the late triassic black shales of the guanling ... - Wiley Online Library

[Palaeontology, Vol. 51, Part 1, 2008, pp. 27–61]

THE LATE TRIASSIC BLACK SHALES OF THE GUANLING AREA, GUIZHOU PROVINCE, SOUTH-WEST CHINA: A UNIQUE MARINE REPTILE ¨ TTE AND PELAGIC CRINOID FOSSIL LAGERSTA by WANG XIAOFENG*, GERHARD H. BACHMANN , HANS HAGDORNà, P. MARTIN SANDER§, GILLES CUNY–, CHEN XIAOHONG*, WANG CHUANSHANG*, CHEN LIDE*, CHENG LONG*, MENG FANSONG* and XU GUANGHONG* *Yichang Institute of Geology and Mineral Resources, Yichang, Hubei 443003, China; e-mail: [email protected] Martin-Luther-Universita¨t Halle-Wittenberg, Institut fu¨r Geologische Wissenschaften, Von-Seckendorff-Platz 3, D-06120 Halle (Saale), Germany; e-mail: [email protected] àMuschelkalkmuseum, D-74653 Ingelfingen, Germany; e-mail: [email protected] §Universita¨t Bonn, Institut fu¨r Pala¨ontologie, Nussallee 8, D-53115 Bonn, Germany; e-mail: [email protected] (corresponding author) –Geologisk Museum, Øster Voldgade 5–7, DK-1350 København K, Denmark; e-mail: [email protected] Typescript received 13 January 2006; accepted in revised form 18 December 2006

hundreds of skeletons, is numerically dominated by three species of ichthyosaurs and four species of thalattosaurs. The thalattosaurs fill a palaeobiogeographic gap between the Alpine thalattosaur faunas and those from western North America. Two species of placodonts are rare finds. As for the thalattosaurs, the placodont occurrences greatly expand the geographic range of the group because placodonts have been known previously only from the Mediterranean region, the Alps and the Muschelkalk Basin. The unique abundance of thalattosaurs contrasts with a scarcity of fishes. Although we suggest that the fauna is authochthonous and inhabited surface waters, it must have represented an unusual ecosystem. However, the possibility remains that both the marine reptiles and the Traumatocrinus colonies were concentrated in the region by currents and do not reflect the biocoenosis.

Abstract: Black shales of the lower member of the Carnian Xiaowa Formation (previously known as the Wayao Member of the Falang Formation or as the Wayao Formation) in the Guanling area, Guizhou Province, south-west China, are yielding a rich marine reptile fauna and exceptional remains of pelagic crinoids. The black shales represent deposition on the drowned southern margin of the Yangtze Platform during a Maximum Flooding Interval. The relatively reduced sedimentation rates led to the formation of the Lagersta¨tte through the accumulation of fossils in the anoxic bottom sediments over a prolonged period of time. Invertebrate fossils represent almost exclusively pelagic forms, such as a diverse ammonite fauna and halobiid bivalves. The spectacular finds of colonies of large (stem lengths > 11 m) crinoids of the genus Traumatocrinus attached to driftwood prove that this crinoid was the first to evolve a pseudoplanktonic life style. The other crinoid is the planktonic roveacrinid Osteocrinus. The marine reptile fauna, represented by probably

Key words: Carnian, south-west China, Guanling area, black shales, Lagersta¨tte, marine reptiles, pelagic crinoids.

The black shales of the Xiaowa Formation, Guanling area, Guizhou Province, south-west China, are a unique Late Triassic (Carnian) Lagersta¨tte (Wang X. et al., 2000b, 2002a, b, 2003a, b). Nowhere else are so many marine reptile skeletons and pelagic crinoids known in such perfect preservation from this time. Comparable preservation and abundance is known only from the Anisian ⁄ Ladinian (Middle Triassic) Besano Formation in the Southern Alps and the Toarcian (late Early Jurassic) Posidonia Shale of Holzmaden, Germany.

Thus, the Xiaowa Formation Lagersta¨tte is very important for understanding the evolution and palaeobiogeography of marine reptiles and crinoids as well as for the sedimentary and taphonomic processes in black shale konservat Lagersta¨tten in general. The geological study of the Guanling area and the earliest discoveries of fossils in the Carnian black shales go back to Hsu¨ (1940) and Hsu¨ and Chen (1944). They collected crinoid specimens that were later described by Mu (1949). This important paper, which included a revision

ª The Palaeontological Association

doi: 10.1111/j.1475-4983.2007.00735.x

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of the genus Traumatocrinus, has long remained almost unnoticed by the scientific community. Mainly since the 1990s, numerous well-preserved articulated skeletons of marine reptiles and complete crinoids have been excavated by local farmers. This initiated extensive research on the fossil vertebrate and invertebrate groups of the Xiaowa and underlying formations (‘Guanling biota’ of the Chinese literature). During the past few years, a research and excavation campaign organized and conducted by WX and his colleagues, in co-operation with the local fossil conservation authorities, has yielded a plethora of articulated marine reptile skeletons and complete crinoid colonies. The rich fossil finds are exhibited in various regional museums in Guanling County and, especially, in the Yichang Institute of Geology and Mineral Resources (YIGMR) in Yichang, Hubei Province. Since 2000, several preliminary reports and research papers on the biota from the Xiaowa and underlying formations have been published (Li J. et al. 2000; Li J. 2001; Wang L. 2000; Wang L. et al. 2001; Wang Y. et al. 2000; Wang X. et al. 2002a, b, 2003a), including a special issue of the Geological Bulletin of China covering the ‘Guanling biota’ (Wang X. et al. 2003b) and a book in Chinese for a general readership (Wang X. et al. 2004). Many of the marine reptile and crinoid specimens, however, are still unprepared or under preparation and have not yet been studied in detail. Since 2002 our Chinese-European research group has been re-investigating the quarries and fossil collections. In the meantime the Chinese team has continued their largescale scientific excavations. A ‘field window’ showing the fossils of the Guanling Lagersta¨tte in their original position on the black shale bedding planes was opened up at Wolonggang (Sleeping Dragon Hill) near the village of Xiaowa by the Guanling research group (Wang X. et al. 2003a, 2004) (Text-figs 1–2). It has now become the ‘National Geological Park for the Guanling Fossil Group’ (NGPGFG). This paper aims at presenting a summary and preliminary synthesis of our joint research, focusing on the stratigraphy, sedimentary environment, fossil content, palaeoecology and taphonomy of the Lagersta¨tte and its implications for the palaeobiogeography and evolutionary biology of the most important vertebrate and invertebrate groups. All specimens mentioned are deposited in the YIGMR, the NGPGFG and the collections of the government of Guanling County (GNG). Many other specimens from the black shales of the Carnian Xiaowa Formation have reached western fossil markets or been acquired by the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) in Beijing and other Chinese public collections. The black shales and their fossils should not be confused with similar Anisian (e.g. Cheng et al. 2004) and Ladinian (Rieppel et al. 2003; Li C. et al. 2004)

T E X T - F I G . 1 . Tectonic map of South China showing the Yangtze Platform and the Nanpangjiang Basin in Late Triassic times. Arrow indicates study area near Guanling; modified from Sun et al. (1989) and Lehrmann et al. (2005).

deposits in south-west Guizhou Province, which have also yielded marine reptile faunas.

GEOGRAPHICAL AND GEOLOGICAL SETTING The Carnian Xiaowa Formation Lagersta¨tte is located in the border area between Yunnan and Guizhou provinces, c. 160 km south-west of Guiyang, the capital of Guizhou Province, and 40 km south-west of Guanling, seat of Guanling County, and covers an area of at least 200 km2 (Text-fig. 2). Stratigraphically the formation occurs in the upper part of a more than 1000-m-thick succession of Anisian–Carnian carbonates and marls (Text-fig. 3; Table 1). The Lower Member of the Xiaowa Formation, hereafter usually referred to as the ‘Lower Xiaowa Formation’, is exposed in a large number of quarries that are operated by local farmers for fossil collecting. The best exposures, where most discoveries have been made, are located in Guanling County at Wolonggang (Sleeping Dragon Hill) near the village of Xiaowa, along the small River Shiao He downstream of the village of Bamaolin near the town of Xinpu, at the villages of Maowa, Hu-

¨ TTE WANG ET AL.: UNIQUE MARINE REPTILE AND PELAGIC CRINOID FOSSIL LAGERSTA

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A

B

T E X T - F I G . 2 . A, geography of the study area showing important quarries (black triangles) in the Lower Xiaowa Formation containing the Lagersta¨tte. The location of Wolonggang quarry in the NGPGFG is marked by the triangle to the south-west of the village of Xiaowa. B, geological map of the study area.

angtutang and Zhuganpo, at the village of Baiyan near the town of Gangwu, and at the village of Liangshui in adjacent Qinglong County (Text-fig. 2). Structurally, the Lagersta¨tte is situated in the centre of the Xinpu Synclinorium, a large, rather complicated north-west–southeast-striking synform exposing Middle–Upper Triassic strata (Wang X. et al. 2003b).

