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The New Ichthyosauriform Chaohusaurus Brevifemoralis (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China

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The New Ichthyosauriform Chaohusaurus Brevifemoralis (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China The new ichthyosauriform Chaohusaurus brevifemoralis (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China Jian-dong Huang1, Ryosuke Motani2, Da-yong Jiang3, Andrea Tintori4, Olivier Rieppel5, Min Zhou3, Xin-Xin Ren1 and Rong Zhang1 1 Department of Research, Anhui Geological Museum, Hefei, Anhui, People’s Republic of China 2 Department of Earth and Planetary Sciences, University of California, Davis, CA, USA 3 Department of Geology, Peking University, Beijing, People’s Republic of China 4 Dipartimento di Scienze della Terra, Università degli Studi di Milano, Milano, Italia 5 Center of Integrative Research, The Field Museum, Chicago, IL, USA ABSTRACT A new species of ichthyosauriform is recognized based on 20 specimens, including nearly complete skeletons, and named Chaohusaurus brevifemoralis. A part of the specimens was previously identified as Chaohusaurus chaoxianensis and is herein reassigned to the new species. The new species differs from existing species of Chaohusaurus in a suite of features, such as the bifurcation of the caudal peak neural spine and a short femur relative to trunk length. The specimens include both complete and partially disarticulated skulls, allowing rigorous scrutiny of cranial sutures. For example, the squamosal does not participate in the margin of the upper temporal fenestra despite previous interpretations. Also, the frontal unequivocally forms a part of the anterior margin of the upper temporal fenestra, forming the most medial part of the anterior terrace. The skull of the holotype largely retains three-dimensionality with the scleral rings approximately in situ, revealing that the eyeball was uncovered in two different directions, that is, laterally and slightly dorsally through the main part of the orbit, and dorsally through the medial Submitted 22 November 2018 Accepted 27 July 2019 extension of the orbit into the skull roof. This skull construction is likely a basal Published 9 September 2019 feature of Ichthyosauromorpha. Phylogenetic analyses place the new species as a Corresponding author sister taxon of Chaohusaurus chaoxianensis. Ryosuke Motani, [email protected] Subjects Paleontology, Taxonomy Academic editor Keywords Early Triassic, Ichthyosauromorpha, Chaohusaurus brevifemoralis, Majiashan, Chaohu, Joseph Gillespie Anhui Province Additional Information and Declarations can be found on INTRODUCTION page 36 The marine reptile clade Ichthyosauromorpha comprises two groups, namely DOI 10.7717/peerj.7561 Ichthyosauriformes that lasted for about 160 million years and spread worldwide Copyright 2019 Huang et al. (McGowan, 1991; Motani et al., 2015a) and Hupehsuchia, which is a small group of Distributed under heavily-built reptiles known only from the Spathian (Lower Triassic) of Hubei Province, Creative Commons CC-BY 4.0 China (Young, 1972; Chen et al., 2014a). The two groups faced contrasting fates at theendoftheEarlyTriassicbuttheywerealmostequallydiverseintheSpathian,when How to cite this article Huang J-d, Motani R, Jiang D-y, Tintori A, Rieppel O, Zhou M, Ren X-X, Zhang R. 2019. The new ichthyosauriform Chaohusaurus brevifemoralis (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China. PeerJ 7:e7561 DOI 10.7717/peerj.7561 they were relatively new to the history of life (Motani et al., 2017). Early ichthyosauriforms from that time period had been known since 1929 based on Grippia longirostris from Spitsbergen (Wiman, 1929, 1933), and three additional species were reported within the same century, namely Chaohusaurus geishanensis from China (Young & Dong, 1972), Utatsusaurus hataii from Japan (Shikama, Kamei & Murata, 1978), and Parvinatator wapitiensis from Canada (Nicholls & Brinkman, 1995). However, none of them was represented by a complete skeleton and it was not until complete body fossils were reported that we learned the bauplan of these animals—at least Chaohusaurus and Utatsusaurus appeared like lizards with flippers (Motani, You & McGowan, 1996; Motani, Minoura & Ando, 1998), unlike later ichthyosaurs that were fish-shaped. Three more species have been recognized since, namely Gulosaurus helmi from Canada (Cuthbertson, Russell & Anderson, 2013a), Cartorhynchus lenticarpus from China (Motani et al., 2015a), and Sclerocormus breviceps from China (Jiang et al., 2016). Thaisaurus chonglakmanii from Thailand (Mazin et al., 1991) may also belong to the list but further study is needed to establish the exact