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Middle Triassic Xingyi Fauna: Showing turnover of marine reptiles from coastal to oceanic environments
Accepted Manuscript Title: Middle Triassic Xingyi Fauna: showing turnover of marine reptiles from coastal to oceanic environments Authors: Hao Lu, Da-Yong Jiang, Ryosuke Motani, Pei-Gang Ni, Zuo-Yu Sun, Andrea Tintori, Shi-Zhen Xiao, Min Zhou, Cheng Ji, Wan-Lu Fu PII: DOI: Reference:
S1871-174X(17)30031-8 http://dx.doi.org/doi:10.1016/j.palwor.2017.05.005 PALWOR 418
To appear in:
Palaeoworld
Received date: Revised date: Accepted date:
17-2-2017 25-4-2017 22-5-2017
Please cite this article as: Lu, Hao, Jiang, Da-Yong, Motani, Ryosuke, Ni, Pei-Gang, Sun, Zuo-Yu, Tintori, Andrea, Xiao, Shi-Zhen, Zhou, Min, Ji, Cheng, Fu, Wan-Lu, Middle Triassic Xingyi Fauna: showing turnover of marine reptiles from coastal to oceanic environments.Palaeoworld http://dx.doi.org/10.1016/j.palwor.2017.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Middle Triassic Xingyi Fauna: showing turnover of marine reptiles from coastal to oceanic environments
Hao Lu a, Da-Yong Jiang
a
*, Ryosuke Motani b, Pei-Gang Ni a, Zuo-Yu Sun a,
Andrea Tintori c, Shi-Zhen Xiao d, Min Zhou a, Cheng Ji e, Wan-Lu Fu f
a
Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education and
Department of Geology and Geological Museum, Peking University, Beijing 100871, China b
Department of Earth and Planetary Sciences, University of California, Davis, CA
95616-8605, USA [E-mail address: [email protected]] c
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via
Mangiagalli 34-20133 Milano, Italy [E-mail address: [email protected]] d
School of Karst Science, Guizhou Normal University, Guiyang 550001, China
[E-mail address: [email protected]] e
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of
Geology and Palaeontology, CAS, Nanjing 210008, China [E-mail address: [email protected]] f
Business School, Tianjin University of Commerce, Tianjin 300134, China [E-mail
address: [email protected]]
* Corresponding author. E-mail address: [email protected]
Abstract The late Ladinian (Middle Triassic) Xingyi Fauna from the Zhuganpo Member of the Falang Formation yields abundant and well-preserved marine reptiles. Bed-by-bed excavation at Wusha in Xingyi of Guizhou Province reveals two marine vertebrate assemblages in a fossiliferous horizons that span 5.1 m in total thickness. The lower assemblage
is
marked
by
the
near-shore
sauropterygians,
including
the
pachypleurosaur Keichousaurus, the nothosaurians Nothosaurus and Lariosaurus, 1
with a strong paleobiogeographic affinity to western Tethys. The upper assemblage consists of oceanic ichthyosaurs and pistosaurs, including the large shastasaurid ichthyosaur Guizhouichthyosaurus, the euichthyosaur Qianichthyosaurus, pistosaurs Yunguisaurus and Wangosaurus, and the thalattosaur Xinpusaurus, with a closer paleobiogeographic affinity to North America. The coastal pachypleurosaur and nothosaurid sauropterygians disappeared in the upper assemblage, suggesting that they were replaced by an oceanic marine reptile community that emerged. The reptilian composition of the upper assemblage is similar to that of the Guanling Biota, which is of the Early Carnian (Late Triassic) in age and thus somewhat younger than the Xingyi Fauna. The ecological turnover of marine reptiles from near-shore to the open ocean community corresponds to the paleoenvironmental changes indicated by lithofacies analysis, δ13C and the global sea level changes.
Keywords: Xingyi Fauna; marine reptile; Middle Triassic; Guizhou; China
1. Introduction The Middle Triassic Xingyi Fauna, containing exceptionally well-preserved marine reptiles and fishes (Young, 1958; Su, 1959; Wang et al., 2001; Li and Jin, 2003), has been providing knowledge on the taxonomy, biodiversity, paleobiology and evolution of Triassic marine reptiles, fishes, ammonoids and other clades, and also on the paleobiogeography (Jiang, 2002; Li et al., 2002; Liu et al., 2003; Cheng et al., 2004, 2006, 2012; Li and Rieppel, 2004; Rieppel et al., 2006, 2010; Li, 2007; Li et al., 2007; Zhao et al., 2008a, 2008b; Sato et al., 2010, 2013; Tintori et al., 2012, 2015; Xu et al., 2012, 2015; Benton et al., 2013; Yang et al., 2013; Ji et al., 2014; Ma et al., 2015; Xue et al., 2015; Zou et al., 2015a, 2015b; Sun et al., 2016; Fig. 1A). The research of Xingyi Fauna dates back to 1957, when the first Mesozoic marine reptile fossil in Asia was found by Hu at Lüyin Village, Dingxiao District, Xingyi City of Guizhou Province, and was named by Young (1958) as Keichousaurus hui. Subsequently, Su (1959) reported three new taxa of fishes from the same locality, including Peltopleurus orientalis, Sinoeugnathus kueichowensis and Asialepidotus 2
shingyiensis. However, over the next four decades, a few reptiles were reported (Young, 1965a, 1965b), while abundant specimens of Keichousaurus and Asialepidotus presumably disappeared into private hands.
Fig. 1 here
The situation changed in the late 1990s, when Keichousaurus hui and Shingyisaurus unexpectus (Nothosaurus sp. following Rieppel, 1998) were redescribed in detail by Lin and Rieppel (1998) and Rieppel (1998), and diverse fossils of reptiles and fishes were excavated from the Zhuganpo Member of Falang Formation (Wang et al., 2001; Sun et al., 2016 (Zhuganpo Formation)). Almost all major clades of Triassic marine reptiles have their representatives found in the Xingyi Fauna. New taxa include the placodont Glyphoderma kangi Zhao et al., 2008a, the eosauropterygian Qianxisaurus chajiangensis Cheng et al., 2012, the nothosaurids Lariosaurus xingyiensis Li et al., 2002 and Nothosaurus youngi Li and Rieppel, 2004, the pistosaurs Yunguisaurus liae Cheng et al., 2006 and Wangosaurus brevirostris Ma et al., 2015, the thalattosaurs Anshunsaurus wushaensis Rieppel et al., 2006, A. huangnihensis Cheng et al., 2007 and Xinpusaurus xingyiensis Li, Z.G. et al., 2016a, the euichthyosaur Qianichthyosaurus wushaensis Yang et al., 2013, the protorosaurs Tanystropheus cf. T. longobardicus (Rieppel et al., 2010), Tanystropheus sp. (Li, 2007), Macrocnemus fuyuanensis Li et al., 2007 and the archosaur Diandongosuchus fuyuanensis Li et al., 2012 and Litorosuchus somnii Li, C. et al., 2016. Some new materials remain under study, such as the large shastasaurid ichthyosaur Guizhouichthyosaurus. The age of Xingyi Fauna was confirmed to be late Ladinian (Middle Triassic) by the ammonoid biostratigraphic correlation and isotope dating (Zou et al., 2015a; Li, Z.G. et al., 2016b; Sun et al., 2016), which proposed that the Xingyi Fauna is the only known late Ladinian (Middle Triassic) Fossil Lagerstätte characterized by exceptionally well-preserved marine reptiles and fishes in the world (Ji et al., 2012). Compared to the Late Ladinian Kalkschieferzone of the famous Area (Stockar et al., 3
2012), the Xingyi Fauna is much more diversified regarding reptiles as in the Alps only Lariosaurus has been recorded (Tintori and Renesto, 1990) owing probably to a somewhat different environment (Lombardo et al., 2012). Since August of 2011, the excavation at Nimaigu village in Wusha of Xingyi City has been carried out by the internationally collaborative team composed of Peking University, the University of California (Davis), Università degli Studi di Milano, supported by City Land and Resources Bureau of Xingyi. More than 1000 square meters of the surface of 5.1 m thick fossil-bearing layers of the Xingyi Fauna were exposed bed-by-bed and exact information of excavated individual fossils were recorded (Fig. 2). A complete stratigraphic section from the Zhuganpo Member up to the base of Wayao Member of Falang Formation was measured, again bed by bed, and rock samples from each bed were collected. The excavation disproved some of the early perceptions about the Xingyi Fauna. Most of the taxa of the Xingyi Fauna have been erected based on specimens originally collected by amateurs without precise and detailed geographic and stratigraphic information. The lack of information resulted in a wrong assumption to consider all taxa to represent a single assemblage from a single fossiliferous stratigraphic zone. Biogeographically, the Xingyi Fauna was known to show a strong affinity to Middle Triassic faunas from the West Tethys and consists mainly of marine reptiles that inhabited nearshore environments (Rieppel et al., 2000, 2010; Li and Jin, 2003; Li et al., 2007). However, the excavation revealed that the fossiliferous level of Xingyi Fauna can be subdivided into two parts.