PALAEOGEOGRAPHICAL SETTING In Triassic times the South China Block (continent) was situated between Panthalassa to the east and the PalaeoTethys to the west. It drifted from a position just north of the equator in the Early Triassic to c. 30N during the Late Triassic (Scotese 2001). From the mid-Triassic

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onwards, its northern margin collided with the North China Block (Huang K. and Opdyke 1996) and its southwestern margin with the Simao Block and the Indochina Block (Metcalfe 1998, 1999; Wang X. et al. 2000a; Scotese 2001; Text-fig. 1). Triassic facies and palaeogeography of the south-western part of the South China Block were summarized recently by Liu and Xu X. (1994) and Lehrmann et al. (2005). From the Early to the Middle Triassic (Ladinian) most of the Guizhou area was located in the south-western part of the vast Yangtze Platform (GBGMR 1987), which stretched across much of the South China Block (Text-fig. 1). In the study area a succession, c. 1000 m thick, of shallow water carbonates with evaporites (Guanling, Yangliujing, Lower and Middle Zhuganpo formations) developed in Middle Triassic times. The southwestern corner of Guizhou, however, was part of the Nanpanjiang Basin (Sun S. et al. 1989; Wu 2003; Lehrmann et al. 2005), adjacent to the Yangtze Platform. This basin developed in the border region between Yunnan, Guizhou and Guangxi provinces and adjacent areas in northern Vietnam. Sedimentary and biostratigraphic evidence suggests that the study area was at the boundary between the Yangtze Platform and the Nanpanjiang Basin, along a north-east–south-west trend running from Qingyan near Guiyang via Anshun and Zhenning to Zhenfeng. The Nanpanjiang Basin has been interpreted in different ways. In the Middle and Late Triassic it can be regarded as having been a foreland basin because the South China Block was involved in convergence and collision along its northern, western and southern sides associated with the Indosinian orogeny (GBGMR 1987; Hsu¨ et al. 1988; Huang and Opdyke 1996; Xu et al. 1996; Metcalfe 1999; Wang X. et al. 2000a; Wu 2003; Lehrmann et al. 2005) (Text-fig. 1). During Middle and early Late Triassic times much of the basin was filled with thick, deep-water, flysch-like turbiditic sands. The Guanling area, located at the south-west platform margin adjacent to the north-west margin of the original Nanpanjiang Basin, was drowned and transformed into a south-west– north-east-running trough that subsided rapidly. It was surrounded by emergent areas on three sides and opened only towards the south-west. The trough contains a succession, c. 100 m thick, comprising the Carnian Upper Zhuganpo Formation and, especially, the Lower Xiaowa Formation with black limestones and shales bearing the fossil Lagersta¨tte described here (Yang and Zhang 2000). In Guanling and adjacent areas the Lower Xiaowa Formation is overlain by some 200 m of increasingly silty and sandy limestones of the Middle and Upper (members of the) Xiaowa Formation as well as several hundred metres of sand- and mudstones of the Laishike Formation.

Later, in Norian times, the South China Block was uplifted and deformed by the intra-continental collision caused by the Indosinian Orogeny (Text-fig. 1). Latest Carnian and early Norian marine deposits are absent on the south-western part of the Yangtze Platform and the original Nanpanjiang Basin. From late Norian to Rhaetian times, littoral swamp deposits were formed in isolated intra-continental coal-bearing basins.

STRATIGRAPHY The Triassic in the Guanling area has been described by several authors (Hsu¨ 1940; Hsu¨ and Chen 1944; Yin 1962; Wang Y. et al. 1963; Chen et al. 1979; GBGMR 1987; Yang S. et al. 1995; Dong et al. 1997) and is considered to be a key area for Triassic stratigraphy in South China. Table 1 gives an overview of the different lithostratigraphical subdivisions and nomenclature as well as their chronostratigraphical correlation. In addition to the inter-

T E X T - F I G . 3 . Stratigraphy and conodont zonation of the Middle and Upper Triassic in south-western Guizhou. Beds labelled GFL denote the upper unit of the Lower Xiaowa Formation containing the Lagersta¨tte.


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T A B L E 1 . History of lithostratigraphical and chronostratigraphical subdivisions of the Triassic in south-western Guizhou, modified from Chen and Cuny (2003). Grey bar, Lower Xiaowa Formation containing the Lagersta¨tte.

national chronostratigraphy, the Chinese Committee of Stratigraphy has proposed a regional chronostratigraphy for China (Chinese Committee of Stratigraphy, CCS, 2002). According to Wang X. et al. (2002b) the Middle and Upper Triassic comprises the following formations in ascending order: Guanling, Yangliujing, Zhuganpo, Xiaowa (containing the fossil Lagersta¨tte) and Laishike (Text-fig. 3). The stratotype sections of these formations are located along Highway 320 from the town of Yongning to the village of Huangtutang, Guanling County (Hsu¨ 1940; Hsu¨ and Chen 1944; Yin 1962; Wang Y. et al. 1963; Chen et al. 1979; GBGMR 1987) (Text-fig. 2A).

and layers with isolated Dadocrinus ossicles. The Guanling Formation yields the bivalves Costatoria goldfussi mansuyi, Leptochondria paradoxica and Pleuromya elongata, and the ammonoids Progonoceratites sp. and P. nanjiangensis. Conodonts of the Neogondolella constricta, Nicoraella germanica and Nicoraella kockeli zones are found in its upper part (Shizishan Member), indicating an early– middle Anisian age (Yang et al. 1995). The sedimentary environment of the formation is interpreted as shallow marine and partly hypersaline.

Yangliujing Formation Guanling Formation Wang Y. et al. (1963) restricted the original Guanling Series (Hsu¨ 1940; Hsu¨ and Chen 1944), or Guanling Formation (Chinese Commission of Geology and Geological Institute, CCGGI, 1956), to its lower part and renamed the dolomites in its upper part the Yangliujing Formation (Table 1). The stratotype section of the revised Guanling Formation is at a roadcut about 1 km south-west of the town of Yongning (GBGMR 1987; Dong et al. 1997) where it is c. 550 m thick and consists mainly of wellbedded grey micritic limestones, dolomitic limestones and dolomites with a bed, 2–3 cm thick, of yellow-green tuff at the base. Bioturbation, thin tempestites and ophiuroid ossicle layers were found in the middle and upper parts. Dolomite breccia, vuggy dolomites, residual marls and rauhwackes occur higher up, indicating evaporates that were dissolved at the surface. The uppermost part is characterized by bedded limestones resembling the German Muschelkalk, with some Hoernesia sp. on bedding-planes

The type section of the Yangliujing Formation is at a roadcut in Yangliujing valley, 2 km south-west of the town of Yongning (Wang Y. et al. 1963; Chen et al. 1979; Dong et al. 1997). It attains a total thickness of some 450 m and consists mainly of thick-bedded grey dolomites and dolomite breccias with gypsum crystals. Fossils are quite rare. Thick-bedded limestones with bivalves and isolated ossicles of Dadocrinus and Holocrinus and conodonts of the Metapolygnathus foliata inclinata Zone were found in the uppermost few metres, suggesting a Ladinian age. This formation has also yielded rare teeth of hybodont sharks (Polyacrodus contrarius and ?Parvodus). Most of it is regarded as a hypersaline deposit.

Zhuganpo Formation The stratotype of the Zhuganpo Formation is located between the villages of Zhuganpo and Wayao, about 4 km south-west of the town of Yongning, and attains a thick-

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ness of c. 75 m. Historically it corresponds to the lower part of the Halobia Beds or Falang Beds of Hsu¨ (1940), Hsu¨ and Chen (1944) and later authors (Chen et al. 1979; GBGMR 1987; Table 1). This formation is widespread in the Wayao, Bamaolin, Xiaowa, Xinpu and Gangwu areas together with the overlying Xiaowa Formation. The Lower and Middle (members of the) Zhuganpo Formation in the stratotype section consist mainly of medium- to thick-bedded, grey micritic limestones intercalated with dolomites and dolomitic limestones in the lower part and thick-bedded bioclastic limestones in the uppermost part. The limestones are characterized by distinctively undulating bedding planes. The conodont Metapolygnathus polygnathiformis has recently been found in the Xiaowa section 3.38 m above the base of the formation (Sun et al. 2005). About 25–50 m above its base, the Zhuganpo Formation contains a few fossils that include the ammonoids Langdaiceras sp. and Xenoprotrachyceras cf. primum, the nautiloids Michelinoceras sp. and Syringonautilus sp. (Xu et al. 2003), the bivalves Daonella sp. and Halobia sp., and the brachiopod Ninglangothyris subcircularis. The Upper (member of the) Zhuganpo Formation consists mainly of sets of light grey, thin-bedded micritic limestones with typically undulating bedding planes. The highest thin-bedded limestone interval is taken as the top of the formation. The individual beds are separated by thin, dark grey marlstones, the bedsets by dm-thick fissile, dark grey marlstones. The limestones are relatively rich in fossils including Halobia sp., articulated brachiopods, isolated Traumatocrinus ossicles, diverse elasmobranch ichthyoliths (Chen and Cuny 2003; see below) and holothurian sclerites (Chen H. et al. 2003). Disarticulated Traumatocrinus remains occur near the top of the formation (Hagdorn et al. 2005). In addition, Yang and Zhao (1998) reported Encrinus liliiformis from the formation. The limestones of the Upper Zhuganpo Formation are usually devoid of marine reptiles. Some show bioturbation with burrowing traces resembling Balanoglossites, an irregularly branching burrow 1–2 cm thick and up to 15 cm deep that commonly occurs in the Middle Triassic of Central Europe. The marlstones contain Daonella sp., Halobia sp., the ammonoids Protrachyceras deprati, P. ladinum and P. yongningense, and the brachiopod Laballa scabrula. The conodont Metapolygnathus polygnathiformis is found in the lowest Upper Zhuganpo Formation (Sun et al. 2005), together with Metapolygnathus navicula, M. foliata inclinata and M. parafoliata (Chen and Wang, 2002; Wang X. et al. 2003a). Metapolygnathus nodosus occurs in the uppermost Zhuganpo Formation together with the above-mentioned conodonts of the M. polygnathiformis Zone, and ranges up into the Xiaowa Formation (Sun et al. 2005).

The limestones and dolomites of the Lower and Middle Zhuganpo Formation are interpreted as relatively shallowwater platform sediments, whereas the micrites of the Upper Zhuganpo Formation are interpreted as deposits of relatively deep, quiet water below the storm-wave base, although benthic brachiopods and bioturbation indicate generally oxygenated bottom waters. The intervals with thin-bedded micritic limestones and dark grey marlstones in the Upper Zhuganpo Formation are thought to represent times of reduced oxygenation and carbonate productivity.