stratigraphy. Chaohusaurus is by far the best-known genus of the Spathian ichthyosauriforms, being represented by dozens of skeletons, and more than 10 scientific papers focusing on the genus have been published (Young & Dong, 1972; Chen, 1985; Motani, You & McGowan, 1996; Motani & You, 1998a, 1998b; Maisch, 2001; Chen et al., 2013; Motani et al., 2015b, 2015c, 2018; Zhou et al., 2017). Three species are currently recognized in the genus. The type species Chaohusaurus geishanensis and the second species Chaohusaurus chaoxianensis are known from the Chaohu fauna in Anhui Province, China. The third species, Chaohusaurus zhangjiawanensis, is from the Nanzhang-Yuan’an fauna in Hubei Province. Of the three species, Chaohusaurus chaoxianensis is the most abundant, whereas the other two are known from only a few specimens each. Most of the specimens of Chaohusaurus chaoxianensis were collected from former limestone quarries in the Majiashan area, located north of the eastern outlet of Chaohu Lake, Anhui Province, China. Multi-year excavations that started in 2010, by a joint team from the Peking University, University of California, Davis, University of Milan, and Anhui Geological Museum, unearthed about 60 marine vertebrate specimens from the area. About 40 of the specimens were tentatively identified as Chaohusaurus chaoxianensis based on the presence of poorly ossified carpals or tarsals, which is among the features that distinguish the species from Chaohusaurus geishanensis (Motani et al., 2015c). A recent examination of morphological variation in these specimens revealed four morphotypes that likely represented females and males of two taxa, which were tentatively referred to as Types A and B (Motani et al., 2018). The two morphotypes were distinguished based on a suite of both qualitative and quantitative morphological characters. There are at least a dozen specimens for each morphotype—numbers that are far greater than those for the other two species of Chaohusaurus. These morphotypes most likely represent different species given that morphological differences do not reflect sexual dimorphism (Motani et al., 2018). Type A (Fig. 1), for which 13 specimens are known, contains theholotypeofChaohusaurus chaoxianensis so these specimens are considered to belong to this species. Type B (Fig. 2), with 21 specimens, had yet to be named or described, Huang et al. (2019), PeerJ, DOI 10.7717/peerj.7561 2/40 Figure 1 The most complete specimen of Chaohusaurus chaoxianensis (AGB6256). (A) Photograph. (B) Approximate bone map. See the section “Osteological abbreviations” for abbreviations. Scale bar is five cm in total. Full-size DOI: 10.7717/peerj.7561/fig-1 although some of the specimens were previously included in descriptive studies of Chaohusaurus chaoxianensis (see “Systematic Paleontology”). It is therefore important to clarify the taxonomic confusion in the existing literature. The purpose of this paper is to describe Type B as a new species of Chaohusaurus, and clarify its diagnostic differences with Chaohusaurus chaoxianensis. MATERIALS AND METHODS Specimens The specimens used in this study are listed in Systematic Paleontology. Most specimens are articulated skeletons lacking some parts of the body. All specimens have undergone at least some degree of compressional deformation during preservation, adding biases to the morphological data. The degree of compression depends on the specimen. Thick bones or bony structure are most prone to the bias from such deformation. For example, when comparing specimens of similar sizes, the shaft of long bones may appear wider in one specimen than the other because of preservational deformation; the wider shaft has been flattened and widened more severely through compaction. In extreme cases, a convex surface my appear slightly concave because of compaction. The extremity of the same bones, on the other hand, seems to suffer less from similar flattening and widening, probably because the average density of the bone in these areas are higher than in the shaft. Apart from long bones, cranial structures are often distorted by compactional deformation because of their thickness. It is therefore important to compare many specimens first to grasp the original morphology of bones before compression. Huang et al. (2019), PeerJ, DOI 10.7717/peerj.7561 3/40 Figure 2 Holotype of Chaohusaurus brevifemoralis sp. nov (AGB7401). (A) Photograph. (B) Approximate bone map. See the section “Osteological abbreviations” for abbreviations. Scale bar is five cm in total. Full-size DOI: 10.7717/peerj.7561/fig-2 Nomenclature The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank Life science identifiers (LSIDs) can be resolved
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