Fig. 2 here
The lower part, yielding the Lower Assemblage recognized in this paper, previously meant the whole “Xingyi Fauna” and known as the typical Ladinian fauna of Tethyan affinity, contains mainly sauropterygians, sharing genera with the lower Anisian Panxian and Luoping faunas, such as Nothosaurus, Lariosaurus and possibly Keichousaurus. In the Upper Assemblage yielded in the upper part of the fossiliferous 4
level, middle-large sized ichthyosaurs, pistosaurs and thalattosaurs appear, including Qianichthyosaurus,
Guizhouichthyosaurus,
Wangosaurus,
Yunguisaurus,
and
Xinpusaurus (Rieppel et al., 2006; Yang et al., 2013; Ma et al., 2015; Li, Z.G. et al., 2016a), among which Qianichthyosaurus, Guizhouichthyosaurus and Xinpusaurus were previously only reported in the younger Guanling Biota (Early Carnian of Late Triassic). These ichthyosaurs are oceanic animals (Ji et al., 2010) and suggest a trans-Pacific affinity (Jiang et al., 2004). Several fishes previously only known in the Carnian (Late Triassic) of the West Tethys were also recorded in this assemblage of the Xingyi Fauna, e.g., flying fishes Thoracopterus Tintori et al. (2012) and stem-group Teleostei Malingichthys Tintori et al. (2015). The subdivision of the fossiliferous level reveals that the Xingyi Fauna indeed comprises two different marine vertebrate assemblages consisting of different marine reptiles and fishes. Ma et al. (2013) gave a brief lithological description of the Nimaigu section and recognized the taxonomic difference between the marine vertebrate assemblages, but they did not point out the faunal turnover and discuss its paleogeographic background. Since that study, additional materials have been found by further excavations and new studies on microfacies and geochemical analysis of the section have been conducted. Therefore, with new information, we are able to detail the stratigraphic distribution of the marine vertebrate fossils, confirm the presence of the two distinct assemblages and correlate them with the lithofacies, δ13C data and the global sea level changes. It can be concluded that the two assemblages suggest a major faunal change, which probably indicated a turnover of ecological setting from nearshore to offshore. These assemblages are possibly related to different ecosystems, representing transitional links between the earlier Panxian-Luoping Fauna (Anisian) (Motani et al., 2008; Jiang et al., 2009; Hu et al., 2011) and the later Guanling Biota (Carnian). It hints a faunal turnover and to confirm it requires a thorough investigation of the biostratigraphy of marine vertebrates in other sections within the area.
2. Material and methods All samples studied in this paper were collected from the middle-upper part of the 5
Zhuganpo Member to the base of the Wayao Member of the Falang Formation, at Nimaigu, Wusha District, Xingyi City of Guizhou Province. The section is measured 48.5 m in thickness, comprising 214 natural layers that correspond from the base Layer 26 to the top Layer 239 according to our field numbering system (Fig. 1B). The fossiliferous level of the Xingyi Fauna ranges from Layer 26 to Layer 53 and is 5.1 m thick (Fig. 1B). More than 800 skeletons of marine reptiles, including sauropterygians, ichthyosaurs and thalattosaurs, were collected during the excavation and were preserved and prepared in the Museum of Xingyi National Geopark. All vertebrate fossils were prepared using air chisels. 94 rock samples were collected and δ13C data measured by MAT-253 Stable Isotope Ratio Mass Spectrometer in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Thin sections for microfacies analysis were made in the Laboratory of Orogenic Belt and Crustal Evolution of Ministry of Education at Peking University. Horizons were recorded and assemblages of marine vertebrates were established following the method of Motani et al. (2008) and Jiang et al. (2009) for the Anisian Panxian Fauna. The analysis and comparison of ecological strategies of global marine reptiles mainly refer to Kelley et al. (2014). Microfacies analysis of carbonates and reconstruction of paleoenvironmental conditions follow Yu (1989) and Flügel (2004).
3. Sequence of the middle-upper Zhuganpo Member, Falang Formation The Nimaigu section is located at 25°09′52″N, 104°47′19″E with an altitude of about 1491 m (Fig. 1B). The description of the section in descending order is as below. Data of ammonoids and zircon U-Pb datings are derived from Zou et al. (2015a) and Li, Z.G. et al. (2016b), respectively.
Overlying: Wayao Member of the Falang Formation Layers 236-239 Dark-grey, thick-bedded muddy limestone, marls, interbedded with black shales, yielding ammonoids Trachyceras multituberculatum and abundant bivalves Halobia. > 16 m (not reach the top) —conformity— 6
Zhuganpo Member of the Falang Formation Layers 227-235 Grey to dark-grey, medium -bedded limestone, containing ammonoid Trachyceras multituberculatum (bed 235) and the ichthyosaur Qianichthyosaurus (bed 230). 3.8 m Layers 194-226 Grey, medium-bedded limestone with cherts intercalated with thick-bedded limestone, containing ammonoid species of Trachyceras. 8.8 m Layers 170-193 Grey, medium- to thick-bedded limestone, intercalated with medium-bedded argillaceous banded limestone. 6.6 m Layers 139-169 Dark-grey, medium-bedded limestone with cherts intercalated with grey to dark-grey thin- to medium-bedded argillaceous banded limestone, containing ammonoids Clionitites sp. indet. (bed 162), Ptychites sp. A (bed 154), Yangites densicostatus (bed 152). The age of 161 layer is 239.8 ± 1.7 Ma through the U-Pb analysis. 6.5 m Layers 97-138 Dark-grey, thin- to medium-bedded argillaceous banded limestone intercalated with gray medium- to thick-bedded limestone, containing ammonoids Yangites densicostatus, Sinomeginoceras wangi (bed 98). 9.7 m Layers 54-96 Grey, thin- to medium-bedded limestone intercalated with thin- to medium-bedded nodular limestone, containing ammonoids Haoceras xingyiense (beds 57, 58, 59, 68, 88), Yangites densicostatus (beds 59, 65), Sinomeginoceras wangi (beds 68, 70), Sinomeginoceras xingyiense (bed 71), Xenoprotrachyceras cf. primum (bed 59), Detoniceras sp. A (beds 72, 88). 7.7 m Layers 26-53 Grey, thin- to medium-bedded argillaceous banded limestone, marl and nodular limestone intercalated with medium- to thick-bedded limestone, yielding abundant marine reptiles and fishes associated with bivalves, ammonoids and crinoids. > 5.1 m (not measured to the bottom)
The sequence of the fossil-bearing layers in descending order is as follows: Layer 53 Dark-grey, medium- to thick-bedded bioclastic micrite, yielding the shastasaurid
ichthyosaur
Guizhouichthyosaurus,
the
thalattosaur
xingyiensis, ammonoid Ptychites, crinoids and bivalves. 26 cm 7
Xinpusaurus
