A small-sized dinocephalosaurid archosauromorph from the Middle Triassic of Yunnan, southwestern China

doi:10.19615/j.cnki.2096-9899.231013

Abstract

Abstract:

Among numerous marine reptiles discovered in the Triassic eastern Tethys, today’s Southern China, Dinocephalosaurus is a bizarre animal comparable to European Tanystropheus in developing a prominently long neck. These two taxa are respectively assigned to Dinocephalosauridae and Tanystropheidae, and the two families and other basal members collectively form an early-diverging clade of Archosauromorpha. Here we report a new archosauromorph specimen, IVPP V18579, excavated from the lower Middle Triassic (Anisian), from Luoping, Yunnan in southwestern China. Compared with all the hitherto known dinocephalosaurids and tanystropheids, this skeletally mature individual is exclusively similar to Dinocephalosaurus in a number of characteristics, particularly with the long posterodorsal process of the premaxilla extending posteriorly beyond the level of the external nares, the concave posterior margin of the anteroposteriorly broad quadrate, and the strongly expanded distal end of the chevron in most of the caudal vertebrae. However, this reptile is much smaller than Dinocephalosaurus and different from Dinocephalosaurus and the other dinocephalosaurid, Pectodens, in many aspects, such as an anteriorly tapering long rostrum, the dentition composed of short conical teeth with less heterodonty, relatively but obviously tall neural spines of the axis and the anterior cervical vertebrae. Our phylogenetic analysis suggests that the new archosauromorph is a dinocephalosaurid, and then we erect Austronaga minuta gen. et sp. nov. based on this specimen. Detailed comparisons in osteological anatomy and the discussion about its potential aquatic adaptation of this new taxon are also provided.

Key words: Luoping, Yunnan, East Tethys, Triassic, marine reptile, Archosauromorpha, Dinocephalosauridae, osteological anatomy

摘要：

现在的中国南方地区在三叠纪时期曾位于特提斯洋东部。在该区域产出的众多海生爬行动物中，恐头龙(Dinocephalosaurus)的形态特化，颈部极度增长，可与时代略晚的长颈龙(Tanystropheus)相比拟。这两种爬行动物分别归属于恐头龙科(Dinocephalosauridae)和长颈龙科(Tanystropheidae), 而这两个科的成员加之另外一些基干物种代表了主龙型类(Archosauromorpha)中一系列早期分异出的支系。报道一件产自云南罗平地区中三叠世安尼期地层的主龙型类成年个体新标本IVPP V18579, 与目前已知的早期主龙型类成员相比，该标本显示出与恐头龙属一致但区别于其他属种的特征，主要包括前颌骨发育有超过外鼻孔后缘的较长后背侧突，方骨前后向较宽大且具有后边缘的凹缺，大部分尾椎上的人字骨带有远端前后向的延展。然而，新标本体型明显小于恐头龙，且显示出与恐头龙属和梳齿龙属(另一恐头龙科成员)不同的鉴别特征，例如吻部窄长、牙齿短锥状且异齿性弱、枢椎和前部颈椎的神经棘显著更高等。基于最新的用于早期主龙型类系统发育分析的特征矩阵，系统发育分析结果确定V18579代表一个新的恐头龙科成员，在恐头龙科中与恐头龙属互为姊妹群关系。于是基于V18579建立了一新属新种：玲珑南蛇龙(Austronaga minuta), 同时对其进行了详细的解剖学特征描述，并讨论了它可能具有的水生适应性。

关键词: 云南罗平, 东特提斯, 三叠纪, 海生爬行动物, 主龙型类, 恐头龙科, 骨骼学解剖

CLC Number:

Q915.864

WANG Wei, LEI Hong, LI Chun. A small-sized dinocephalosaurid archosauromorph from the Middle Triassic of Yunnan, southwestern China. Vertebrata Palasiatica, 2024, 62(1): 13-32.

王维, 雷洪, 李淳. 2024, 62(1): 13-32, 云南中三叠世一恐头龙科小型主龙型类. 古脊椎动物学报.