Xiaowa Formation The stratotype of the Xiaowa Formation is along a small road from Xiaowa to Huangtutang villages, about 7 km west-south-west of the town of Yongning. It is some 180 m thick and can be subdivided into three members. The name Xiaowa Formation was suggested by Wang X. et al. (2002b) for the original ‘Wayao Member’ of GBGMR (1987) (Table 1) and ‘Wayao Formation’ of Yang et al. (1995) because the name ‘Wayao’ is preoccupied by a Palaeogene unit on Hainan Island, South China (Zheng et al. 1999, pp. 104–105) and the stratotype at the Wayao section is poorly exposed. Historically, the Xiaowa Formation corresponds to the middle part of the Halobia Beds or Falang Beds (Table 1). Lower Member of Xiaowa Formation. The Lower Member of the Xiaowa Formation has a total thickness of 12 m. It can be subdivided into two units (Text-figs 3–4). The lower, 5-m-thick unit consists of medium- to thick-bedded, grey bioclastic micrites with yellow-green shale interbeds. The micrites commonly have sharp bases and grade upwards into dm-thick grey to dark grey laminated marlstones (Text-fig. 5A). The limestones are mostly biopelmicrites or biomicrites containing rare marine reptiles. Isolated Traumatocrinus ossicles and the bivalves Halobia subcomata and Daonella bifurcata are abundant. The conodonts Metapolygnathus nodosus and M. polygnathiformis are relatively rare. Ammonoids include Protrachyceras deprati, P. costulatum, P. cf. douvillei and Clionites cf. zeilleri, which are mainly found in the lowermost 3 m of the formation. Other fossils found are ostracods and diverse elasmobranch ichthyoliths (Chen and Cuny 2003). The fossil content suggests an early Carnian age, although the conodont Metapolygnathus nodosus implies late Carnian. The depositional environment of the limestones was quiet and deeper than during deposition of the Zhuganpo Formation, but still with generally oxygenated bottom waters. The interbedded, dark grey, laminated marlstones are thought to represent times of more reduced oxygenation and carbonate productivity.


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T E X T - F I G . 4 . Lithological log of Wolonggang quarry section near the village of Xiaowa, with natural gamma radiation in counts per second (cps).

The upper unit is 7 m thick and consists of greyish to black, thin- to medium-bedded laminated mudstone with argillaceous limestone interbeds containing crinoids (Text-fig. 5B). It begins with c. 1 m of thick-bedded, dark grey, marly, laminated micritic limestones. Most of the well-preserved articulated marine vertebrate and Traumatocrinus skeletons and the driftwood logs described below occur on the bedding planes. The next 2 m are characterized by conspicuously dark grey to black shaly marlstones and marly shales and are rich in Halobia and Daonella, and the ammonoids Trachyceras multituberculatum, T. cf. aon, Paratrachyceras cf. hofmanni, P. douvillei and Hauerites cf. himalayanus. The uppermost 3.5–4 m of the upper unit of the Lower Member consist of dark grey laminated marlstones with the conodonts Metapolygnathus nodosus and M. polygnathiformis. Well-weathered quarry faces in the Bamaolin section indicate dm-thick rhythms of more limy and more marly beds. Thin-

sections of the limestones show marly biomicrites and biopelmicrites with Halobia and ostracods. The limestones contain some quartz silt in the uppermost part. The upper unit represents anoxic conditions with much reduced carbonate productivity. The black shaly interval seems to represent the greatest water depth. The quartz silt is thought to be wind-blown. The natural gamma radiation of the Lower Xiaowa Formation was measured at the Wolonggang section with a hand-held scintillometer and a gamma ray log was constructed (cf. Aigner et al. 1995). The readings are between some 60 and 120 counts per second (cps), with the highest values of about 90–120 cps in the black shales in the lower part of the upper unit (Text-fig. 4). The radioactivity is thought to be mainly owing to the presence of the isotope 40K in the clay minerals. Wang X. et al. (2003a) and Chen X. et al. (2003) determined that the content of organic matter and the

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Xiaowa Fm middle member lower member (upper unit)

hut

wall

shaly black marlstones

A

B

T E X T - F I G . 5 . A, Lower Member of the Xiaowa Formation (lower unit), Bamaolin section (well-weathered natural cliff of Xiao He River). B, Lower Member of the Xiaowa Formation (upper unit) and boundary with Middle Member, Wolonggang quarry section near the village of Xiaowa. An exposed bedding plane with reptiles, driftwood and Traumatocrinus colonies is visible in the foreground. This is overlain by a conspicuous black shale interval. One of the huts built to protect in situ specimens is on the righthand side. Photograph taken in August 2004. Scale bars represent 0.5 m.

elements Fe, Mn, Sr, Ba, Co, Rb, Ga and B. Corg is between some 0.5 per cent to < 0.1 per cent without an increase in the black shale interval. From the petroleum geology point of view this would correspond to a lean source rock (Tissot and Welte 1984). Middle and Upper members of the Xiaowa Formation. The Middle Member of the Xiaowa Formation is separated by a sharp boundary from the Lower Member (Text-figs 4, 5B). It is c. 140 m thick and consists of medium- to thick-bedded, laminated, grey limestones interbedded with medium to thick grey laminated marlstones. The lamination is mainly owing to thin layers of quartz silt. In the whole of the study area the basal part is characterized by conspicuously deformed strata, locally up to 20 m thick (Text-fig. 6). The deformation seems to have been caused by widespread synsedimentary slumping, which is often associated with large listric slump planes at the base of the Middle Member. Additional dm-thick deformed beds occur higher up in the section (Text-fig. 7A).

The Upper Member of the Xiaowa Formation attains a thickness of c. 30 m. It consists of laminated limestones without significant marlstone interbeds that up-section incorporate increasing amounts of silt and fine sand (Text-fig. 7B). The laminated limestones contain some dm-thick deformed intervals. The laminae consist of mmthick layers of quartz silt commonly with sharp bases and tops. The silty laminae are sometimes indistinctly graded and occasionally rippled. Halobia sp. and the ammonoids Buchites cf. aldrovandii, Protrachyceras sp., Sirenites cf. senticosus and Trachyceras sp. are found in the uppermost part of the Middle Member in the Xiaowa section (Xu et al. 2003), suggesting a late early Carnian age. Fossils, such as Halobia sp. and ammonoids, are rare in the Upper Member. There is no bioturbation. The laminated silty limestones are considered to be deposits of relatively deep water. The quartz silt is thought to be wind-blown in part or associated with distal turbidites and been reworked by weak currents. Up-section


A

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B

middle member lower member

Xiaowa Fm T E X T - F I G . 6 . Strongly deformed (slumped) base of Middle Member of the Xiaowa Formation. A, Bamaolin section, exposure in a natural cliff of Xiao He River; scale bar represents 1 m. B, Bamaolin section, exposure in a fossil quarry; person for scale.

A

B

T E X T - F I G . 7 . A, Middle Member of Xiaowa Formation with deformed (slumped) interval 0.6 m thick, Xiaowa section (roadcut); scale bar represents 0.5 m. B, Upper Member of the Xiaowa Formation, basal part, Xiaowa section (roadcut); length of hammer handle, 0.3 m.

there is a general trend of shallowing and a higher input of quartz silt and fine sand. The reason for this input is possibly related to the early Indosinian orogenic movements during the Middle Triassic that affected much of the Yangtze Platform (e.g. Huang and Opdyke 1996; Metcalfe 1999). These movements may also have caused seismic activity that triggered the slumps. A sudden input of clastics in early Carnian (Cordevolian) times is characteristic for large parts of the Tethys and adjacent epicontinental areas and is

referred to as the ‘Reingraben turning point’ (Schlager and Schoellnberger 1974; Brandner 1984).

Laishike Formation This formation, named by Chen et al. (1979; see also GBGMR 1987; Yang et al. 1995), is equivalent to the upper part of the Falang Formation as revised by Wang

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Y. et al. (1963; Table 1). The stratotype is located at the village of Laishike, 4 km south-west of Longchang, Zhenfeng County, Guizhou Province, and is about 700 m thick. In the Guanling area, only the lowest part of the formation is exposed along the road from Xiaowa to Huangtutang. The boundary with the underlying Xiaowa Formation is transitional. The Laishike Formation consists of thin- and mediumto thick-bedded, laminated grey silty and sandy limestones with interbeds of silty and sandy mudstones. The lamination is owing to a high content of quartz silt and fine sand. There are some dm-thick deformed intervals. The bivalves Halobia rugosoides and the ammonoids Trachyceras sp. and Protrachyceras sp. were found in the Duanqiao section of the Guanling area by GBGMR (1987). The formation is regarded here as late early Carnian in age based on the fossils found in the underlying strata at the Duanqiao section. There seems to be a trend towards a higher input of quartz silt and sand owing to the land around the trough continuing to rise. The formation is generally regarded as a flysch-like deposit (Lehrmann et al. 2005).

CYCLICITY AND SEQUENCE STRATIGRAPHY The Zhuganpo Formation as a whole shows a deepeningupward trend. Its uppermost thick-bedded limestones and dark grey marlstones are cyclic and grade, without a significant break, into the Lower Xiaowa Formation. Its lower unit, cyclic medium- to thick-bedded, grey limestones and dm-thick grey to dark grey laminated marlstones, indicates further deepening, well below the stormwave base. The interbedded laminated dark grey marlstones in both Upper Zhuganpo and Lower Xiaowa formations represent cyclical variations of oxygenation and carbonate productivity. The black shaly interval in the Lower Xiaowa Formation seems to correspond to the greatest water depth, anoxic conditions and much reduced carbonate productivity. The upper part of this formation reflects cyclical increases in carbonate productivity. The Middle and Upper Xiaowa Formation, as well as the basal Laishike Formation, with their predominantly silty limestones seem to have been deposited in increasingly shallow water. In terms of sequence stratigraphy, the Zhuganpo Formation and the lower unit of the Lower Member of the Xiaowa Formation comprise a Transgressive Systems Tract (TST). The black shales are a good candidate for the maximum flooding interval. Beds rich in crinoids and marine reptiles as well as high gamma-ray readings in and around this interval suggest relatively reduced sedimentation rates and condensation. The uppermost part of

the Lower Xiaowa Formation consists of dm-thick, cyclically bedded, laminated marly limestones and marlstones. This part, as well as the Middle and Upper Xiaowa Formation, represents a Highstand Systems Tract (HST). This interpretation as one sequence is in agreement with Wei et al. (1996); it corresponds to their Triassic Transgressive-Regressive Sequence 7 in the GuizhouGuangxi region. However, the cyclicity and sequence stratigraphy of the Xiaowa Formation needs additional work in detail and on a regional scale.