Layer 52 Dark-grey, thin-bedded muddy micrite with bioclastics (1-3 cm each layer), yielding crinoids and fossil fishes. 10-12 cm Layer 51 Grey, thin-bedded laminated micrite. 2 cm Layer 50 Light-grey, medium to thick-bedded muddy micrite with bioclastics, yielding the ichthyosaur Qianichythyosaurus and Guizhouichthyosaurus, the thalattosaur Xinpusaurus and bivalves. 60 cm Layer 49 Dark-grey, thin-bedded muddy micrite with bioclastics, yielding fragments of marine reptiles, fishes, ammonoids Parasturis and Ptychites, bivalves and crinoids. 5 cm Layer 48 Grey, medium-bedded laminated micrite, yielding fishes, bivalves and ammonoid Detoniceras. 13 cm Layer 47 Grey, thin-bedded muddy micrite with bioclastics, yielding fishes and bivalves. We obtained an age of 240.8 ± 1.8 Ma through the U-Pb analysis. 4 cm Layer 46 Grey, medium- to thick-bedded muddy micrite, yielding the ichthyosaur Qianichythyosaurus, the thalattosaur Xinpusaurus, ammonoid Ptychites, fishes and crinoids. 11 cm Layer 45 Upperpart: dark-grey thick-bedded muddy micrite; lower part: grey medium-bedded muddy micrite, yielding the fragmentary thalattosaur skeletons and fishes. 70 cm Layer 44 Dark-grey, thin-bedded muddy micrite, yielding the ichthyosaur candal vertebra, the pachypleurosaur Keichousaurus, ammonoid Ptychites and Detoniceras, fishes, crinoids and bivalves. 32 cm Layer 43 Grey, medium- to thick-bedded muddy micrite, yielding ammonoid Ptychites, fishes and bivalves. 33 cm Layer 42 Medium-bedded laminated micrite, yielding the fragmentary ichthyosaur skeletons, fishes, ammonoids Parasturia and Detoniceras. 7 cm Layer 41 Grey, medium-bedded muddy micrite, yielding the fragmentary ichthyosaur skeletons, fishes. 30 cm Layer 40 Grey, medium-bedded muddy micrite, yielding fishes and bivalves. 10 cm Layer 39 Grey, medium- to thick-bedded muddy micrite, yielding fishes, bivalves, 8
ammonoids Ptychites and Detoniceras. 20 cm Layer 38 Grey, medium- to thick-bedded muddy micrite with bioclastics, yielding fishes, bivalves. 32 cm Layer 37 Dark-grey, laminated limestone with bioclastics, yielding the pistosauroid Wangosaurus and Yunguisaurus, fishes, bivalves. 13 cm Layer 36 Grey, medium- to thick-bedded muddy micrite, yielding rich bivalve fossils. 36 cm Layer 35 Dark-grey, laminated muddy micrite (1-4 cm each layer), yielding the pachypleurosaur Keichousaurus, fishes. 30 cm Layer 34 Greyish-yellow, thin-bedded muddy micrite; yielding the pachypleurosaur Keichousaurus. 10 cm Layer 33 Yellowish, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus, fishes. 10 cm Layer 32 Dark-grey, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus and fishes. 19 cm Layer 31 Dark-yellow, laminated muddy micrite (1-3 cm each layer), yielding the pachypleurosaur Keichousaurus and fishes. 20 cm Layer 30 Grey, medium-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus, the nothosaurids Nothosaurus youngi and Lariosaurus xingyiensis. 13-15 cm Layer 29 Grey, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus, fishes. 4 cm Layer 28 Dark-grey, thin to medium-bedded muddy micrite; yielding the pachypleurosaur Keichousaurus and fishes. 14 cm Layer 27 Grey, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus. 4.5 cm Layer 26 Grey, thin to medium-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus. 19 cm
4. Correlation, dating, and two major faunal horizons of Xingyi Fauna and 9
Guanling Biota yielded at the Nimaigu Section The fossiliferous level of the Xingyi Fauna at the Nimaigu section lies in the middle part of the Zhuganpo Member of the Falang Formation; its top is about 48 m below the base of the Wayao Member of the Falang Formation (Sun et al., 2016; Fig. 1B). Zou et al. (2015a) recognized three ammonoid biozones at this section, including Haoceras xingyiensis Zone, Trachyceras beds and Trachyceras multituberculatum Zone in ascending order. The Haoceras xingyiensis Zone, whose base is about 0.6 m above the fossiliferous level of the Xingyi Fauna (from Layer 57 to Layer 88), can be partly correlated to the North American lower Sutherlandi Zone, which indicates that the age of the Xingyi Fauna is middle Late Ladinian (Zou et al., 2015a). This is also supported by the U-Pb dating on the Zircon samples from tuff at the bottom of Layer 47, 240.8 ± 1.8 Ma, according to the 2016 International Chronostratigraphic Chart (Li, Z.G. et al., 2016b). The Trachyceras beds corresponding to Layers 197 to 235, and the Trachyceras multituberculatum Zone which is above Layer 236, are attributed to the lower Carnian with confidence as Trachyceras is an undisputed Carnian genus (Tozer, 1967, 1994; Mietto and Manfrin, 1995; Balini et al., 2001, 2012; Mietto et al., 2012). Recently, a complete skeleton of the ichthyosaur Qianichthyosaurus, an important marine reptile taxon originally found and previously only known in the younger Guanling Biota, was excavated from Layer 230 (attributing to the Trachyceras beds) in the upper part of the Zhuganpo Member, about 2.5 m below the stratigraphic horizon of the Guanling Biota (in the Trachyceras multituberculatum Zone, Fig. 1B). This new finding probably indicates that the Nimaigu section possibly yields two major faunal horizons, i.e., the middle Late Ladinian Xingyi Fauna and the above younger Carnian Guanling Biota. However, between the two major faunal horizons, three ammonoid biozones of upper Ladinian and the lowest zone of the Carnian (namely Maclearni, Sutherlandi and Daxatina Zones in ascending order) have not been established at this section (Tozer, 1967, 1994; Zou et al., 2015a).
5. Two assemblages of the Xingyi Fauna The fossiliferous level of the Xingyi Fauna ranges from Layer 26 to Layer 53 at 10
the Nimaigu section. Based on the fossils that we recorded, the sauropterygian materials only appeared in the lower part of the fossiliferous level, including nothosaurians (Layer 30) and Keichousaurus (Layers 26 to 35, and 44). Among them, specimens of Keichousaurus were abundant and appeared continuously between Layers 26 and 35 but rarely seen above that range, with only two specimens excavated from Layer 44. Two well-articulated sauropterygian skeletons, i.e., Nothosaurus youngi and Lariosaurus xingyiensis, were collected from Layer 30. Ichthyosaurs, thalattosaurs, and pistosaurs were collected from the upper part of the fossiliferous level (Layers 37 to 53). They include several well-articulated ichthyosaur skeletons
from
Layer
42
(large
shastasaurid
ichthyosaurs),
Layer
46
(Qianichthyosaurus wushaensis Ji et al. in Yang et al., 2013), and Layer 50 (Guizhouichthyosaurus sp.). Wangosaurus and other pistosaur sauropterygian materials, which can be described as Yunguisaurus, were collected from Layer 37. The thalattosaur Xinpusaurus xingyiensis Li, Z.G. et al., 2016a appeared in Layers 45, 50, and 53 (Fig. 3). Based on the stratigraphic distribution of marine reptiles, two different assemblages of marine reptiles could be recognized. The lower marine reptile assemblage occurs in the beds has been found from Layer 26 to Layer 36, with a total thickness of 1.82 m. These layers are mainly thin grey argillaceous limestone, and gradually changed to medium argillaceous limestone upwards (Zou et al., 2015b). The marine reptiles of this assemblage are abundant and comprise mostly coastal sauropterygians, associated with plenty of fish fossils. Keichousaurus hui and Asialepidotus shingyiensis are the dominated taxa for this assemblage, and there are no large pelagic groups such as large ichthyosaurs found so far. Previous researches mistook this lower assemblage as the whole composition of the Xingyi Fauna, e.g., Wang et al. (2008) who proposed that among the marine reptiles in the Xingyi community, nothosaurians dominate the fauna and considered the whole fauna as a single assemblage.
