Formation, North Xiakou section, upper surface. F, G, Palaeophycus tubularis on the upper bedding plane of the lower part of Member II of the Jialingjiang Formation, South Xiakou section (F) and the middle part of Member III of the Daye Formation, North Xiakou section (G), upper surface.

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Fig. 8. Lower Triassic trace fossils at the South and North Xiakou sections (coin diameter is 2

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cm). A, Paleodictyon isp. from the middle part of Member III of the Daye Formation, South

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Xiakou section, upper surface. B, C, Planolites montanus from the middle part of Member II of the Jialingjiang Formation, South Xiakou section and the middle part of Member III of the Daye Formation, upper surface. D, E, Thalassinoides isp. on the upper bedding plane of the

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upper part of Member II of the Jialingjiang Formation, South Xiakou section, upper surface. F,

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Planolites montanus and Skolithos isp. from the middle part of Member II of the Jialingjiang Formation, South Xiakou section, cross section. G, Skolithos isp. from the upper part of

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Member II of the Jialingjiang Formation, South Xiakou section, cross section.

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Fig. 9. Size of trace fossils in the Daye and Jialingjiang formations. A, Arenicolites isp. from Member III of the Daye Formation. B, Arenicolites isp. from Member II of the Jialingjiang

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Formation. C, Gordia molassica from Member III of the Daye Formation. D, Didymaulichnus lyelli from Member III of the Daye Formation. E, Didymaulichnus lyelli from Member II of the Jialingjiang Formation. F, Paleodictyon isp. from Member II of the Daye Formation. G, Thalassinoides isp. from Member II of the Jialingjiang Formation.

Fig. 10. Size of trace fossil Palaeophycus tubularis in the Daye and Jialingjiang formations. A, lower Member III of the Daye Formation. B, middle Member III of the Daye Formation. C, Member IV of the Daye Formation. D, lower Member II of the Jialingjiang Formation. E, middle Member II of the Jialingjiang Formation. F, upper Member II of the Jialingjiang

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ACCEPTED MANUSCRIPT Formation. G, burrow size of Palaeophycus tubularis variations through the Early Triassic. Note the line with quadrates shows the maximum diameter values and the line with diamonds

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shows the mean diameter values.

Fig. 11. Size range of trace fossil Planolites montanus in the Daye and Jialingjiang formations. A, Member II of the Daye Formation. B, lower Member III of the Daye Formation. C, middle

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Member III of the Daye Formation. D, upper Member III of the Daye Formation. E, Member

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IV of the Daye Formation. F, lower Member II of the Jialingjiang Formation. G, middle Member II of the Jialingjiang Formation. H, upper Member II of the Jialingjiang Formation. I,

D

burrow size of Planolites montanus variations through the Early Triassic. Note the line with

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quadrates shows the maximum diameter values and the line with diamonds shows the mean

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diameter values.

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Fig. 12. Ichnofaunas from the Lower Triassic of South China. A, Member II of the Daye Formation; B, Member III of the Daye Formation; C, Member IV of the Daye Formation; D, Member II of the Jialingjiang Formation.

Fig. 13. A, burrow size variations of trace fossils in the Daye and Jialingjiang formations. B, tiering levels indicated by the maximum penetration depths in the Daye and Jialingjiang formations. D2, Member II of the Daye Formation; D3, Member III of the Daye Formation; D4, Member IV of the Daye Formation; J2, Member II of the Jialingjiang Formation.

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ACCEPTED MANUSCRIPT Table 1. Lower Triassic ichnotaxa from the studied sections and other areas of the Middle Yangtze region. Locality

Ichnotaxa

Revision

References

Member II of the Daye Formation

Xingshan, Hubei

Planolites montanus

Member III of the Daye Formation

Xingshan, Hubei

Arenicolites isp.

Member III of the Daye Formation

Xingshan, Hubei

Chondrites filiformis

Member III of the Daye Formation

Xingshan, Hubei

Circulichnis montanus

Member III of the Daye Formation

Xingshan, Hubei

Cochlichnus kochi

Member III of the Daye Formation

Xingshan, Hubei

Didymaulichnus lyelli

Member III of the Daye Formation

Xingshan, Hubei

Diplocraterion parallelum

This study

Member III of the Daye Formation

Xingshan, Hubei

Gordia molassica

This study

Member III of the Daye Formation

Xingshan, Hubei

Mammillichnis? isp.

This study

Member III of the Daye Formation

Xingshan, Hubei

Oldhamia radiate

This study

Member III of the Daye Formation

Xingshan, Hubei

Oldhamia isp.