FOSSIL CONTENT Brachiopods Triassic brachiopods in the Guanling area are relatively rare and have not been studied before. Zeng (in Wang X. et al. 2004) first reported the brachiopods as part of the discussion of benthic palaeocommunities. Re-study shows that they are most common in the Middle Zhuganpo Formation, and consist of Pseudokoninckina xinpuensis, Ninglangothyris subcircularis, Sanqiaothyris sp. nov., ?Aulacothyris sp. nov. 1 and ?Aulacothyris sp. nov. 2. In the Upper Zhuganpo Formation, we have found Laballa scabrula, Laballa sp. nov., Norella sp. nov. 1 and Norella sp. nov. 2 (Zeng 2006). In the Lower Xiaowa Formation only two species, Similingula cf. lipoldi and ?Crania sp., have been found so far; these are associated with fossil drift wood, suggesting that they are allochthonous. Pseudokoninckina xinpuensis is a new genus and species recently described by Zeng (2006); the other new species will be formally described elsewhere in due course.

Bivalves The Guanling fossil Lagersta¨tte contains many bivalves. The fauna is dominated by the thin-shelled Halobia and Daonella. Most bivalves occur in the Lower Xiaowa Formation, but a few are found in the underlying Zhuganpo Formation and overlying Middle and Upper Xiaowa Formation. The main species include Daonella bifurcata, D. indica, Halobia brachyotis, H. kui, H. planicosta, H. rugosoides and H. subcomata. Additionally, rare Asoella sp., Angustella sp., Krumbeckiella sp. and Plagiostoma sp. occur (Li C. et al. 2004). The Halobia subcomata-Daonella bifurcata Assemblage Zone, restricted to the Lower Xiaowa Formation, is similar to the H. subcomata-D. varifurcata Assemblage Zone reported by Chen et al. (1992) from Guangxi, and considered by them to be latest Ladinian in age. It is indicative of a deep-water depositional environment. However, the occurrence of some characteristic Late Triassic (Carnian) bivalves, e.g. Angustella sp. and


Krumbeckiella sp., together with conodonts of the Metapolygnathus nodosus Zone and ammonoids of the Protrachyceras costulatum and Trachyceras multituberculatum zones in the Lower Xiaowa Formation, suggest that the Halobia subcomata-Daonella bifurcata Assemblage Zone is of early Carnian age and slightly younger than the latest Ladinian beds with Halobia lommeli and D. indica in the western Tethys (Yin H. 2003).

Ammonoids In the sections at Bamaolin, Xiaowa, Wolonggang and Zhuganpo, ammonoid abundance and diversity is relatively low in the Zhuganpo Formation and lowermost Xiaowa Formation. Most ammonoids occur in the upper (black shale) part of the Lower Xiaowa Formation together with marine reptiles, crinoid colonies and bivalves. Ammonoid preservation is not very good because the shells are strongly compressed. A gradual up-section increase in abundance and diversity is interpreted as a general trend towards increasing water depth. From the Zhugangpo to the Xiaowa formations, five ammonoid zones can be recognized: Xenoprotrachyceras primum, Protrachyceras deprati, Protrachyceras costulatum, Trachyceras multituberculatum and Sirenites cf. senticosus (Xu et al. 2003). The first two of these are within the Middle and Upper Zhugangpo Formation and can be correlated with the same zones proposed by Wang (1983) for south-west Guizhou, and with the Reitziites reitzii–Protrachyceras archelaus zones of the western Tethys (Brack et al. 2002), and are probably Ladinian in age. The P. costulatum Zone is defined by the FAD (first appearance datum) of the index fossil in the lowermost Xiaowa Formation, which is 2.5–2.8 m above the base of the formation and dominated by the index species together with P. cf. douvillei and Clionites cf. zeilleri. The Trachyceras multituberculatum Zone occurs in the middle and upper parts of the Lower Xiaowa Formation. It is defined by the FAD of the index fossil 5 m above the base of the formation in the Xiaowa and Bamaolin sections, together with marine reptiles, crinoid colonies and bivalves. Other ammonoids in this zone are Trachyceras cf. aon, T. sinensis, Paratrachyceras cf. hofmanni, Protrachyceras douvillei, P. ladinum, P. longiangense and Hauerites cf. himalayanus. The zone in the study area is likely to be equivalent to the earliest Carnian Trachyceras aon Zone in the Southern Alps, based on the occurrence of T. cf. aon and P. cf. hofmanni (Brack et al. 2002). The Sirenites cf. senticosus Zone, located in the lower to middle parts of the Middle Xiaowa Formation, is dominated by the index fossil. Associated ammonoids are Buchites cf. aldrovandii, Pro-

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trachyceras sp., Sibyllites cf. tenuispinosus, Sibyllites sp. and Trachyceras sp.

Echinoderms Articulated colonies of the large, pseudoplanktonic crinoid Traumatrocrinus are arguably the most conspicuous faunal element in the black shales of the Lower Xiaowa Formation. As a result of their usually benthic lifestyle, echinoderms in general, and crinoids in particular, are bound to well-oxygenated bottom water with nutrientrich currents. Hence, black shales would seem to be rather unlikely sources of echinoderms. However, among the most spectacular echinoderm Lagersta¨tten are the Sinemurian Black Ven Marls of the Dorset coast of southern England and the Toarcian Posidonienschiefer of southwest Germany, both of which have been studied for more than 200 years (Seilacher et al. 1968; Simms 1986, 1999; Wignall and Simms 1990; Hess 1999). The latter unit has yielded large, complete colonies of the pentacrinitid crinoids Pentacrinites and Seirocrinus still attached to driftwood. Seilacher et al. (1968) proposed a pseudoplanktonic lifestyle for Seirocrinus, which is now generally accepted. In connection with a colony of the shortstemmed Pentacrinites fossilis from southern England, Simms (1986) demonstrated how its raft gradually sank and finally covered the colony. A pseudoplanktonic lifestyle was originally suggested for Traumatocrinus from Guanling County by Hagdorn (1998) based on the overall morphological similarities with these Jurassic pentacrinitids. Pseudoplanktonic Traumatocrinus. Stems of this large crinoid were originally described by Dittmar (1866) from the Carnian Hallstatt Limestones of the Salzkammergut (Austria) as Porocrinus. They were renamed Traumatocrinus by Wo¨hrmann (1889) and later considered a junior synonym of Encrinus (Moore et al. 1978). Articulated specimens of Traumatocrinus, with their crowns beautifully preserved, have been collected in Guizhou Province before. They enabled Mu (1949) to give a full morphological description of this remarkable crinoid and to establish the family Traumatocrinidae, which he thought was related to the Encrinidae. Mu erected several new species, which do not substantially differ in their stem characters from the genotype. After Mu (1949), Kristan-Tollmann and Tollmann (1983) were the first authors to recognize the importance of Traumatocrinus and to discuss its world-wide stratigraphical and geographical occurrence on the basis of specimens from the town of Yongning and from Zhenfeng County (Guizhou Province). As it is not the aim of this paper to discuss systematics and taxonomy, the Guanling traumatocrinids are not attributed

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to any of the existing Traumatocrinus species, the number of which has increased during the last few years (Yu et al. 2000; Wang B. et al. 2002; Wang C. et al. 2003; Wang X. et al. 2004). Fragmentary specimens from Afghanistan and Iran caused some authors to include this derived crinoid within the Flexibilia (Klikushin 1983) or Cladida (Kristan-Tollmann 1991). Recently Mu’s (1949) ideas were revalidated based on complete specimens from Guanling: Traumatocrinus is a specialized offshoot of the Anisian ⁄ Ladinian Encrinidae, which is placed in its own family, Traumatocrinidae, together with its sister genus Vostocovacrinus, which differs in having uniserial arms (Hagdorn 1995, 1998, 2001) and is known from the boreal realm (Yeltysheva and Polyarnaya 1986). Inclusion of the Traumatocrinidaea in the Encrinida is also confirmed by Cassianocrinus and Zardinicrinus from the Upper Ladinian ⁄ Lower Carnian Cassian Formation of the Southern Alps; these are derived encrinids with some traumatocrinid characters (Hagdorn 2004). Recent research has focused on the palaeoecology, palaeobiogeography, evolutionary biology, functional morphology and taphonomy of Traumatocrinus in comparison with that of the Jurassic pseudoplanktonic crinoids Seirocrinus and Pentacrinites (Hagdorn et al. 2002, 2004, 2007; Seilacher and Hauff 2004). Its morphology, occurrence in black shales and Tethys-wide distribution (Kristan-Tollmann and Tollmann 1983) initially suggested a pseudoplanktonic lifestyle (Hagdorn 1998). This was confirmed by the Traumatocrinus colonies discovered by our working group (Text-figs 8, 9A–C), which are still attached to coalified driftwood with their attachment structures preserved (Wang C. et al. 2003; Wang X. et al. 2003a, b, 2004, 2006; Hagdorn et al. 2004, 2007). Traumatocrinus colonies have been found at several levels in the Lower Xiaowa Formation, 5–11 m above its base. On a bedding plane of c. 200 m2 at Wolonggang (NGPGFG), a large colony together with its 3.3-m-long driftwood log (Text-fig. 9A–C), a small colony with a 0.5m-long piece of wood, and two groups of additional individuals isolated from their raft, have been exposed together with marine reptile skeletons (Hagdorn et al. 2004; Wang X. et al. 2006). On another bedding plane c. 30 cm above this level, a colony attached to a 1.9-m-long log has been exposed. The Wolonggang exposure shows four additional colonies in cross section, some of them together with their coalified driftwood raft. The YIGMR collections hold complete colonies with stems exceeding 11 m in length and covering more than 15 m2, and numerous slabs with parts of colonies that were not completely recovered. According to these occurrences, Traumatocrinus colonies are certainly much more common in the Xiaowa Lagersta¨tte than Seirocrinus and Pentacrinites colonies in the Posidonia Shale.