Figures/Tables 6

Fig. 1 Holotype of Austronaga minuta (IVPP V18579) discovered from Luoping, Yunnan, China, with the skull, anterior cervical vertebrae, and most of the caudal vertebrae preserved on a block

Fig. 2 Skull and anterior cervical vertebrae (A) with interpretative line drawings (B, C) of the holotype of Austronaga minuta (IVPP V18579) from the Middle Triassic in Luoping, Yunnan, China Abbreviations: an. angular; ar. articular; atic. atlas intercentrum; atpl. atlas pleurocentrum; atn. atlas neural arch; ax. axis (pleurocentrum); axic. axis intercentrum; axn. axis neural spine; bo. basioccipital; cop. coronoid process; cr(3-5). (third to fifth) cervical rib; crat. cervical rib of atlas; crax. cervical rib of axis; cv(3-6). (third to sixth) cervical vertebrae (centra); cv(3-5)n. (third to fifth) cervical vertebral neural arches; d. dentary; ept. ectopterygoid; f. frontal; ipv. interpterygoid vacuity; j. jugal; la. lacrimal; m. maxilla; mte. maxillary teeth; n. nasal; p. parietal; pbs. parabasisphenoid; pfor. parietal foramen; pl. palatine; pm. premaxilla; po. postorbital; prf. prefrontal; pt. pterygoid; q. quadrate; rap. retroarticular process; so. supraoccipital; sq. squamosal; sran. surangular

Table 1 Measurements of the holotype of Austronaga minuta (IVPP V18579) from the Middle Triassic in Luoping, Yunnan, China (mm)

Item	Length	Item	Length (centrum)	Height at midpoint
Skull	47.5	Atlas	-	-
Snout	24.1	Axis	3.1^*	6.4
Orbit approximately length	15.9	Cervical V3	6.1	5.6
Temporal region	7.5	Cervical V4	9.0	5.1
Mandible	46.6	Cervical V5	10.4	5.0

Fig. 3 Middle region of the tail (A) with interpretative line drawings (B) of representative caudal vertebrae in the holotype of Austronaga minuta (IVPP V18579) from the Middle Triassic in Luoping, Yunnan, China Abbreviations: cav. caudal vertebra (centrum); cavn. caudal vertebral neural arch; ch. chevron

Fig. 4 Phylogenetic results showing the position of Austronaga minuta in Dinocephalosauridae and other relationships within Archosauromorpha A. strict consensus tree; B. majority-rule reduced consensus trees(nodes with bootstrap proportion > 50% remained with values labeled above)

Appendix Scores of character states for the holotype of Austronaga minuta

char. 1-50	11?30?1111	??[0/1]0000???	?0?0000-01	11002?0000	???0?00001
char. 51-100	???00?????	??????0?1?	?0?1??0???	?????????1	0?????-??1
char. 101-150	-------?1?	??????????	??????????	??????????	???000?1?0
char. 151-200	?01?10?11?	01100001[0/4]0	00010?1220	21??210211	1?00????2[1/2]
char. 201-250	??????????	??????????	?03???????	??????????	??????????

