Largocephalosaurus is an extinct genus of basal saurosphargid, a marine reptile known from the Middle Triassic (Anisian age) Guanling Formation of Yunnan and Guizhou Provinces, southwestern China. It contains a type species, Largocephalosaurus polycarpon, and a second species L. qianensis. L. polycarpon was initially interpreted as an eosauropterygian sauropterygian closely related to European pachypleurosaurs and nothosaurids. However following additional preparation of the postcranial skeleton and the discovery of a second better known species L. qianensis, Li et al. (2014) reinterpreted Largocephalosaurus as a basal member of the family Saurosphargidae and a close relative of Saurosphargis and Sinosaurosphargis. According to Li et al. (2014), saurosphargids did not belong to Sauropterygia, although they were closely related to it, forming its sister taxon.
Discovery The type species of Largocephalosaurus, L. polycarpon is known only from the holotype WIGM SPC V 1009 a nearly complete and articulated skull and skeleton missing most of its tail and measuring more than 113 centimetres (3.71 ft) in length, housed at the Wuhan Institute of Geology and Mineral Recourses. SPC V 1009 was collected from Member II of the Guanling Formation, dating to the Pelsonian substage of the mid-late Anisian stage of the early Middle Triassic, about 243 million years ago, at Luoping County of Yunnan Province. While L. polycarpon was being studied at the WIGM, three specimens of what was later named L. qianensis were prepared at the Geological Museum of Peking University and the Institute of Vertebrate Paleontology and Paleoanthropology. These three specimens were collected in 2008 about 100 km northeast of the type locality of L. polycarpon at Xinmin District of Panxian County, southwestern Guizhou Province. They, like SPC V 1009, came from Member II of the Guanling Formation, dating to the late Anisian. Out of these specimens, IVPP V 15638, a nearly complete skeleton exposed in view from below with only the back of the tail missing, was chosen to represent the holotype of L. qianensis. The other two specimen are referable to L. qianensis and include GMPKU-P-1532-A, a skull with a neck vertebra in view from above, and GMPKU-P-1532-B, an incomplete postcranial skeleton in view from above, with the back portion of the mandible preserved, but missing most of the tail. All specimens of Largocephalosaurus came from the Lagerstätte discovered during a 2007 geological mapping project with a diverse record of marine life called Luoping Biota, that yielded, apart from Largocephalosaurus, various invertebrates, fish, basal ichthyosaurs, Atopodentatus, the advanced saurosphargid Sinosaurosphargis, and several species of eosauropterygian, including both pachypleurosaurs and nothosaurids.
Etymology Largocephalosaurus was first described and named by Long Cheng, Xiaohong Chen, Xiongwei Zeng and Yongjian Cai in 2012 with the type species Largocephalosaurus polycarpon. The generic name is said to be derived from Latin largus, meaning "large", and cephalus, meaning "head", and from Greek sauros, meaning "lizard", a common suffix for genus names of extinct reptiles. In classical Latin, the proper word for "head" is caput, while in ancient Greek kephalē (κεφαλή) was used for "head". Kephalos (Κέφαλος) was the first name of various Greek mythological and historical figures that was rendered in Latin as Cephalus. The specific name polycarpon is said to be derived from Latin poly, meaning "many", plus "carpon", meaning "ossified carpals", in reference to the eleven carpal ossifications the forelimb of L. polycarpon shows, resulting in the unique hand bone formula of 2-3-4-5-5. In classical Latin the proper word for "many" is multus, while in ancient Greek they used the word polys (πολύς). A second species L. qianensis was named by Chun Li, Da-Yong Jiang, Long Cheng, Xiao-Chun Wu and Olivier Rieppel in 2014. Its specific name is derived from Qian, a shortened alternative spelling for Guizhou, plus the Latin suffix -ensis meaning "from", in reference to the different occurrence of the species - while the specimens of L. qianensis were discovered in Guizhou Province, the only known specimen of L. polycarpon came from the neighboring Yunnan Province.
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