Traumatocrinus was attached to its wooden substrate by countless cirrus-like rootlets originating from columnals of the distalmost part of the stem. The rootlets consist of barrel-shaped elements with multiradiate articulation facets (Text-fig. 10A–E). In the same way that ivy twines, these irregularly growing and anastomosing root cirri encrusted the surface of the log. Clusters of these structures must have tightly attached even large bundles of crinoids to their rafts. Sometimes single individuals and whole groups of crinoids were found embedded with their holdfasts but devoid of driftwood. These occurrences must have resulted from loss of fixation followed by sinking to the sea-floor. As in the Lower Jurassic black shales (Hess 1999; Simms 1999), the size of Traumatocrinus colonies depends on the size of their driftwood raft. Even very small pieces of wood may have been colonized by crinoids (Textfig. 9D). Obviously, the planktonic larvae did not select large drifting logs. However, the cargo of growing colonies made smaller rafts sink sooner and these preserve juvenile and semi-adult individuals that would otherwise have been excluded from the fossil record, at least as complete individuals in the black shales. In contrast to the Pentacrinites and Seirocrinus rafts, which may have carried different generations of crinoids (Hess 1999; Simms 1999), the Traumatocrinus colonies that we have studied consisted of individuals with crowns of the same size, indicating that each log was colonized by a single generation. Logs more than 3 m long were able to provide lift to large Traumatocrinus colonies of more than 150 individuals. Traumatocrinus is among the largest and most spectacular of invertebrate animals, surpassed only by the Toarcian Seirocrinus, which attained stem lengths of 20 m (Hess 1999). The distribution of the crinoids on the log surfaces in the Lower Xiaowa Formation clearly shows that Traumatocrinus larvae preferred to attach themselves to the ends of the logs (Text-figs 9A–C, 11). From the variation of stem lengths within a single colony, we conclude that the animals positioned their crowns at different water depths for more efficient and complete filtering of the water column (Text-fig. 11; see also Hagdorn et al. 2004). For such intracolonial competition and effective plankton filtering, individual animals must have been able to accelerate the growth of their stems. The limited drifting time of a log must have required rapid growth and early reproduction by large numbers of planktotrophic larvae that settled on any suitable drifting substrate. On the other hand, abundant driftwood in the black shales without crinoids indicates that the chances of Traumatocrinus larvae meeting a drifting log in the open sea were rather limited. Seilacher et al. (1968) demonstrated that, contrary to benthic crinoids, pseudoplanktonic Seirocrinus stems had a rather stiff proximal part and a more flexible distal part


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A

B C

T E X T - F I G . 8 . A–C, Traumatocrinus sp. A, crown with proximal stem in bell position; arms branch isotomously into four biserial major branches that show additional, endotomous, biserial, pinnulated armlets, distally with axillary spines; note also interbrachial plating; stem straight, bent like a walking stick only in the proximal part; YIGMR TH0001. B, crown in bell position; stem embedded in rope-like soft condition; YIGMR TH00050. C, crown with radially spread arms in star position; stem straight with proximal end bent in the manner of a walking stick; YIGMR TH00051. Scale units are in cm.

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A

B

C

D

A–D, Traumatocrinus colonies. A–C, large Wolonggang colony, WMTH-10, consisting of a driftwood log 3.3 m long with crinoids attached to both ends; stem lengths range from c. 1 to > 7.4 m, allowing plankton filtering of the colony at different water depths; most of the stems straight, bent in the manner of a walking stick below crowns; crowns of longest individuals not preserved. A, compressed driftwood; the longest stems (extending past the person) are twisted around each other. B, overview of bedding plane with crinoids outlined with chalk; driftwood has been covered by rubble for protection; length of tape measure is 200 cm. C, detail of the above, showing stems attached to end of driftwood; WMTH-10; scale bar represents 10 cm. D, small piece of driftwood with 25 juvenile individuals; YIGMR XT3WH-11; scale units are in cm.

TEXT-FIG. 9.


A

B

C

D

E

T E X T - F I G . 1 0 . A–E, Traumatocrinus roots. A, bedding plane with patchy coaly remains of a piece of driftwood; crinoid stem fragments and roots with anastomosing rootlets (‘root cirri’); note openings of intercolumnar fossulae; YIGMR TH00052; height of image, 42 mm. B, cluster of distal stem ends with decomposed rootlets; YIGMR TH00053; scale units are in cm. C, distal stem end with articulated, anastomosing rootlets in different states of decomposition; different part of slab YIGMR TH00052; width of image, 28 mm. D, detail of B; width of image, 65 mm. E, cluster of terminal stems of two large individuals with anastomosing rootlets and round openings left by the crinoid between the rootlets; note the extremely low columnals with intercolumnar fossulae; top of Zhuganpo Formation, Bamaolin; YIGMR TH00054; width of image, 78 mm.

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Its pseudoplanktonic mode of life combined with its planktotrophic larvae allowed Traumatocrinus to disperse over the entire Palaeotethys (Kristan-Tollmann and Tollmann 1983). This lifestyle also explains why the distribution of Traumatocrinus is independent of facies, as indicated by its occurrence in red, deep-water limestones of the Hallstatt type, grey limestones and marls, and even in bituminous black shales. Traumatocrinidae are the first crinoids to have evolved the strategy of pseudoplanktonic drifting on a wooden raft. The strategy evolved once more during Late Triassic times and was realized by crinoids over a period of at least 50 myr until the Middle Jurassic (Hagdorn et al. 2007).

T E X T - F I G . 1 1 . Reconstruction of the large Wolonggang Traumatocrinus colony WMTH-10 (Text-fig. 9A–C). Wind makes the log drift in the surface water. The crinoid filtration fans reach down into the more slowly moving deeper water layers and are directed towards the plankton with the dorsal sides of the arms. Modified from Hagdorn et al. (2007).

to prevent entangling of the animals. In Traumatocrinus colonies, individuals less than 150 cm long usually show straight stems, with their most proximal 10–15 cm bent like the handle of a walking stick (Text-figs 8A–B, 10A). This is also the case in the very long stems of two colonies from Wolonggang (Hagdorn et al. 2004). These were obviously embedded in a stiff, rod-like condition. In other colonies, the stems were embedded in narrow loops indicating a soft, rope-like condition (Text-fig. 8B). Because these individuals do not have different morphological characters, unlike different species, their preservation indicates variations in stem flexibility. This may be a result of mutable collagens that probably ran in long strings through the systems of longitudinal tubes surrounding the central canal of the Traumatocrinus stem. These tubuli are connected by intercolumnar fossulae with the stem surface, the openings of which gave this crinoid its original name, Porocrinus. From the taphonomic data we conclude that the crinoids were able to both stiffen and soften their stems, possibly depending on, or reacting to, changing current velocities, or even to move actively to improve filtration efficiency while drifting with the current. Seilacher and Hauff’s (2004) model of long-stemmed tow-net filter feeders only works as long as the raft at the water surface drifts faster than the filtration fans of the animals several metres deeper.

Pelagic Roveacrinidae. Another type of ‘pelagic’ crinoid is represented by the roveacrinids, very small, stemless crinoids with delicate arms that enabled them to swim. Their best known representative is Saccocoma from the Tithonian (Late Jurassic) Solnhofen Lithographic Limestone. Roveacrinids first occur in late Middle Triassic (Ladinian) limestones and marls of the Alps and other parts of the Tethys. Their tiny ossicles are usually obtained by washing and etching of sediment, but we discovered semi-articulated specimens on weathered slabs of bituminous shale on the waste tips of quarries at Wolonggang. The slabs unequivocally originate from the black shales of the Lower Xiaowa Formation and are densely covered by remains of roveacrinids together with bivalves and ammonoids (Text-fig. 12A–C). Currently, we are not able to locate the precise position of these slabs in the section because discovery of the tiny ossicles requires weathering of the bedding planes for months or years. There may be only a single mm-thick roveacrinid horizon or several such horizons. The roveacrinids are referable to Osteocrinus (KristanTollmann, 1970), subfamily Somphocrininae. This genus is characterized by variably elongate centrodorsals and very long, bone-shaped brachials. At least two species are present: Osteocrinus cf. O. virgatus with a short, coneshaped centrodorsal, and Osteocrinus cf. O. spinosus with a very long, thin centrodorsal. On one slab, the longitudinal brachials appear to show a preferred orientation (Text-fig. 12A); on most other slabs, a general direction cannot be observed. In addition to abundant isolated elements, some cups, arm fragments, and even associated ossicles of more complete individuals in different stages of decomposition were recovered. The taphonomic evidence suggests a mass mortality event affecting oxygenated higher parts of the water column. Because until now only isolated material of Triassic roveacrinids has been found, all reconstructions are hypothetical. The Lower Xiaowa Formation may thus offer the chance of finding complete specimens of these enigmatic crinoids, so that their


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A B

C T E X T - F I G . 1 2 . A–C, Osteocrinus spp. A, bedding plane with disarticulated ossicles; note preferred orientation; YIGMR CROS001; width of image, 20 mm. B, arm fragment of Osteocrinus sp. in initial state of disarticulation; YIGMR CROS002; width of SEM image, 6 mm. C, Osteocrinus cf. virgatus, low cone-shaped centrodorsal with radials and long bone-shaped first primibrachials in association; YIGMR CROS003; width of SEM image, 6 mm.

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functional morphology can be studied and they can be compared with the Jurassic saccocomids. Roveacrinids also contribute to the age calibration of the Lower Xiaowa Formation because in late Ladinian ⁄ early Carnian times there was a Tethys-wide roveacrinid mass occurrence (Kristan-Tollmann 1970, 1977). In the latest Ladinian Sina Formation of north-east Iran, Traumatocrinus also occurs together with an Osteocrinus species with a short, cone-shaped centrodorsal (O. aghdarbandensis; Kristan-Tollmann 1991). Holothurians. The bedding plane with the roveacrinids also yielded a few questionable holothurian sclerites. The flat, perforated plates, which are attributable to the parafamily Calclamnidae, have not been studied in detail. However, the underlying limestones of the Zhuganpo Formation have yielded five paragenera with six paraspecies and two unidentified paraspecies of holothurian sclerites (Chen H. et al. 2003): Acanthotheelia, Calclamnella, Eocaudina, Tetravirga and Theelia, which belong to the extant holothurian orders Apodida, Aspidochirotida and Dendrochirotida. Among these, Theelia and possibly Acanthotheelia belong to the family Chiridotidae. These holothurians were epibenthic and ⁄ or endobenthic, whereas the paragenus Tetravirga, of the family Synallactidae, was possibly nektobenthic (Dr Mike Reich, Go¨ttingen, pers. comm. Dec. 2004).