References 52

[1]	Bassani F, 1886. Sui fossili e sull’eta degli schisti bituminosi triasici di Besano in Lombardia. Atti Soc Ital Sci Nat Mus Civ Stor Nat Milano, 29: 15 -72
[2]	Cheng L, Chen X H, Shang Q H et al., 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaften, 101: 251-259 DOI PMID
[3]	Cong L Y, Hou L H, Wu X C et al., 1998. The Gross Anatomy of Alligator sinensis Fauvel. Beijing: Science Press. 1-388
[4]	de Oliveira T M, Pinheiro F L, Stock Da-Rosa A A et al., 2020. A new archosauromorph from South America provides insights on the early diversification of tanystropheids. PLoS One, 15: e0230890
[5]	Dilkes D W, 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Philos Trans R Soc, 353: 501-541 DOI URL
[6]	Ezcurra M D, 2016. The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms. PeerJ, 4: e1778, doi: 10.7717/peerj.1778
[7]	Fraser N C, Rieppel O, 2006. A new protorosaur (Diapsida) from the Upper Buntsandstein of the Black Forest, Germany. J Vert Paleont, 26: 866-871 DOI URL
[8]	Fraser N C, Rieppel O, Li C, 2013. A long-snouted protorosaur from the Middle Triassic of southern China. J Vert Paleont, 33: 1120-1126 DOI URL
[9]	Goloboff P A, Catalano S A, 2016. TNT version 1.5, including a full implementation of phylogenetic morphometrics. Cladistics, 32: 221-238 DOI PMID
[10]	Gottmann-Quesada A, Sander P M, 2009. A redescription of the early archosauromorph Protorosaurus speneri Meyer, 1832, and its phylogenetic relationships. Palaeontogr Abt A, 287: 123-220 DOI URL
[11]	Griffin C T, Stocker M R, Colleary C et al., 2021. Assessing ontogenetic maturity in extinct saurian reptiles. Biol Rev, 96(2): 470-525 DOI URL
[12]	Huang J D, Motani R, Jiang D Y et al., 2019. The new ichthyosauriform Chaohusaurus brevifemoralis (Reptilia, Ichthyosauromorpha) from Majiashan, Chaohu, Anhui Province, China. PeerJ, 7: e7561, doi: 10.7717/peerj.7561
[13]	Huene F R, 1946. Die grossen Stamme der Tetrapoden in den geologischen Zeiten. Biol Zentr, 65: 268-275
[14]	Jaquier V P, Fraser N C, Furrer H et al., 2017. Osteology of a new specimen of Macrocnemus aff. M. fuyuanensis (Archosauromorpha, Protorosauria) from the Middle Triassic of Europe: potential implications for species recognition and paleogeography of tanystropheid protorosaurs. Front Earth Sci, 5: 1-28 DOI URL
[15]	Jiang D Y, Motani R, Hao W C et al., 2008. First record of Placodontoidea (Reptilia, Sauropterygia, Placodontia) from the Eastern Tethys. J Vert Paleont, 28: 904-908 DOI URL
[16]	Jiang D Y, Rieppel O, Fraser N C et al., 2011. New information on the protorosaurian reptile Macrocnemus fuyuanensis Li et al., 2007, from the Middle/Upper Triassic of Yunnan, China. J Vert Paleont, 31: 1230-1237 DOI URL
[17]	Jiang D Y, Motani R, Huang J D et al., 2016. A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Sci Rep, 6: 26232, doi: 10.1038/srep26232
[18]	Li C, 1999. Ichthyosaur from Guizhou, China. Chinese Sci Bull, 44: 1318-1321 DOI URL
[19]	Li C, 2003. First record of protorosaurid reptile (order: Protorosauria) from the Middle Triassic of China. Acta Geol Sin, 77: 419-423 DOI URL
[20]	Li C, Rieppel O, LaBarbera M C, 2004. Triassic marine reptile with extremely long neck. Science, 305: 1913 DOI URL
[21]	Li C, Wu X C, Cheng Y N et al., 2006. An unusual archosaurian from the marine Triassic of China. Naturwissenschaften, 93: 200-206 DOI URL
[22]	Li C, Zhao L J, Wang L T, 2007. A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication. Sci China Ser D: Earth Sci, 50(11): 1601-1605 DOI URL
[23]	Li C, Rieppel O, Wu X C et al., 2011. A new Triassic marine reptile from southwestern China. J Vert Paleont, 31: 303-312 DOI URL
[24]	Li C, Rieppel O, Cheng L et al., 2016. The earliest herbivorous marine reptile and its remarkable jaw apparatus. Sci Adv, 2: e1501659
[25]	Li C, Fraser N C, Rieppel O et al., 2017a. A new diapsid from the Middle Triassic of southern China. J Paleont, 91: 1306-1312 DOI URL
[26]	Li C, Rieppel O, Fraser N C, 2017b. Viviparity in a Triassic marine archosauromorph reptile. Vert PalAsiat, 55: 210-217
[27]	Liu J, Organ C L, Benton M J et al., 2017. Live birth in an archosauromorph reptile. Nat Commun, 8: 1-8 DOI
[28]	Meyer H V, 1847-1855. Die Saurier des Muschelkalkes mit Rücksicht auf die Saurier aus buntem Sandstein und Keuper. In: Keller H ed. Zur Fauna der Vorwelt, zweite Abtheilung. Frankfurt am Main: Heinrich Keller. 1-560