Fig. 3 here 11
The upper marine reptile assemblage occurs in thick grey medium-thick limestone beds with a total thickness of 3.28 m, corresponding to Layers 37 to 53. In this assemblage the typical oceanic marine reptiles such as the large shastasaurid ichthyosaur Guizhouichthyosaurus and euichthyosaur Qianichthyosaurus appeared, associated with the thalattosaur Xinpusaurus, the pistosaurs Wangosaurus brevirostris and Yunguisaurus, the flying fish Thoracopterus wushaensis Tintori et al. (2012) as well as Marcopoloichthys and plenty of ammonoids and Daonella bivalves. Based on the descriptions above, it is clear that an evident transition from coastal to pelagic ecotype occurred between the lower and upper assemblages (Fig. 2). In the lower assemblage, almost all typical marine reptiles are coastal taxa, e.g., Keichousaurus, Nothosaurus, and Lariosaurus. Viviparity of Keichousaurus has been well-documented and its fully aquatic mode of life was also argued for in terms of its anatomical features of its hip-joints (Cheng et al., 2004; Ji et al., 2010). Therefore, the lower assemblage should be represented by near-shore reptiles of fully aquatic and of various degree of terrestriality. Furthermore, the cyamodontoidea Glyphoderma and other taxa with even stronger terrestrial affinity such as the protorosaurs Macrocnemus, Fuyuansaurus and Tanystropheus were reported from the Zhuganpo Member of Falang Formation at Jiyangshan of Fuyan in Yunnan or other sites close to Wusha. The strata of the aforementioned taxa can be correlated with the layers of the lower assemblage of the Nimaigu Section based on our first hand investigation and therefore they should belong to the assemblage. In contrast, taxa from the upper assemblage show characteristics of open-sea marine reptiles. The distinct tailbend is an important feature for ichthyosaur to cruise in the open ocean, and Qianichthyosaurus wushaensis excavated from this assemblage developed the distinct tailbend (Yang et al., 2013; Fig. 3). Also, a large Guizhouichthyosaurus, reaching 15 m in total length, has been reported in the local community. The large size alone does not guarantee an open sea habitat because the modern gray whales (Eschrichtius robustus) may reach 15 m but are restricted to the coastal waters (Jefferson et al., 1993). However, the vast majority of such large marine tetrapods are oceanic, so it is 12
more likely that this large Guizhouichthyosaurus was oceanic at least to some degree. The change of paleogeographic affinity accompanied the ecological change. All taxa of the lower assemblage show a strong phylogenetic affinity with the West Tethyan Fauna (Rieppel et al., 2000, 2010; Jiang et al., 2009; Ma et al., 2013; Liu et al., 2014; Fig. 4). In the upper assemblage, Guizhouichthyosaurus, Qianichthyosaurus and Xinpusaurus, which were previously only known in the Guanling Biota (Early Carnian, Late Triassic), are discovered for the first time in the Late Ladinian strata. The occurrence of these taxa in the upper assemblage of the Xingyi Fauna reveals a close affinity to the younger Guanling Biota (Carnian, Late Triassic) (Jiang et al., 2004, 2005; Wang et al., 2009), which provided physical evidence that the Triassic biosphere had undergone a full recovery from the end Permian mass extinction and reached the peak of biodiversity (Jiang et al., 2005). The ichthyosaurs from the upper assemblage also presents a close phylogenetic affinity with the Carnian (Late Triassic) North-American taxa, such as Guizhouichthyosaurus vs Shastasaurus and Qianichthyosaurus vs Toretocnemus, which indicates a closer paleogeographical relationship between South China and North America (Ji et al., 2016; Fig. 4). Regarding its paleobiogeographic relationship to the North America, therefore, the upper assemblage of the Xingyi Fauna shows the distinguished difference from the lower assemblage, with the taxa which were most likely cruise into the open ocean.
Fig. 4 here
6. Correspondence to global paleoenvironmental changes Marine reptiles emerged in the late Early Triassic and radiated in the Middle Triassic, and the coastal taxa reached the greatest abundance in the Ladinian (Erwin, 1994; Sun et al., 2012; Benton et al., 2014; Motani et al., 2014). Afterward, the evolution of the marine reptiles presented a drastic change around the Ladinian-Carnian boundary, as open-ocean cruisers emerged while most coastal taxa became extinct (Kelley et al., 2014). The Xingyi Fauna is the only example that truly presents this change within its sequence from the lower assemblage to the upper 13
assemblage. It presents a physical evidence to pin down the timing of the transition within the short time frame in the Late Ladinian and not at the Ladinian/Carnian boundary. Global regression (Yin, 1982; Kelley et al., 2014; Fig. 2) and the shrinkage of the nearshore regions probably were the physical factors that led to the decline of coastal taxa and fueled the appearance of pelagic taxa (Kelley et al., 2014; Fig. 2). The same reasoning applies to the appearance of pelagic taxa in the upper assemblage of the Xingyi Fauna. δ13C data is an important marker of paleoenvironmental events (Payne et al., 2004). The curve of δ13C of the Nimaigu Section exhibits a sudden negative excursion at fossil beds (Zou et al., 2015b; Fig. 2) that also ends abruptly toward the top of these beds. Similar excursions are also present in other sections in the Xingyi area (Cheng, 2015), and at the Aghia Marina section and Monte San Giorgio section from the West Tethys (Muttoni et al., 2014). It suggests that a large-scale change in ocean chemistry occurred at least across the Tethyan region, and possibly globally, coinciding with the ecological transition of marine reptiles and global marine regression. A 2-3 mm thick bentonite bed was preserved in Layer 47 of the Nimaigu section, and the volcanic sediments also developed in the Monte San Giorgio area (Muttoni et al., 2014), which suggested the strong volcanic activities during this age. Therefore, the volcanic activities, global regression, and the paleoenvironmental change indicated by the negative excursion of δ13C, accompanied the ecological transition of the marine reptiles from the coast to open ocean, as revealed by the sequence from the lower assemblage to the upper assemblage of the Xingyi Fauna.
7. Conclusion The Nimaigu Section in Xingyi possibly includes two major fossiliferous units, i.e., the middle Late Ladinian Xingyi Faunal unit and the above younger Carnian Guanling Biotic unit; the latter is represented by its basal part and occurs about 45-m above the former. The fossiliferous unit of the Xingyi fauna, which is 5.1 m thick, can be subdivided into two subunits containing two well-distinguished reptilian assemblages, i.e., the lower assemblage consisting mainly of nearshore taxa, with a 14
strong affinity to coeval European marine reptiles (western Tethys), and the upper assemblage mainly of pelagic taxa with a closer relationship to the reptiles of Guanling Biota and North American (eastern Pacific) clades. This change within the Xingyi Fauna suggests a major faunal turnover, which may have indicated the transition of ecological types from nearshore to offshore in association with the paleoenvironmental changes such as global sea level variation and volcanic activities.
Acknowledgements Yun-Zhong Wang, Tian-Fen Hu, and Tetsuya Sato helped prepare the marine reptiles that were collected during excavations. Peng-Fei Yang, Hun-Qin Lin, Le-Tian Ma, Xiao-Dong Zou, Wen-Qi Chen, Li Jiang, and Zhi-Guang Li (students graduated from Peking University) participated the joint excavations during 2009–2012. This study was financially supported by Science and Technology Project in Guizhou Province of China under Support No. 2094 [2016], Ministry of Science and Technology (grant 2016YFC0503301), NSFC (grants 41572008, 41372016, 40920124002), State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (No. 123107, 143108), and China Geological Survey (No. 121201102000150012-09). Chun Li, Xiao-Chun Wu, and the editors are appreciated for their constructive comments, which significantly improved the manuscript.
15
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Fig. 1. (A) Geological setting; paleogeographic map of southern China during the Middle Triassic (modified after Liu et al., 2013). (B) Photograph of Nimaigu section, Zhuganpo Member of Falang Formation, Late Ladinian.
25
Fig. 2. Two different assemblages of marine reptiles in Xingyi fanua. Upper assemblage consisted of mainly pelagic groups: (A) Guizhouichthyosaurus, scale bar = 40 cm; (B) Yunguisaurus, scale bar = 20 cm; (C) Qianichthyosaurus, scale bar = 20 cm; (D) Wangosaurus brevirostris, scale bar = 20 cm; (E) “fly fish” Thoracopterus, scale bar = 4 cm; (F) Xinpusaurus xingyiensis, scale bar = 20 cm. Lower assemblage consisted of mainly nearshore reptiles: (G) Keichousaurus hui, scale bar = 4 cm; (H) Asialepidotus shingyiensis, scale bar = 4 cm; (I) Nothosaurus yangi, scale bar = 20 cm; (J) Lariosaurus xingyiensis, scale bar = 10 cm.
26
Fig. 3. (A) Marine reptile and species diversity in the early Mesozoic (Triassic–Early Jurassic); species is subdivided into ecological groups: black bars show number of durophagous species, blue bars show number of pelagic species, gray bar shows all others, and combined bars represent total marine reptile diversity within each time bin (quoted from Kelley et al., 2014); third-order transgression/regression cycles (~5-10 Ma) and second-order sea-level trends (~50 Ma) are taken from Hardenbol et al. (1998) and Kelley et al. (2014). (B) Carbon isotope curves from the Nimaigu section (quoted from Zou et al., 2015b). (C) The stratigraphic distribution of marine reptiles, fishes and ammonoids at the Nimaigu section.