Conodonts Middle and Late Triassic conodonts from south-western Guizhou have been studied by Yang et al. (1995), Chen and Wang (2002) and Sun et al. 2005. Chen and Wang (2002) subdivided the conodonts recovered from the Lower Xiaowa Formation and the underlying Zhuganpo Formation (see also description of these formations) into the Metapolygnathus polygnathiformis and M. nodosus zones. The former zone is defined by the first appearance of the index species, 3.38 m above the base of the Zhuganpo Formation (Sun et al. 2005). This species extends through most of the Zhuganpo Formation in association with M. foliata foliata, M. foliata inclinata, M. navicula navicula, and forms that are transitional between the last two species (Wang X. et al. 2003a). The overlying M. nodosus Zone is defined by the first appearance of the index species in the uppermost Zhuganpo Formation, about 2.5 m below the top of the formation. This ranges up into the Xiaowa Formation in low numbers. In comparison with global Triassic series and stage boundaries (Ogg 2002, 2004; Table 1) and the Upper Triassic conodont biochronology (Orchard 1991), the conodont index fossils suggest that the Guanling biota is of late Carnian age. This contradicts the early Carnian age indicated by the ammonoid fauna (see above). We are currently

unable to resolve this discrepancy. Because other biostratigraphic indicators (halobiid bivalves, Osteocrinus, plants) also point to an early Carnian age for the uppermost Zhuganpo and Lower Xiaowa formations, we consider these beds to be of early Carnian age.

Fishes A diverse assemblage of elasmobranch dermal denticles (Chen and Cuny 2003) together with rare conodonts of the M. nodosus Zone was recovered from the uppermost limestone bed of the Zhuganpo Formation in the section near the village of Wayao. This bed has also yielded a pharyngeal tooth of an unidentified actinopterygian fish. The dermal denticles include the parataxa Annulicorona pyramidalis, Glabrisubcorona cf. arduidevexa, Lobaticorona cf. floriditurris, Parvicorona dacrysulca, Sacrisubcorona cf. circabasis, and paragenera A and B (Chen and Cuny 2003). Annulicorona pyramidalis was also found in the bed immediately above this limestone and, as well as Parvicorona dacrysulca, also higher up in the lowest limestone interbeds of the Lower Xiaowa Formation at the Xiaowa section together with ammonoids of the Protrachyceras costulatum Zone. No elasmobranch teeth were recovered; as a result it is impossible positively to identify these animals and to estimate elasmobranch diversity in the Xiaowa Formation. However, elasmobranchs appear to be restricted to the lowermost part of the formation because neither dermal denticles nor teeth have been recorded higher in the section. Osteichthyes are exceedingly rare in the Lower Xiaowa Formation by comparison with marine reptiles, which is rather unusual. Only two bony fish, compared to more than ten marine reptile skeletons, were found during the excavations conducted by our team at the Xiaowa locality, and none was found in the Wolonggang quarry. The other specimens in the collections were obtained from local farmers and many appear to be composites. The precise level or levels from which these fossils were collected is unknown. Because of the presence of composite specimens, the total number of fish individuals recovered so far is difficult to determine, but it is not much greater than ten, and thus dramatically lower than the number of marine reptile specimens. There are at least four different genera of bony fish. The most common is 60–120 cm long and easily identified by its heavily ornamented scales; it appears to be referable to Asialepidotus. Asialepidotus shigyiensis was first found in the Zhugangpo Formation in the Xinyi (= Shigyi) area (Su 1959). Further preparation of the material will be required to determine whether the same species is involved, its stratigraphic range therefore extending into


the Xiaowa Formation. Insufficient preparation has also hampered the study of the other fish fossils from the Lower Xiaowa Formation. Two specimens (YIGMR TF0001–2) c. 20 cm long possess rod-like, smooth flank scales similar to those of Peltopleurus. However, the scales appear to be much thinner, the pelvic fins are better developed, and the dorsal fin is less well developed and more posteriorly situated than in Peltopleurus. The anal fin is not preserved. The skull also appears to be more elongated than in Peltopleurus. These specimens are thus likely to represent a new taxon (Wang X. et al. 2003a, pl. 2, fig. 4). The possibility that it might be referable to Pholidopleurus xiaowaensis (Liu et al. 2006) needs further study. A specimen recently found (YIGMR TRF 0020), c. 150 cm in length, appears to be devoid of scales and quite similar to Birgeria. Confirmation of this identification must await full preparation, but Birgeria has already been recorded from the Xiaowa Formation in the Guanling area (Liu et al. 2006) and the Middle–Upper Triassic of South China in general (Chang and Miao 2004). Finally, a single specimen (YIGMR SPC V 30020) c. 60 cm long seems to differ from these taxa. The scales appear to be smooth and smaller than in Asialepidotus.

Marine reptiles The marine reptiles from the Lower Xiaowa Formation of the Guanling area have been known to science for only a very short time, with the first reports appearing in 1999. The fossils were separately reported by Li (1999, 2000, 2001), Liu (1999), Li et al. (2000) and Yin et al. (2000), and further studied by Rieppel et al. (2000, 2005), Liu and Rieppel (2001, 2005), Luo and Yu (2002), Nicholls et al. (2002), Cheng (2003), Chen and Cheng (2003), Jiang et al. (2004), Cheng et al. (2006) and Rieppel and Liu (2006). The formation has yielded the best-preserved and most abundant Upper Triassic marine reptile fauna so far known. It consists of ichthyosaurs, thalattosaurs and placodonts. Remarkably, the otherwise exceedingly rare thalattosaurs are as abundant and diverse as the ichthyosaurs in this fauna. However, other marine reptiles commonly known from Triassic black shales are absent, as are any terrestrial reptiles. Both ichthyosaurs and thalattosaurs are known from many specimens. The census in the YIGMR and Guanling County collections and in the field at Wolonggang (NGPGFG) taken for this study resulted in 63 skeletons comprising 31 ichthyosaurs and 32 thalattosaurs. Estimates of the total number of skeletons excavated from the Lower Xiaowa Formation must range as high as a few hundred specimens. Placodonts, on the other hand, are much scarcer, with a minimum of six specimens known.

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A general problem with the marine reptiles in the Xiaowa Formation is the rather complicated and confused taxonomy of the material. This is partly because a large number of specimens became available for study in a very short time, since about 1996. This resulted in several names being given to the same taxon. The most influential, but also the most problematic, of the systematic papers is that by Yin et al. (2000) in which several new taxa of ichthyosaurs were named. The major problem lies with the holotype specimens of these taxa, which were deposited with the Guizhou Geological Survey (Guiyang, capital of Guizhou Province) but were collected by a businessman. They are currently considered by the Guanling government to be an illegal collection and cannot be accessed by the scientific community. Ichthyosaurs. There are three different genera and species of ichthyosaurs, all of which are more derived than the Mixosauridae and Cymbospondylidae. The species comprise the small to medium-sized Qianichthyosaurus zhoui Li, 1999 (Text-fig. 13A–C), the medium-sized to large Ghuizhouichthyosaurus tangae Cao and Luo, in Yin et al. 2000 (Text-fig. 14A–C) and the large Guanlingichthyosaurus liangae Yin, in Yin et al. 2000. Qianichthyosaurus is, by virtue of its smaller size, by far the most common, including juvenile individuals and a pregnant female (Text-fig. 13B). Based on fin anatomy, the species is most closely related to the basal euichthyosaur Toretocnemus from the Carnian of North America, although Qianichthyosaurus is more derived than Toretocnemus in the shape of its ischium and in the extended length of its front flipper (Nicholls et al. 2002). In their phylogenetic analysis of Ichthyosauria, Maisch and Matzke (2000) found Qianichthyosaurus to be the sister group of Toretocnemus and included both in the family Toretocnemidae, a conclusion that was supported by Nicholls et al. (2002). Mixosaurus guanlingensis Yin, 2000 is probably a junior synonym of Qianichthyosaurus zhoui, although McGowan and Motani (2003) suggested that it may be a valid taxon. The second most common ichthyosaur is the shastasaurid Ghuizhouichthyosaurus tangae (Text-fig. 14A–C), if specimens of its probable junior synonyms Cymbospondylus asiaticus Li and You, 2002 and Panjiangsaurus epicharis Chen and Cheng, 2003 are also counted. The largest and rarest is the remarkably small-headed Guanlingichthyosaurus liangae, which also probably belongs to the Shastasauridae. A likely junior synonym of G. liangae is Typicusichthyosaurus tsaihuae. This taxon was also erected by Yin et al. in 2000 but both share an unusually small skull. Because of their early Carnian age, the ichthyosaurs from the Xiaowa Formation are of special importance in understanding ichthyosaur evolution, this having


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A–C, ichthyosaur Qianichthyosaurus zhoui Li, 1999. A, complete skeleton, YIGMR XTwQ-3. B, nearly complete skeleton, 1.28 m long; note embryos below rib cage of mother; YIGMR TR00040. C, skull in right lateral view; YIGMR TR00047. Scale bars represent 10 cm in A–B, 5 cm in C.

TEXT-FIG. 13.


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B

C T E X T - F I G . 1 4 . A–C, ichthyosaur Ghuizhouichthyosaurus tangae Cao and Luo, in Yin et al. 2000. A, probable junior synonym, holotype of Panjiangsaurus epicharis, complete skeleton, 5.4 m long; YIGMR TR00001. B, skull in left lateral view, with anterior part of the rostrum missing; GNG DQ-46. C, specimen preserved and exposed in situ, pelvic and hind limb bones; Wolonggang, WLTR0010. Scale bars represent 30 cm in A, 10 cm in B–C.