[29]	Miedema F, Spiekman S N F, Fernandez V et al., 2020. Cranial morphology of the tanystropheid Macrocnemus bassanii unveiled using synchrotron microtomography. Sci Rep, doi: 10.1038/s41598-020-68912-4
[30]	Motani R, Jiang D Y, Chen G B et al., 2015. A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature, 517: 485-488 DOI
[31]	Nosotti S, 2007. Tanystropheus longobardicus (Reptilia, Protorosauria): re-interpretations of the anatomy based on new specimens from the Middle Triassic of Besano (Lombardy, northern Italy). Mem Soc Ital Sci Nat Mus Civ Stor Nat Milano, 35: 1-88
[32]	Osborn H F, 1903. The reptilian subclasses Diapsida and Synapsida and the early history of the Diaptosauria. Mem Am Mus Nat Hist, 1: 449-519
[33]	Peyer B, 1930. Tanystropheus longobardicus Bass. sp. (Vorläufige Mitteilung). Centr Mine Geol Paläont B, 8: 336-337
[34]	Pol D, Escapa I H, 2009. Unstable taxa in cladistic analysis: identification and the assessment of relevant characters. Cladistics, 25: 1-13 DOI URL
[35]	Pritchard A C, Nesbitt S J, 2017. A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida. R Soc Open Sci, 4170499170499, doi: 10.1098/rsos.170499
[36]	Renesto S, 1994. A new prolacertiform reptile from the Late Triassic of Northern Italy. Riv Ital Paleontol Stratigr, 100: 285-306
[37]	Renesto S, Dalla Vecchia F M, 2000. The unusual dentition and feeding habits of the prolacertiform reptile Langobardisaurus (Late Triassic, northern Italy). J Vert Paleont, 20: 622-627 DOI URL
[38]	Renesto S, Saller F, 2018. Evidences for a semi aquatic life style in the Triassic diapsid reptile Tanystropheus. Riv Ital Paleontol Stratigr, 124: 23-34
[39]	Rieppel O, 1989a. The hind limb of Macrocnemus bassanii (Nopcsa) (Reptilia, Diapsida): development and functional anatomy. J Vert Paleont, 9: 373-387 DOI URL
[40]	Rieppel O, 1989b. Helveticosaurus zollingeri Peyer (Reptilia, Diapsida) skeletal paedomorphosis, functional anatomy and systematic affinities. Palaeontogr Abt A, 208: 123-152
[41]	Rieppel O, 2000. Paraplacodus and the phylogeny of the Placodontia (Reptilia: Sauropterygia). Zool J Linn Soc, 130: 635-659 DOI URL
[42]	Rieppel O, Li C, Fraser N C, 2008. The skeletal anatomy of the Triassic protorosaur Dinocephalosaurus orientalis, from the middle Triassic of Guizhou Province, southern China. J Vert Paleont, 28: 95-110 DOI URL
[43]	Romer A S, 1956. Osteology of the Reptiles. Chicago and London: University of Chicago Press. 1-772
[44]	Scheyer T M, Wang W, Li C et al., 2020. Osteological re-description of Macrocnemus fuyuanensis (Archosauromorpha, Tanystropheidae) from the middle Triassic of China. Vert PalAsiat, 58: 169-187
[45]	Shang Q H, Wu X C, Li C, 2020. A new Ladinian nothosauroid (Sauropterygia) from Fuyuan, Yunnan Province, China. J Vert Paleont, 40: e1789651 DOI URL
[46]	Spiekman S N F, Scheyer T M, 2019. A taxonomic revision of the genus Tanystropheus (Archosauromorpha, Tanystropheidae). Palaeontol Electron, 22: 1-46
[47]	Spiekman S N F, Neenan J M, Fraser N C et al., 2020a. Aquatic habits and niche partitioning in the extraordinarily long-necked Triassic reptile Tanystropheus. Curr Biol, 30: 1-7 DOI URL
[48]	Spiekman S N F, Neenan J M, Fraser N C et al., 2020b. The cranial morphology of Tanystropheus hydroides (Tanystropheidae, Archosauromorpha) as revealed by synchrotron microtomography. PeerJ, 8: e10299 DOI URL
[49]	Spiekman S N F, Fraser N C, Scheyer T M, 2021. A new phylogenetic hypothesis of Tanystropheidae (Diapsida, Archosauromorpha) and other “protorosaurs”, and its implications for the early evolution of stem archosaurs. PeerJ, 9: e11143 DOI URL
[50]	Spiekman S N F, Wang W, Zhao L J et al., in review. Dinocephalosaurus orientalis Li, 2003: a remarkable marine archosauromorph from the Middle Triassic of Southwestern China. Earth Env Sci Tran Roy Soc Edinburgh
[51]	Wang W, Shang Q H, Cheng L et al., 2022. Ancestral body plan and adaptive radiation of sauropterygian marine reptiles. iScience, 25: 12, doi: 10.1016/j.isci.2022.105635
[52]	Wang W, Spiekman S N F, Zhao L J et al., 2023. A new long-necked archosauromorph from the Guanling Formation (Anisian, Middle Triassic) of southwestern China and its implications for neck evolution in tanystropheids. Anat Rec, doi: 10.1002/ar.25216