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Fig. 4. Marine reptiles palaeobiogeographical distribution in the Late Ladinian (modified from C. Scotese’s paleomap project; http://www.scotese.com).
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S1871-174X(17)30031-8 http://dx.doi.org/doi:10.1016/j.palwor.2017.05.005 PALWOR 418
To appear in:
Palaeoworld
Received date: Revised date: Accepted date:
17-2-2017 25-4-2017 22-5-2017
Please cite this article as: Lu, Hao, Jiang, Da-Yong, Motani, Ryosuke, Ni, Pei-Gang, Sun, Zuo-Yu, Tintori, Andrea, Xiao, Shi-Zhen, Zhou, Min, Ji, Cheng, Fu, Wan-Lu, Middle Triassic Xingyi Fauna: showing turnover of marine reptiles from coastal to oceanic environments.Palaeoworld http://dx.doi.org/10.1016/j.palwor.2017.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Middle Triassic Xingyi Fauna: showing turnover of marine reptiles from coastal to oceanic environments
Hao Lu a, Da-Yong Jiang
a
*, Ryosuke Motani b, Pei-Gang Ni a, Zuo-Yu Sun a,
Andrea Tintori c, Shi-Zhen Xiao d, Min Zhou a, Cheng Ji e, Wan-Lu Fu f
a
Laboratory of Orogenic Belt and Crustal Evolution, Ministry of Education and
Department of Geology and Geological Museum, Peking University, Beijing 100871, China b
Department of Earth and Planetary Sciences, University of California, Davis, CA
95616-8605, USA [E-mail address: [email protected]] c
Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via
Mangiagalli 34-20133 Milano, Italy [E-mail address: [email protected]] d
School of Karst Science, Guizhou Normal University, Guiyang 550001, China
[E-mail address: [email protected]] e
State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of
Geology and Palaeontology, CAS, Nanjing 210008, China [E-mail address: [email protected]] f
Business School, Tianjin University of Commerce, Tianjin 300134, China [E-mail
address: [email protected]]
* Corresponding author. E-mail address: [email protected]
Abstract The late Ladinian (Middle Triassic) Xingyi Fauna from the Zhuganpo Member of the Falang Formation yields abundant and well-preserved marine reptiles. Bed-by-bed excavation at Wusha in Xingyi of Guizhou Province reveals two marine vertebrate assemblages in a fossiliferous horizons that span 5.1 m in total thickness. The lower assemblage
is
marked
by
the
near-shore
sauropterygians,
including
the
pachypleurosaur Keichousaurus, the nothosaurians Nothosaurus and Lariosaurus, 1
with a strong paleobiogeographic affinity to western Tethys. The upper assemblage consists of oceanic ichthyosaurs and pistosaurs, including the large shastasaurid ichthyosaur Guizhouichthyosaurus, the euichthyosaur Qianichthyosaurus, pistosaurs Yunguisaurus and Wangosaurus, and the thalattosaur Xinpusaurus, with a closer paleobiogeographic affinity to North America. The coastal pachypleurosaur and nothosaurid sauropterygians disappeared in the upper assemblage, suggesting that they were replaced by an oceanic marine reptile community that emerged. The reptilian composition of the upper assemblage is similar to that of the Guanling Biota, which is of the Early Carnian (Late Triassic) in age and thus somewhat younger than the Xingyi Fauna. The ecological turnover of marine reptiles from near-shore to the open ocean community corresponds to the paleoenvironmental changes indicated by lithofacies analysis, δ13C and the global sea level changes.
Keywords: Xingyi Fauna; marine reptile; Middle Triassic; Guizhou; China
1. Introduction The Middle Triassic Xingyi Fauna, containing exceptionally well-preserved marine reptiles and fishes (Young, 1958; Su, 1959; Wang et al., 2001; Li and Jin, 2003), has been providing knowledge on the taxonomy, biodiversity, paleobiology and evolution of Triassic marine reptiles, fishes, ammonoids and other clades, and also on the paleobiogeography (Jiang, 2002; Li et al., 2002; Liu et al., 2003; Cheng et al., 2004, 2006, 2012; Li and Rieppel, 2004; Rieppel et al., 2006, 2010; Li, 2007; Li et al., 2007; Zhao et al., 2008a, 2008b; Sato et al., 2010, 2013; Tintori et al., 2012, 2015; Xu et al., 2012, 2015; Benton et al., 2013; Yang et al., 2013; Ji et al., 2014; Ma et al., 2015; Xue et al., 2015; Zou et al., 2015a, 2015b; Sun et al., 2016; Fig. 1A). The research of Xingyi Fauna dates back to 1957, when the first Mesozoic marine reptile fossil in Asia was found by Hu at Lüyin Village, Dingxiao District, Xingyi City of Guizhou Province, and was named by Young (1958) as Keichousaurus hui. Subsequently, Su (1959) reported three new taxa of fishes from the same locality, including Peltopleurus orientalis, Sinoeugnathus kueichowensis and Asialepidotus 2
shingyiensis. However, over the next four decades, a few reptiles were reported (Young, 1965a, 1965b), while abundant specimens of Keichousaurus and Asialepidotus presumably disappeared into private hands.
Fig. 1 here
The situation changed in the late 1990s, when Keichousaurus hui and Shingyisaurus unexpectus (Nothosaurus sp. following Rieppel, 1998) were redescribed in detail by Lin and Rieppel (1998) and Rieppel (1998), and diverse fossils of reptiles and fishes were excavated from the Zhuganpo Member of Falang Formation (Wang et al., 2001; Sun et al., 2016 (Zhuganpo Formation)). Almost all major clades of Triassic marine reptiles have their representatives found in the Xingyi Fauna. New taxa include the placodont Glyphoderma kangi Zhao et al., 2008a, the eosauropterygian Qianxisaurus chajiangensis Cheng et al., 2012, the nothosaurids Lariosaurus xingyiensis Li et al., 2002 and Nothosaurus youngi Li and Rieppel, 2004, the pistosaurs Yunguisaurus liae Cheng et al., 2006 and Wangosaurus brevirostris Ma et al., 2015, the thalattosaurs Anshunsaurus wushaensis Rieppel et al., 2006, A. huangnihensis Cheng et al., 2007 and Xinpusaurus xingyiensis Li, Z.G. et al., 2016a, the euichthyosaur Qianichthyosaurus wushaensis Yang et al., 2013, the protorosaurs Tanystropheus cf. T. longobardicus (Rieppel et al., 2010), Tanystropheus sp. (Li, 2007), Macrocnemus fuyuanensis Li et al., 2007 and the archosaur Diandongosuchus fuyuanensis Li et al., 2012 and Litorosuchus somnii Li, C. et al., 2016. Some new materials remain under study, such as the large shastasaurid ichthyosaur Guizhouichthyosaurus. The age of Xingyi Fauna was confirmed to be late Ladinian (Middle Triassic) by the ammonoid biostratigraphic correlation and isotope dating (Zou et al., 2015a; Li, Z.G. et al., 2016b; Sun et al., 2016), which proposed that the Xingyi Fauna is the only known late Ladinian (Middle Triassic) Fossil Lagerstätte characterized by exceptionally well-preserved marine reptiles and fishes in the world (Ji et al., 2012). Compared to the Late Ladinian Kalkschieferzone of the famous Area (Stockar et al., 3
2012), the Xingyi Fauna is much more diversified regarding reptiles as in the Alps only Lariosaurus has been recorded (Tintori and Renesto, 1990) owing probably to a somewhat different environment (Lombardo et al., 2012). Since August of 2011, the excavation at Nimaigu village in Wusha of Xingyi City has been carried out by the internationally collaborative team composed of Peking University, the University of California (Davis), Università degli Studi di Milano, supported by City Land and Resources Bureau of Xingyi. More than 1000 square meters of the surface of 5.1 m thick fossil-bearing layers of the Xingyi Fauna were exposed bed-by-bed and exact information of excavated individual fossils were recorded (Fig. 2). A complete stratigraphic section from the Zhuganpo Member up to the base of Wayao Member of Falang Formation was measured, again bed by bed, and rock samples from each bed were collected. The excavation disproved some of the early perceptions about the Xingyi Fauna. Most of the taxa of the Xingyi Fauna have been erected based on specimens originally collected by amateurs without precise and detailed geographic and stratigraphic information. The lack of information resulted in a wrong assumption to consider all taxa to represent a single assemblage from a single fossiliferous stratigraphic zone. Biogeographically, the Xingyi Fauna was known to show a strong affinity to Middle Triassic faunas from the West Tethys and consists mainly of marine reptiles that inhabited nearshore environments (Rieppel et al., 2000, 2010; Li and Jin, 2003; Li et al., 2007). However, the excavation revealed that the fossiliferous level of Xingyi Fauna can be subdivided into two parts.