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been the time that the Neoichthyosauria (Sander, 2000) must have originated. Another interesting aspect is the potential for soft-part preservation in the Xiaowa ichthyosaurs, which would be especially valuable in understanding the evolution of ichthyosaur locomotion. Also of palaeobiological interest are two kinds of gastric contents found in Guanlingichthyosaurus. One specimen, the holotype of Panjiangsaurus epicharis, has two clusters of gastroliths that are mainly composed of silicified limestone and quartzite (Cheng et al. 2006). The other gastric content is composed mainly of fish scales and fragments of bivalves and brachiopods, and presumably represents the remains of food, although these invertebrates are unlikely to have been part of the regular diet of this ichthyosaur. Placodonts. The placodonts from the Xiaowa Formation are typical representatives of the armoured Cyamodontidae, which arose in the Middle Triassic and have a good Late Triassic record in the western Tethys, particularly in the Alps. Before the Chinese finds became known, the easternmost placodont records were from Israel. The Chinese occurrences thus greatly extended the restricted western Tethyan distribution of the Placodontia. Currently two taxa are known from the formation: Sinocyamodus xinpuensis Li, 2000 and Psephochelys polyosteoderma Li and Rieppel, 2002 (Text-fig. 15A–C). The phylogenetic relationships of these within the Cyamodontidae are not yet resolved. Thalattosaurs. There are three different genera and four species of thalattosaurs, the small Xinpusaurus with two species, the large Anshunsaurus huangguoshuensis Liu, 1999 (Text-figs 16A–B, 17C–D), and a medium to large as yet undescribed taxon. As is the case for Qianichthyosaurus, by virtue of its smaller size Xinpusaurus is easily the most common thalattosaur, representatives including many juveniles. There are two species, the common Xinpusaurus suni Yin, in Yin et al. 2000 (Text-fig. 17A) and the much scarcer longirostrine X. bamaolinensis Cheng, 2003 (Text-fig. 17B). This name has priority over Xinpusaurus kohi Jiang et al., 2004. The skull and rostral morphology of X. suni was recently described in detail by Rieppel et al. (2005) and Rieppel and Liu (2006). According to Liu and Rieppel (2005), Xinpusaurus is a basal member of the Thalattosauroidea and not closely related to Nectosaurus from the Carnian of California (contra Jiang et al. 2004). Anshunsaurus is known from several good skeletons, reaching a maximum length of 3 m (Text-figs 16A–B, 17C), making it the largest thalattosaur known. Our own examination of the postcranium of Anshunsaurus huangguoshuensis in the YIGMR and Guanling County Collections indicates that this taxon bears a striking

resemblance to the thalattosaur Askeptosaurus italicus from the Anisian ⁄ Ladinian boundary beds of Monte San Giorgio in the Southern Alps. This similarity had already been noted by Rieppel et al. (2000) for the skull and was recently corroborated by Liu and Rieppel (2005) for the entire skeleton on material at the Institute of Vertebrate Palaeontology and Palaeoanthropology, Bejing. Not surprisingly, in their species-level phylogenetic analysis of thalattosaur interrelationships, Liu and Rieppel (2005) found the two taxa to be sister groups. The thalattosaur fauna from the Xiaowa Formation is striking not only because of the abundance and diversity of the material but also because of its biogeographic importance. Previously, thalattosaurs were only known from the western Tethyan (Switzerland, Italy, Germany) and eastern Pacific (Nevada, USA; British Columbia, Canada) realms (Nicholls and Brinkman 1993), and not from the western Pacific or eastern Tethyan realms. The discovery of the first Anshusaurus huangguoshuensis (Liu 1999) in the Guanling biota (Rieppel et al. 2000) filled this gap, suggesting that thalattosaurs essentially enjoyed a cosmopolitan distribution and possibly were able to cross the very large Panthalassa Ocean. However, no clear hyptheses of dispersal routes emerge from the phylogenetic analysis of thalattosaur interrelationships by Liu and Rieppel (2005). Marine reptile palaeobiogeography. South China in Late Triassic times was at the boundary between the eastern Tethyan and western Pacific biogeographic provinces (Rieppel et al. 2003). Although the systematic affinities of the marine reptiles from the Xiaowa Formation have yet to be understood fully, certain patterns in their biogeography are apparent: some are most closely related to taxa from the western Tethys (Anshunsaurus, the placodonts) while others are most closely related to eastern Pacific taxa (Qianichthyosaurus), a distribution previously noted by Rieppel et al. (2000). This is also in accordance with the pattern observed by Rieppel (1999) and Rieppel et al. (2003) for Middle Triassic marine reptiles, specifically sauropterygians, from China. With the invertebrate and conodont faunas being of limited diversity, abundance and quality of finds, biostratigraphic information provided by the marine reptiles is potentially important. Among the ichthyosaurs, the absence of the exclusively Early and Middle Triassic Mixosauridae and Cymbospondylidae is noteworthy. From a systematic point of view, the affinities of the ichthyosaurs from the Xiaowa Formation lie with those of the Carnian Hosselkus Limestone of California. Among the thalattosaurs, Anshunsaurus, with its sister-group relationship to Askeptosaurus, would argue for a Middle Triassic age. Based on the phylogenetic analysis of Thalat-


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Placodont Psephochelys polyosteoderma Li and Rieppel, 2002. A, complete skeleton preserved in dorsal view; YIGMR TR00043. B, complete skeleton preserved in ventral view; YIGMR TR00046. C, complete skeleton preserved in dorsal view; YIGMR TR00005. Scale bars represent 10 cm.

TEXT-FIG. 15.

tosauria by Liu and Rieppel (2005), Xinpusaurus, being a member of their Thalatosauroidea, is uninformative biostratigraphically. This group includes a taxon from the Upper Triassic of North America that is less derived than Xinpusaurus, and several more derived taxa that are either

from the Middle Triassic of Europe or the Upper Triassic of North America. The placodont genera and species do not contribute much to the age issue either, because Cyamodontidae are known from the western Tethys throughout the Middle and Upper Triassic. Resolution

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T E X T - F I G . 1 6 . A–B, thalattosaur Anshunsaurus huangguoshuensis Liu, 1999. A, photograph of nearly complete skeleton in ventral view; YIGMR TR00042. B, drawing of the skeleton. Scale bar represents, 30 cm.


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D

A, thalattosaur Xinpusaurus suni Yin, in Yin et al. 2000, complete skeleton GNG DQ-10. B, thalattosaur Xinpusaurus bamaolinensis Cheng, 2003 (= Xinpusaurus kohi Jiang et al., 2004), holotype, complete skeleton 1.4 m long; YIGMR SPCV30015. C–D, thalattosaur Anshunsaurus huangguoshuensis Liu, 1999. C, skull of a juvenile in dorsal view; YIGMR TR00006. D, skull of a small juvenile in dorsal view; YIGMR TR00043. Scale bars represent 10 cm in A–B, 5 cm in C, 2 cm in D.

TEXT-FIG. 17.

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may improve with a better understanding of cyamodontid ingroup phylogeny. In conclusion, among marine reptiles, ichthyosaurs best constrain the age of the Xiaowa Formation and suggest a Carnian age because Euichthyosauria are not known to have occurred before the Carnian elsewhere and typical Middle Triassic ichthyosaurs are lacking.

Plants Most of the plant material found in the Xiaowa Formation is drift wood, which is remarkably common as small pieces and large logs up to 3.3 m in length on bedding planes in the black shales. Most pieces are more or less compressed laterally and coalified or preserved as ‘jet’, and show a polygonal pattern of calcified cracks. It is not possible to attribute the wood to any group of plants. Only one piece, which was three-dimensionally preserved in a limestone nodule, has its cellular structure and annual rings perfectly preserved. It is definitely a gymnosperm and also shows a small amount of bark and a cluster of Traumatocrinus roots attached that filled the space of a crack deep into the heartwood. Additionally, a few specimens of well-preserved leaflets of the pteridophyte Ctenozamites sarrani and the horsetail Equisetites arenaceus have been found in the upper part of the Lower Xiaowa Formation. On the specimen of Equisetites there are encrustations of small indeterminate bivalves that are similar to the Muschelkalk ‘Placunopsis’; these indicate that the plant had been drifting in the sea for some time. However, this specimen is not encrusted by crinoids, possibly because Equisetites was not lignified. The two plant species are considered to be representive elements of the early Carnian Abropteris-Pteriphyllum longifolium Assemblage Zone in South China (Sun et al. 1995). Since its initial discovery in Vietnam (Zeiller 1903), Ctenozamites sarrani has been widely recorded from the Late Triassic coal-bearing strata of Guangdong (Xiaoping Formation), Yunnan (Yipinglan Formation), western Hubei (Jiuligang Formation), Shanxi and Inner Mongolia (Yanchang Formation) in China (Meng et al. 2003). It has not yet been found in the Middle Triassic. Associated Equisetites arenaceus usually occurs in Late Triassic (Carnian–Norian) strata in south-west China (Xu et al. 1979; Wu 1982) and in the Ladinian and Carnian (Lower and Middle Keuper) of Germany, France and Switzerland (Dobruskina 1982; Meng et al. 2003; Kelber 2005). It has also been recorded from the Middle Triassic Badong Formation in western Hunan, China, and the Upper Muschelkalk in France (Meng et al. 2000). The plant remains thus suggest an early Carnian age for the Lower Xiaowa Formation.

TAPHONOMY AND DEPOSITIONAL ENVIRONMENT Like other black shales containing kerogen, the Lower Xiaowa Formation is a konservat Lagersta¨tte that preserves complete and articulated multi-element skeletons of pelagic faunal elements, namely vertebrates and pelagic crinoids. Generally, the percentage of complete and fully articulated skeletons is very high compared to other black shales, whereas isolated bones and traumatocrinid sclerites are only rarely found. A taphonomic analysis of these fossils provides valuable data for understanding the depositional environment and origin of the deposit.

Evidence from the sediment The fossiliferous horizons are dark ⁄ light laminated sediments rich in organic matter. Many show crinkle lamination from densely packed daonellid bivalves (Text-fig. 18C). A lack of bioturbation and of unquestionably benthic faunal elements indicates deposition under anoxic or dysoxic conditions. Some of the lamination may also represent mats of bacteria that covered skeletons of crinoids and vertebrates and protected them from disarticulation. Apparently the anoxic zone extended well into the water column (see above). This compares closely with many other konservat Lagersta¨tten, e.g. the Toarcian Posidonia Shale of south-west Germany.