This study

Member III of the Daye Formation

Xingshan, Hubei

Member III of the Daye Formation

Xingshan, Hubei

Member III of the Daye Formation

Xingshan, Hubei

Member III of the Daye Formation Member III of the Daye Formation Member III of the Daye Formation

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Formation

MA

NU

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This study This study This study This study This study This study

This study

Palaeophycus tubularis

This study

Palaeophycus heberti

This study

Xingshan, Hubei

Paleodictyon isp.

This study

Guangji, Hubei

Megagrapton isp.

Yang et al.(1992, p. 4, pl. 1, fig. 5)

Guangji, Hubei

Phycodes isp.

Yang et al.(1992, p. 4, pl. 1, fig. 3)

Member III of the Daye Formation

Guangji, Hubei

Teichichnus isp.

Member III of the Daye Formation

Guangji, Hubei

Thalassinoides isp.

Yang et al.(1992, p. 4, pl. 1, fig. 4)

Member IV of the Daye Formation

Xingshan, Hubei

Arenicolites isp.

This study

Member IV of the Daye Formation

Xingshan, Hubei

Diplocraterion parallelum

This study

Member IV of the Daye Formation

Xingshan, Hubei

Planolites montanus

This study

Member IV of the Daye Formation

Xingshan, Hubei

Palaeophycus tubularis

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Arenicolites isp.

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Circulichnis montanus

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Didymaulichnus lyelli

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Diplocraterion isp.

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Palaeophycus tubularis

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Planolites montanus

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Skolithos isp.

This study

Member II of the Jialingjiang Formation

Xingshan, Hubei

Thalassinoides isp.

This study

Member II of the Jialingjiang Formation

Huangshi, Hubei

Helminthopsis abeli

Ma et al.(2008,fig.3K)

Member II of the Jialingjiang Formation

Huangshi, Hubei

Palaeophycus tubularis

Ma et al.(2008,fig.3D)

Member II of the Jialingjiang Formation

Huangshi, Hubei

Palaeophycus cf. heberti

Ma et al.(2008,fig.3E)

Member II of the Jialingjiang Formation

Huangshi, Hubei

Phycodes palmatus

Ma et al.(2008,fig.3A, B)

Member II of the Jialingjiang Formation

Huangshi, Hubei

Planolites beverleyensis

Member II of the Jialingjiang Formation

Huangshi, Hubei

Planolites isp.

Member II of the Jialingjiang Formation

Huangshi, Hubei

Rhizocorallium cf. jenense

Ma et al.(2008,fig.3C)

Member II of the Jialingjiang Formation

Huangshi, Hubei

Scalarituba cf. missouriensis

Ma et al.(2008,fig.3H,N)

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Planolites montanus

Planolites montanus

Yang et al.(1992, p. 4, pl. 1, fig. 2)

Ma et al.(2008,fig.3F, M) Planolites montanus

Ma et al.(2008,fig.3G, I, L)

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ACCEPTED MANUSCRIPT Member II of the Jialingjiang Formation

Guangji, Hubei

Chondrites isp.

Palaeophycus tubularis

Yang et al.(1992, p. 5, pl. 1, fig. 6)

Member II of the Jialingjiang Formation

Guangji, Hubei

Palaeophycus isp.

Member II of the Jialingjiang Formation

Guangji, Hubei

Protopaleodictyon isp.

Member III of the Jialingjiang Formation

Guangji, Hubei

Asteriacites isp.

Yang et al.(1992, p. 7, pl. 1, fig. 7)

Member III of the Jialingjiang Formation

Guangji, Hubei

Helminthopsis isp.

Yang et al.(1992, p. 6, pl. 1, fig. 8)

Member III of the Jialingjiang Formation

Guangji, Hubei

Phycodes isp.

Member III of the Jialingjiang Formation

Guangji, Hubei

Hormosiroidea isp.

Member III of the Jialingjiang Formation

Guangji, Hubei

Planolites isp.

Member III of the Jialingjiang Formation

Guangji, Hubei

Rhizocorallium isp.

Yang et al.(1992, p.5) Yang et al.(1992, p.4–5)

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Paleodictyon isp.

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Yang et al.(1992, p. 6, pl. 1, fig. 3)

Yang et al.(1992, p. 6, pl. 1, fig. 1) Yang et al.(1992, p. 7, pl. 1, fig. 10)

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D

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NU

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Diplocraterion isp.

Yang et al.(1992, p. 8, pl. 1, fig. 9)

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Figure 1

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Figure 2

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Figure 3

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Figure 7

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Figure 8

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Highlights

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 We report 17 ichnospecies from the Lower Triassic of the Three Gorges area.

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 Smithian trace fossils are characterized by simple, small, horizontal burrows.  Spathian trace fossils are diverse and abundant with complex burrow

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networks.

 Recovery pattern of ichnofossils is likely structured by the changes of refuge

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zone.

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