Fig. 2 here
The lower part, yielding the Lower Assemblage recognized in this paper, previously meant the whole “Xingyi Fauna” and known as the typical Ladinian fauna of Tethyan affinity, contains mainly sauropterygians, sharing genera with the lower Anisian Panxian and Luoping faunas, such as Nothosaurus, Lariosaurus and possibly Keichousaurus. In the Upper Assemblage yielded in the upper part of the fossiliferous 4
level, middle-large sized ichthyosaurs, pistosaurs and thalattosaurs appear, including Qianichthyosaurus,
Guizhouichthyosaurus,
Wangosaurus,
Yunguisaurus,
and
Xinpusaurus (Rieppel et al., 2006; Yang et al., 2013; Ma et al., 2015; Li, Z.G. et al., 2016a), among which Qianichthyosaurus, Guizhouichthyosaurus and Xinpusaurus were previously only reported in the younger Guanling Biota (Early Carnian of Late Triassic). These ichthyosaurs are oceanic animals (Ji et al., 2010) and suggest a trans-Pacific affinity (Jiang et al., 2004). Several fishes previously only known in the Carnian (Late Triassic) of the West Tethys were also recorded in this assemblage of the Xingyi Fauna, e.g., flying fishes Thoracopterus Tintori et al. (2012) and stem-group Teleostei Malingichthys Tintori et al. (2015). The subdivision of the fossiliferous level reveals that the Xingyi Fauna indeed comprises two different marine vertebrate assemblages consisting of different marine reptiles and fishes. Ma et al. (2013) gave a brief lithological description of the Nimaigu section and recognized the taxonomic difference between the marine vertebrate assemblages, but they did not point out the faunal turnover and discuss its paleogeographic background. Since that study, additional materials have been found by further excavations and new studies on microfacies and geochemical analysis of the section have been conducted. Therefore, with new information, we are able to detail the stratigraphic distribution of the marine vertebrate fossils, confirm the presence of the two distinct assemblages and correlate them with the lithofacies, δ13C data and the global sea level changes. It can be concluded that the two assemblages suggest a major faunal change, which probably indicated a turnover of ecological setting from nearshore to offshore. These assemblages are possibly related to different ecosystems, representing transitional links between the earlier Panxian-Luoping Fauna (Anisian) (Motani et al., 2008; Jiang et al., 2009; Hu et al., 2011) and the later Guanling Biota (Carnian). It hints a faunal turnover and to confirm it requires a thorough investigation of the biostratigraphy of marine vertebrates in other sections within the area.
2. Material and methods All samples studied in this paper were collected from the middle-upper part of the 5
Zhuganpo Member to the base of the Wayao Member of the Falang Formation, at Nimaigu, Wusha District, Xingyi City of Guizhou Province. The section is measured 48.5 m in thickness, comprising 214 natural layers that correspond from the base Layer 26 to the top Layer 239 according to our field numbering system (Fig. 1B). The fossiliferous level of the Xingyi Fauna ranges from Layer 26 to Layer 53 and is 5.1 m thick (Fig. 1B). More than 800 skeletons of marine reptiles, including sauropterygians, ichthyosaurs and thalattosaurs, were collected during the excavation and were preserved and prepared in the Museum of Xingyi National Geopark. All vertebrate fossils were prepared using air chisels. 94 rock samples were collected and δ13C data measured by MAT-253 Stable Isotope Ratio Mass Spectrometer in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. Thin sections for microfacies analysis were made in the Laboratory of Orogenic Belt and Crustal Evolution of Ministry of Education at Peking University. Horizons were recorded and assemblages of marine vertebrates were established following the method of Motani et al. (2008) and Jiang et al. (2009) for the Anisian Panxian Fauna. The analysis and comparison of ecological strategies of global marine reptiles mainly refer to Kelley et al. (2014). Microfacies analysis of carbonates and reconstruction of paleoenvironmental conditions follow Yu (1989) and Flügel (2004).
3. Sequence of the middle-upper Zhuganpo Member, Falang Formation The Nimaigu section is located at 25°09′52″N, 104°47′19″E with an altitude of about 1491 m (Fig. 1B). The description of the section in descending order is as below. Data of ammonoids and zircon U-Pb datings are derived from Zou et al. (2015a) and Li, Z.G. et al. (2016b), respectively.
Overlying: Wayao Member of the Falang Formation Layers 236-239 Dark-grey, thick-bedded muddy limestone, marls, interbedded with black shales, yielding ammonoids Trachyceras multituberculatum and abundant bivalves Halobia. > 16 m (not reach the top) —conformity— 6
Zhuganpo Member of the Falang Formation Layers 227-235 Grey to dark-grey, medium -bedded limestone, containing ammonoid Trachyceras multituberculatum (bed 235) and the ichthyosaur Qianichthyosaurus (bed 230). 3.8 m Layers 194-226 Grey, medium-bedded limestone with cherts intercalated with thick-bedded limestone, containing ammonoid species of Trachyceras. 8.8 m Layers 170-193 Grey, medium- to thick-bedded limestone, intercalated with medium-bedded argillaceous banded limestone. 6.6 m Layers 139-169 Dark-grey, medium-bedded limestone with cherts intercalated with grey to dark-grey thin- to medium-bedded argillaceous banded limestone, containing ammonoids Clionitites sp. indet. (bed 162), Ptychites sp. A (bed 154), Yangites densicostatus (bed 152). The age of 161 layer is 239.8 ± 1.7 Ma through the U-Pb analysis. 6.5 m Layers 97-138 Dark-grey, thin- to medium-bedded argillaceous banded limestone intercalated with gray medium- to thick-bedded limestone, containing ammonoids Yangites densicostatus, Sinomeginoceras wangi (bed 98). 9.7 m Layers 54-96 Grey, thin- to medium-bedded limestone intercalated with thin- to medium-bedded nodular limestone, containing ammonoids Haoceras xingyiense (beds 57, 58, 59, 68, 88), Yangites densicostatus (beds 59, 65), Sinomeginoceras wangi (beds 68, 70), Sinomeginoceras xingyiense (bed 71), Xenoprotrachyceras cf. primum (bed 59), Detoniceras sp. A (beds 72, 88). 7.7 m Layers 26-53 Grey, thin- to medium-bedded argillaceous banded limestone, marl and nodular limestone intercalated with medium- to thick-bedded limestone, yielding abundant marine reptiles and fishes associated with bivalves, ammonoids and crinoids. > 5.1 m (not measured to the bottom)
The sequence of the fossil-bearing layers in descending order is as follows: Layer 53 Dark-grey, medium- to thick-bedded bioclastic micrite, yielding the shastasaurid
ichthyosaur
Guizhouichthyosaurus,
the
thalattosaur
xingyiensis, ammonoid Ptychites, crinoids and bivalves. 26 cm 7
Xinpusaurus