Evidence from other invertebrates All members of the invertebrate fauna are nektonic (ammonoids), planktonic (somphocrinids), or pseudoplanktonic (Traumatocrinus and halobiid bivalves), with the exception of rare allochthonous benthic brachiopods. Fossils of other unequivocally benthic animals or traces of their burrowing activity have not yet been observed, and the laminae are entirely undisturbed. Ammonoids are usually concentrated on beddings planes of the black shales in ‘beer mat preservation’ typical for such sediments (Seilacher et al. 1976). They are devoid of periostracum (Text-fig. 19A), in contrast to those from the Posidonia Shale. The palaeoecology of the halobiid bivalve Daonella is connected with a scientific controversy that has lasted for decades. In the latest contribution (Schatz 2005) it is regarded as a benthic bivalve, while other authors regard it as pseudoplanktonic, being attached by its byssus threads to drifting objects. Although driftwood is very abundant in the black shales, logs with halobiid bivalves attached have not yet been found, unlike logs in the Black


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T E X T - F I G . 1 8 . A–C, taphonomy of Traumatocrinus. A, crown being decomposed on upper side of bedding plane with sclerites still associated, stem still articulated and proximally bent like a walking stick; Wolonggang, WMTH-15; width of image, c. 45 cm. B, stems of different individuals in advanced state of decomposition, probably under oxygenated bottom conditions, on bedding plane of uppermost Zhuganpo Formation; small quarry 800 m north-west of Wolonggang; length of scale bar, 10 cm. C, pluricolumnal of distal stem covered by laminated black shale of Lower Xiaowa Formation; crinkle lamination from densely packed daonellid bivalves; acetate peel; width of image, 22 mm.

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A

B T E X T - F I G . 1 9 . A–B, taphonomy of black shales in Lower Xiaowa Formation: mollusc evidence. A, Trachyceras multituberculatum in beer-mat preservation on bedding plane; YIGMR IVA001. B, densely packed, compressed Daonella sp. with calcitic shell preserved; YIGMR IVB001. Scale units are in cm.


Ven Marl or the Posidonia Shale, which were definitely bivalve-encrusted (Seilacher 1990; Wignall and Simms 1990). The halobiid bivalves cover certain bedding planes in dense packages, suggesting low sedimentation rates or, if one were to follow Schatz (2005), benthic colonization episodes (Text-fig. 19B). Ward (2006) went further in suggesting that halobiid bivalves were specifically adapted to low oxygen conditions, making use of organic matter in the sediment on the sea-floor by harbouring chemosymbionts.

Evidence from the crinoids In common with crowns of Pentacrinites from the Dorset Coast and of Seirocrinus and Pentacrinites from the Posidonia Shale, Xiaowa Traumatocrinus crowns form cmthick lenses of calcite. Beautifully preserved and fully articulated crowns and clusters of crowns are found on their lower surfaces, but in different stages of decay on the upper surfaces (Simms 1986, 1999; Hagdorn et al. 2005). In many cases, the cups are still attached to the stems while the arms are more or less disarticulated but largely remain associated, forming a sheet of brachials and pinnulars (Text-fig. 18A). Most of the crowns were embedded laterally with their arms compressed (Textfig. 8A–B), but some are preserved with their oral sides down and their arms radially spread (Text-fig. 8C). When the three-dimensional filter fan of the living animal, resembling a plicated coffee filter, was compressed by sediment overburden, the polygonally plated tegmen was laterally torn along the interbrachial plating (Text-fig. 8A). This selective decay pattern shows that the crinoid crowns were not immediately covered by sediment when they reached the sea-floor but rather were exposed for some time in the water column. We did not find any evidence for deep muddy sediment on the sea-floor into which the crinoids could have sunk or been pressed by their stiff stems when the logs approached it; all of the specimens we studied were embedded strictly horizontally on a single bedding plane. Moreover, there was no evidence of either the crinoids or the logs having been dragged along the sea-floor. From this we conclude that the sea-floor was at least semi-consolidated. We suggest that bacterial mats covered the crinoid skeletons after a while, preventing further decay and disarticulation. Because even tiny elements of the crinoid crowns, such as pinnulars and brachials, were not washed away, the bottom conditions must normally have been stagnant with only very weak bottom currents. The colonies from Wolonggang and a very large colony with individuals exceeding 11 m (Text-fig. 9A–C, sample WMTH-10) indicate that the state of the mutable collagen in the Traumatocrinus stems determined their pose

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on the sea-floor. However, both straight stems and stems embedded in large loops provide evidence that the crinoids reached the sea-floor prior to the driftwood. There are great uncertainties about the drifting time of a log encrusted by crinoids, and the sinking process and its duration. The good preservation of the crinoid skeletons, which today will decompose within a few days in warm water (Scha¨fer and Craig 1972), suggests a relatively short sinking time. However, this time may have been prolonged by the dysaerobic and quiet bottom-water conditions. Sinking time also depends on water depth, salinity (freshwater influx) and additional loading of the log by other encrusters, such as daonellid bivalves. However, driftwood densely covered by bivalves, as commonly found in the Jurassic Posidonia Shale, has not yet been observed in the Xiaowa Formation. The fact that logs with attached Traumatocrinus colonies are nearly as abundant as marine reptile skeletons again suggests low sedimentation rates. Unlike most benthic echinoderm Lagersta¨tten, the black shales are not an obrutional konservat Lagersta¨tte, rather they are a stagnation Lagersta¨tte typical for black shales that preserved crinoid skeletons under stagnant bottom conditions devoid of scavengers (Seilacher 1970). In the uppermost part of the Zhuganpo Formation we found several occurrences that may represent colonies of Traumatocrinus in a state of progressive decomposition of their crowns and even their stems (Text-fig. 18B). This reflects oxygenated bottom conditions, also suggested by the bioturbation and presence of articulated brachiopods. From these observations we conclude that driftwood logs with crinoids attached originated from the open sea of the Nanpanjiang Basin and were embedded under anaerobic bottom conditions in the Xiaowa Formation, either almost completely articulated or in an initial state of decomposition, but under aerobic bottom conditions in the Zhuganpo Formation in an advanced state of decomposition. We have found disarticulated Traumatocrinus remains only in a 5-m-thick section of the uppermost Zhuganpo Formation and the lower part of the Lower Xiaowa Formation (Text-fig. 4). This occurrence is similar to the distribution of Pentacrinites fossilis colonies in a 2-m-thick section of the Black Ven Formation of the Dorset coast and of Seirocrinus subangularis in the dmthick ‘Fleins’ of the Posidonia Shale. Simms (1986, 1999) explained this restricted occurrence as a response to gyrate current systems comparable to the Sargasso Sea that did not release driftwood once it was caught by a gyre. If this model is to be applied here, the drowned margin of the Yangtze Platform, or parts of it, acted as a giant fossil trap, both condensing and preserving multi-element skeletons over a long period of time. However, this does not explain the abundance of marine reptile skeletons because active swimmers could have escaped such a gyre.

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Evidence from fishes and flora Fish (including elasmobranch) remains are extremely rare in the Lower Xiaowa Formation, with not many more than ten finds preserved in the collections, compared to the hundreds of marine reptile skeletons and Traumatorcrinus colonies. In most other marine black shales, fishes are more common than marine reptiles by an order of magnitude. Their scarcity suggests that the surface waters were unsuitable for them. Similarly, higher plants are only represented by a few finds, which derive from a typical terrestrial Triassic flora, but the good preservation is consistent with quiet-water, anoxic depositional conditions.

Evidence from marine reptiles Marine reptiles are the most conspicuous fossils in the Lower Xiaowa Formation and offer much information about the taphonomy of the Lagersta¨tte. The high density of skeletons (about one per 100 m2 in the horizon that is most widely exposed at Wolonggang) is interpreted as an effect of the relatively low rate of sedimentation, which is also indicated by the relatively low carbonate content of the sediment. As in other laminated black shales with marine reptiles, the skeletons are preserved in their entirety, without elements missing, but with varying degrees of disarticulation of the skull, tail, ribs, gastralia and distal extremities. This suggests that the reptile carcasses did not drift for long, otherwise skeletal elements would have been lost (Scha¨fer and Craig 1972). Once a carcass was deposited on the sea-floor, decomposition combined with the effects of gravity, microbial mats and possibly weak bottom currents led to the observed disarticulation patterns. All ontogenetic stages appear to be present in the abundant Qianichthyosaurus and Xinpusaurus suni, from juveniles to large specimens, including a pregnant female of the ichthyosaur. The less abundant species also show size variation. These observations suggest attritional mortality, i.e. sampling of a living population by various causes of death, not catastrophic mortality.

Taphonomic and environmental hypotheses The Lower Xiaowa Formation in the Guanling area was deposited in a quiet, stagnant environment, which appears to have been far away from any coastline, as indicated by the scarcity of terrestrial plants and lack of terrestrial reptiles. In all other Mesozoic laminated black shales of similarly high sampling density, these are present as rare components. Abundant driftwood does not con-

tradict this conclusion because the robust logs could have travelled long distances. The formation is also unusual because of the very limited diversity of invertebrates and marine vertebrates, including the markedly scarce fish. Based on these observations, two taphonomic hypotheses, both of which involve stagnant bottom waters, can be formulated: (1) either the fossil deposit reflects an ecologically stressed community of very limited diversity but high abundance in the aerated surface waters; or (2) it resulted from preferential drifting of dead invertebrates, logs with crinoids, and vertebrate carcasses into an area with dysoxic or anoxic surface waters. If the taphocoenosis originated from life in the surface waters above the anoxic zone, the question arises as to what the ecological restrictions comprised. Poor oxygenation of the surface waters would explain the scarcity of fish, which as gill breathers would have been more affected by low oxygen levels than the marine reptiles. Although low dissolved oxygen levels might seem to be difficult to reconcile with the abundance of ammonoids, some Late Triassic ammonoid clades might have been well adapted to poorly oxygenated water because of their efficient respiratory system (Ward 2006). Ward (op. cit.) suggested that poor oxygenation of marine surface waters was a global phenomenon during the Late Triassic caused by the lowest atmospheric oxygen content (