 |
| Chusaurus xiangensis Liu, Cheng, Stubbs, Moon, Benton, Yan & Tian, 2023 |
Abstract
Neck elongation has appeared independently in several tetrapod groups, including giraffes and sauropod dinosaurs on land, birds and pterosaurs in the air, and sauropterygians (plesiosaurs and relatives) in the oceans. Long necks arose in Early Triassic sauropterygians, but the nature and rate of that elongation has not been documented. Here, we report a new species of pachypleurosaurid sauropterygian, Chusaurus xiangensis gen. et sp. nov., based on two new specimens from the Early Triassic Nanzhang-Yuan’an Fauna in the South China Block. The new species shows key features of its Middle Triassic relatives, but has a relatively short neck, measuring 0.48 of the trunk length, compared to > 0.8 from the Middle Triassic onwards. Comparative phylogenetic analysis shows that neck elongation occurred rapidly in all Triassic eosauropterygian lineages, probably driven by feeding pressure in a time of rapid re-establishment of new kinds of marine ecosystems.
Keywords: Marine reptile, Eosauropterygia, Mesozoic, Body plan, Nanzhang-Yuan’an Fauna
Superorder Sauropterygia Owen, 1860
Order Eosauropterygia Rieppel, 1994
Family Pachypleurosauridae Nopcsa, 1928
 |
| The two specimens of Chusaurus xiangensis gen. et sp. nov. (A) Photograph of WGSC V 1901, mostly in dorsal view. (B) Interpretive drawing of WGSC V 1901. (C) Photograph of WGSC V 1702, dorsally exposed. (D) Mould and part of the skeleton from the counterpart of WGSC V 1702. (E) Interpretive drawing of WGSC V 1702. The black arrow in B indicates the abrupt inversion of the cervical vertebral column. Skull elements are marked by a black dashed line in D, and shadow in E, and represent the shape of the mould in D marked by a white dashed line. White arrows indicate the corresponding parts in C-E. Abbreviations: CdV, caudal vertebra; CV, cervical vertebra; DR, dorsal rib; DV, dorsal vertebra; ph, phalanx; SV, sacral vertebra. Scale bar = 2 cm |
 |
| Selected details of the two specimens. (A) Skull of the holotype, ventrally exposed. (B) Interpretative drawing of A. (C) Coracoid of WGSC V 1702. (D) Pectoral region of the holotype in dorsal view. (E) Pelvic and anterior caudal region of the holotype in dorsal view. (F) Pelvic and anterior caudal region of WGSC V 1702 in dorsal view. G-J. Forelimb and hindlimb of the holotype in dorsal view. K-N. Forelimb and hindlimb of WGSC V 1702 in dorsal view. Dashed line represents conjectural borderline. Shade in B represents unidentified elements. Abbreviations: ang, angular; as, astragalus; at, atlas; ax, axis; cal, calcaneum; cbr, ceratobranchial; CdR, caudal rib; ch, chevron; cl, clavicle; co, coracoid; CR, cervical rib; d, dentary; dc, distal carpal;dt, distal tarsal; DV, dorsal vertebra; ecg, ectepicondylar groove; enf, entepicondylar foramen; f, femur; fi, fibula; hu, humerus; il, ilium; in, intermedium; j, jugal; m, maxilla; mc, metacarpal; mt, metatarsal; ph, phalanx; pl, palatine; pm, premaxilla; pob/ept?, postorbital or ectopterygoid; pt, pterygoid; pu, pubis; ra, radius; rap, retroarticular process; sc, scapula; SR, sacral rib; SV, sacral vertebra; ti, tibia; ul, ulna; uln, ulnare. v, vomer. Scale bar in B-L, N = 5 mm. Scale division in A, M = 1 mm |
Genus Chusaurus, gen. nov.
Chusaurus xiangensis gen. et sp. nov.
Distribution: Olenekian (Early Triassic), South China.
Type locality and horizon: Songshugou Village, Nanzhang County, Xiangyang City, Hubei Province, P. R. China. Member II of the Jialingjiang Formation; Spathian, late Olenekian, Early Triassic.
Etymology: The generic name is derived from the Chu Kingdom in Chinese history, which dominated around the area of the fossil site. The specific epithet is derived from “Xiang”, referring to Xiangyang City where the fossil was discovered.
Diagnosis: A small-sized pachypleurosaurid eosauropterygian with at least 17 cervical, 16 dorsal, 3 sacral, and about 40 caudal vertebrae; neck length about half the trunk length; pterygoid flanges well developed and longitudinally oriented; anteriormost caudal vertebral neural spines obviously heightened compared to presacral and sacral vertebrae; dorsal and anterior caudal ribs highly pachyostotic with expanding proximal end; posteriormost caudal ribs shortened and round; chevron well developed in the caudal region; clavicle short and stout; scapula elongate and without broadly expanded anterior portion; scapular blade is short and robust; coracoid without concave anteromedial margin; iliac blade well developed with a posterior process; little interspace between the manual and pedal digits; the proximal phalanges short and flattened; 6 maximum carpal and tarsal ossifications; manual phalangeal format 2-3-4-5-3; pedal phalangeal format 2-3-4-5-4.
Qi-Ling Liu, Long Cheng, Thomas L. Stubbs, Benjamin C. Moon, Michael J. Benton, Chun-Bo Yan and Li Tian. 2023. Rapid Neck Elongation in Sauropterygia (Reptilia: Diapsida) revealed by A New basal pachypleurosaur from the Lower Triassic of China. BMC Ecology and Evolution. 23: 44. DOI: 10.1186/s12862-023-02150-w
phys.org/news/2023-09-plesiosaurs-neck-length-gaining-vertebrae.html