Layer 52 Dark-grey, thin-bedded muddy micrite with bioclastics (1-3 cm each layer), yielding crinoids and fossil fishes. 10-12 cm Layer 51 Grey, thin-bedded laminated micrite. 2 cm Layer 50 Light-grey, medium to thick-bedded muddy micrite with bioclastics, yielding the ichthyosaur Qianichythyosaurus and Guizhouichthyosaurus, the thalattosaur Xinpusaurus and bivalves. 60 cm Layer 49 Dark-grey, thin-bedded muddy micrite with bioclastics, yielding fragments of marine reptiles, fishes, ammonoids Parasturis and Ptychites, bivalves and crinoids. 5 cm Layer 48 Grey, medium-bedded laminated micrite, yielding fishes, bivalves and ammonoid Detoniceras. 13 cm Layer 47 Grey, thin-bedded muddy micrite with bioclastics, yielding fishes and bivalves. We obtained an age of 240.8 ± 1.8 Ma through the U-Pb analysis. 4 cm Layer 46 Grey, medium- to thick-bedded muddy micrite, yielding the ichthyosaur Qianichythyosaurus, the thalattosaur Xinpusaurus, ammonoid Ptychites, fishes and crinoids. 11 cm Layer 45 Upperpart: dark-grey thick-bedded muddy micrite; lower part: grey medium-bedded muddy micrite, yielding the fragmentary thalattosaur skeletons and fishes. 70 cm Layer 44 Dark-grey, thin-bedded muddy micrite, yielding the ichthyosaur candal vertebra, the pachypleurosaur Keichousaurus, ammonoid Ptychites and Detoniceras, fishes, crinoids and bivalves. 32 cm Layer 43 Grey, medium- to thick-bedded muddy micrite, yielding ammonoid Ptychites, fishes and bivalves. 33 cm Layer 42 Medium-bedded laminated micrite, yielding the fragmentary ichthyosaur skeletons, fishes, ammonoids Parasturia and Detoniceras. 7 cm Layer 41 Grey, medium-bedded muddy micrite, yielding the fragmentary ichthyosaur skeletons, fishes. 30 cm Layer 40 Grey, medium-bedded muddy micrite, yielding fishes and bivalves. 10 cm Layer 39 Grey, medium- to thick-bedded muddy micrite, yielding fishes, bivalves, 8
ammonoids Ptychites and Detoniceras. 20 cm Layer 38 Grey, medium- to thick-bedded muddy micrite with bioclastics, yielding fishes, bivalves. 32 cm Layer 37 Dark-grey, laminated limestone with bioclastics, yielding the pistosauroid Wangosaurus and Yunguisaurus, fishes, bivalves. 13 cm Layer 36 Grey, medium- to thick-bedded muddy micrite, yielding rich bivalve fossils. 36 cm Layer 35 Dark-grey, laminated muddy micrite (1-4 cm each layer), yielding the pachypleurosaur Keichousaurus, fishes. 30 cm Layer 34 Greyish-yellow, thin-bedded muddy micrite; yielding the pachypleurosaur Keichousaurus. 10 cm Layer 33 Yellowish, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus, fishes. 10 cm Layer 32 Dark-grey, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus and fishes. 19 cm Layer 31 Dark-yellow, laminated muddy micrite (1-3 cm each layer), yielding the pachypleurosaur Keichousaurus and fishes. 20 cm Layer 30 Grey, medium-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus, the nothosaurids Nothosaurus youngi and Lariosaurus xingyiensis. 13-15 cm Layer 29 Grey, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus, fishes. 4 cm Layer 28 Dark-grey, thin to medium-bedded muddy micrite; yielding the pachypleurosaur Keichousaurus and fishes. 14 cm Layer 27 Grey, thin-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus. 4.5 cm Layer 26 Grey, thin to medium-bedded muddy micrite, yielding the pachypleurosaur Keichousaurus. 19 cm
4. Correlation, dating, and two major faunal horizons of Xingyi Fauna and 9
Guanling Biota yielded at the Nimaigu Section The fossiliferous level of the Xingyi Fauna at the Nimaigu section lies in the middle part of the Zhuganpo Member of the Falang Formation; its top is about 48 m below the base of the Wayao Member of the Falang Formation (Sun et al., 2016; Fig. 1B). Zou et al. (2015a) recognized three ammonoid biozones at this section, including Haoceras xingyiensis Zone, Trachyceras beds and Trachyceras multituberculatum Zone in ascending order. The Haoceras xingyiensis Zone, whose base is about 0.6 m above the fossiliferous level of the Xingyi Fauna (from Layer 57 to Layer 88), can be partly correlated to the North American lower Sutherlandi Zone, which indicates that the age of the Xingyi Fauna is middle Late Ladinian (Zou et al., 2015a). This is also supported by the U-Pb dating on the Zircon samples from tuff at the bottom of Layer 47, 240.8 ± 1.8 Ma, according to the 2016 International Chronostratigraphic Chart (Li, Z.G. et al., 2016b). The Trachyceras beds corresponding to Layers 197 to 235, and the Trachyceras multituberculatum Zone which is above Layer 236, are attributed to the lower Carnian with confidence as Trachyceras is an undisputed Carnian genus (Tozer, 1967, 1994; Mietto and Manfrin, 1995; Balini et al., 2001, 2012; Mietto et al., 2012). Recently, a complete skeleton of the ichthyosaur Qianichthyosaurus, an important marine reptile taxon originally found and previously only known in the younger Guanling Biota, was excavated from Layer 230 (attributing to the Trachyceras beds) in the upper part of the Zhuganpo Member, about 2.5 m below the stratigraphic horizon of the Guanling Biota (in the Trachyceras multituberculatum Zone, Fig. 1B). This new finding probably indicates that the Nimaigu section possibly yields two major faunal horizons, i.e., the middle Late Ladinian Xingyi Fauna and the above younger Carnian Guanling Biota. However, between the two major faunal horizons, three ammonoid biozones of upper Ladinian and the lowest zone of the Carnian (namely Maclearni, Sutherlandi and Daxatina Zones in ascending order) have not been established at this section (Tozer, 1967, 1994; Zou et al., 2015a).
5. Two assemblages of the Xingyi Fauna The fossiliferous level of the Xingyi Fauna ranges from Layer 26 to Layer 53 at 10
the Nimaigu section. Based on the fossils that we recorded, the sauropterygian materials only appeared in the lower part of the fossiliferous level, including nothosaurians (Layer 30) and Keichousaurus (Layers 26 to 35, and 44). Among them, specimens of Keichousaurus were abundant and appeared continuously between Layers 26 and 35 but rarely seen above that range, with only two specimens excavated from Layer 44. Two well-articulated sauropterygian skeletons, i.e., Nothosaurus youngi and Lariosaurus xingyiensis, were collected from Layer 30. Ichthyosaurs, thalattosaurs, and pistosaurs were collected from the upper part of the fossiliferous level (Layers 37 to 53). They include several well-articulated ichthyosaur skeletons
from
Layer
42
(large
shastasaurid
ichthyosaurs),
Layer
46
(Qianichthyosaurus wushaensis Ji et al. in Yang et al., 2013), and Layer 50 (Guizhouichthyosaurus sp.). Wangosaurus and other pistosaur sauropterygian materials, which can be described as Yunguisaurus, were collected from Layer 37. The thalattosaur Xinpusaurus xingyiensis Li, Z.G. et al., 2016a appeared in Layers 45, 50, and 53 (Fig. 3). Based on the stratigraphic distribution of marine reptiles, two different assemblages of marine reptiles could be recognized. The lower marine reptile assemblage occurs in the beds has been found from Layer 26 to Layer 36, with a total thickness of 1.82 m. These layers are mainly thin grey argillaceous limestone, and gradually changed to medium argillaceous limestone upwards (Zou et al., 2015b). The marine reptiles of this assemblage are abundant and comprise mostly coastal sauropterygians, associated with plenty of fish fossils. Keichousaurus hui and Asialepidotus shingyiensis are the dominated taxa for this assemblage, and there are no large pelagic groups such as large ichthyosaurs found so far. Previous researches mistook this lower assemblage as the whole composition of the Xingyi Fauna, e.g., Wang et al. (2008) who proposed that among the marine reptiles in the Xingyi community, nothosaurians dominate the fauna and considered the whole fauna as a single assemblage.
Fig. 3 here 11
The upper marine reptile assemblage occurs in thick grey medium-thick limestone beds with a total thickness of 3.28 m, corresponding to Layers 37 to 53. In this assemblage the typical oceanic marine reptiles such as the large shastasaurid ichthyosaur Guizhouichthyosaurus and euichthyosaur Qianichthyosaurus appeared, associated with the thalattosaur Xinpusaurus, the pistosaurs Wangosaurus brevirostris and Yunguisaurus, the flying fish Thoracopterus wushaensis Tintori et al. (2012) as well as Marcopoloichthys and plenty of ammonoids and Daonella bivalves. Based on the descriptions above, it is clear that an evident transition from coastal to pelagic ecotype occurred between the lower and upper assemblages (Fig. 2). In the lower assemblage, almost all typical marine reptiles are coastal taxa, e.g., Keichousaurus, Nothosaurus, and Lariosaurus. Viviparity of Keichousaurus has been well-documented and its fully aquatic mode of life was also argued for in terms of its anatomical features of its hip-joints (Cheng et al., 2004; Ji et al., 2010). Therefore, the lower assemblage should be represented by near-shore reptiles of fully aquatic and of various degree of terrestriality. Furthermore, the cyamodontoidea Glyphoderma and other taxa with even stronger terrestrial affinity such as the protorosaurs Macrocnemus, Fuyuansaurus and Tanystropheus were reported from the Zhuganpo Member of Falang Formation at Jiyangshan of Fuyan in Yunnan or other sites close to Wusha. The strata of the aforementioned taxa can be correlated with the layers of the lower assemblage of the Nimaigu Section based on our first hand investigation and therefore they should belong to the assemblage. In contrast, taxa from the upper assemblage show characteristics of open-sea marine reptiles. The distinct tailbend is an important feature for ichthyosaur to cruise in the open ocean, and Qianichthyosaurus wushaensis excavated from this assemblage developed the distinct tailbend (Yang et al., 2013; Fig. 3). Also, a large Guizhouichthyosaurus, reaching 15 m in total length, has been reported in the local community. The large size alone does not guarantee an open sea habitat because the modern gray whales (Eschrichtius robustus) may reach 15 m but are restricted to the coastal waters (Jefferson et al., 1993). However, the vast majority of such large marine tetrapods are oceanic, so it is 12
more likely that this large Guizhouichthyosaurus was oceanic at least to some degree. The change of paleogeographic affinity accompanied the ecological change. All taxa of the lower assemblage show a strong phylogenetic affinity with the West Tethyan Fauna (Rieppel et al., 2000, 2010; Jiang et al., 2009; Ma et al., 2013; Liu et al., 2014; Fig. 4). In the upper assemblage, Guizhouichthyosaurus, Qianichthyosaurus and Xinpusaurus, which were previously only known in the Guanling Biota (Early Carnian, Late Triassic), are discovered for the first time in the Late Ladinian strata. The occurrence of these taxa in the upper assemblage of the Xingyi Fauna reveals a close affinity to the younger Guanling Biota (Carnian, Late Triassic) (Jiang et al., 2004, 2005; Wang et al., 2009), which provided physical evidence that the Triassic biosphere had undergone a full recovery from the end Permian mass extinction and reached the peak of biodiversity (Jiang et al., 2005). The ichthyosaurs from the upper assemblage also presents a close phylogenetic affinity with the Carnian (Late Triassic) North-American taxa, such as Guizhouichthyosaurus vs Shastasaurus and Qianichthyosaurus vs Toretocnemus, which indicates a closer paleogeographical relationship between South China and North America (Ji et al., 2016; Fig. 4). Regarding its paleobiogeographic relationship to the North America, therefore, the upper assemblage of the Xingyi Fauna shows the distinguished difference from the lower assemblage, with the taxa which were most likely cruise into the open ocean.
Fig. 4 here
6. Correspondence to global paleoenvironmental changes Marine reptiles emerged in the late Early Triassic and radiated in the Middle Triassic, and the coastal taxa reached the greatest abundance in the Ladinian (Erwin, 1994; Sun et al., 2012; Benton et al., 2014; Motani et al., 2014). Afterward, the evolution of the marine reptiles presented a drastic change around the Ladinian-Carnian boundary, as open-ocean cruisers emerged while most coastal taxa became extinct (Kelley et al., 2014). The Xingyi Fauna is the only example that truly presents this change within its sequence from the lower assemblage to the upper 13
assemblage. It presents a physical evidence to pin down the timing of the transition within the short time frame in the Late Ladinian and not at the Ladinian/Carnian boundary. Global regression (Yin, 1982; Kelley et al., 2014; Fig. 2) and the shrinkage of the nearshore regions probably were the physical factors that led to the decline of coastal taxa and fueled the appearance of pelagic taxa (Kelley et al., 2014; Fig. 2). The same reasoning applies to the appearance of pelagic taxa in the upper assemblage of the Xingyi Fauna. δ13C data is an important marker of paleoenvironmental events (Payne et al., 2004). The curve of δ13C of the Nimaigu Section exhibits a sudden negative excursion at fossil beds (Zou et al., 2015b; Fig. 2) that also ends abruptly toward the top of these beds. Similar excursions are also present in other sections in the Xingyi area (Cheng, 2015), and at the Aghia Marina section and Monte San Giorgio section from the West Tethys (Muttoni et al., 2014). It suggests that a large-scale change in ocean chemistry occurred at least across the Tethyan region, and possibly globally, coinciding with the ecological transition of marine reptiles and global marine regression. A 2-3 mm thick bentonite bed was preserved in Layer 47 of the Nimaigu section, and the volcanic sediments also developed in the Monte San Giorgio area (Muttoni et al., 2014), which suggested the strong volcanic activities during this age. Therefore, the volcanic activities, global regression, and the paleoenvironmental change indicated by the negative excursion of δ13C, accompanied the ecological transition of the marine reptiles from the coast to open ocean, as revealed by the sequence from the lower assemblage to the upper assemblage of the Xingyi Fauna.
7. Conclusion The Nimaigu Section in Xingyi possibly includes two major fossiliferous units, i.e., the middle Late Ladinian Xingyi Faunal unit and the above younger Carnian Guanling Biotic unit; the latter is represented by its basal part and occurs about 45-m above the former. The fossiliferous unit of the Xingyi fauna, which is 5.1 m thick, can be subdivided into two subunits containing two well-distinguished reptilian assemblages, i.e., the lower assemblage consisting mainly of nearshore taxa, with a 14
strong affinity to coeval European marine reptiles (western Tethys), and the upper assemblage mainly of pelagic taxa with a closer relationship to the reptiles of Guanling Biota and North American (eastern Pacific) clades. This change within the Xingyi Fauna suggests a major faunal turnover, which may have indicated the transition of ecological types from nearshore to offshore in association with the paleoenvironmental changes such as global sea level variation and volcanic activities.
Acknowledgements Yun-Zhong Wang, Tian-Fen Hu, and Tetsuya Sato helped prepare the marine reptiles that were collected during excavations. Peng-Fei Yang, Hun-Qin Lin, Le-Tian Ma, Xiao-Dong Zou, Wen-Qi Chen, Li Jiang, and Zhi-Guang Li (students graduated from Peking University) participated the joint excavations during 2009–2012. This study was financially supported by Science and Technology Project in Guizhou Province of China under Support No. 2094 [2016], Ministry of Science and Technology (grant 2016YFC0503301), NSFC (grants 41572008, 41372016, 40920124002), State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS) (No. 123107, 143108), and China Geological Survey (No. 121201102000150012-09). Chun Li, Xiao-Chun Wu, and the editors are appreciated for their constructive comments, which significantly improved the manuscript.
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Fig. 1. (A) Geological setting; paleogeographic map of southern China during the Middle Triassic (modified after Liu et al., 2013). (B) Photograph of Nimaigu section, Zhuganpo Member of Falang Formation, Late Ladinian.
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Fig. 2. Two different assemblages of marine reptiles in Xingyi fanua. Upper assemblage consisted of mainly pelagic groups: (A) Guizhouichthyosaurus, scale bar = 40 cm; (B) Yunguisaurus, scale bar = 20 cm; (C) Qianichthyosaurus, scale bar = 20 cm; (D) Wangosaurus brevirostris, scale bar = 20 cm; (E) “fly fish” Thoracopterus, scale bar = 4 cm; (F) Xinpusaurus xingyiensis, scale bar = 20 cm. Lower assemblage consisted of mainly nearshore reptiles: (G) Keichousaurus hui, scale bar = 4 cm; (H) Asialepidotus shingyiensis, scale bar = 4 cm; (I) Nothosaurus yangi, scale bar = 20 cm; (J) Lariosaurus xingyiensis, scale bar = 10 cm.
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Fig. 3. (A) Marine reptile and species diversity in the early Mesozoic (Triassic–Early Jurassic); species is subdivided into ecological groups: black bars show number of durophagous species, blue bars show number of pelagic species, gray bar shows all others, and combined bars represent total marine reptile diversity within each time bin (quoted from Kelley et al., 2014); third-order transgression/regression cycles (~5-10 Ma) and second-order sea-level trends (~50 Ma) are taken from Hardenbol et al. (1998) and Kelley et al. (2014). (B) Carbon isotope curves from the Nimaigu section (quoted from Zou et al., 2015b). (C) The stratigraphic distribution of marine reptiles, fishes and ammonoids at the Nimaigu section.
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Fig. 4. Marine reptiles palaeobiogeographical distribution in the Late Ladinian (modified from C. Scotese’s paleomap project; http://www.scotese.com).
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