Rapid neck elongation in Sauropterygia (Reptilia: Diapsida) revealed by a new basal pachypleurosaur from the Lower Triassic of China

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: Body plan; Eosauropterygia; Marine reptile; Mesozoic; Nanzhang-Yuan’an Fauna.

Fig. 1

Summary geological maps. (A) The general location of the South China block in the Early Triassic (modified from Benton et al. [24]). (B) Tectonic map showing major blocks of South China [24], with the site of Nanzhang-Yuan’an Fauna (NYF). (C) Simplified geological map of Nanzhang and Yuan’an counties with distributions of Triassic marine reptiles (modified from Li et al. [16] and Yan et al. [22]). ‘Old land’ is coloured orange, shallow seas light blue, and deep marine basins dark blue in A, B. Abbreviations: Є-S, Cambrian-Silurian; D-P, Devonian-Permian; T_1d, Daye Formation, Lower Triassic; T_1j, Jialingjiang Formation, Lower Triassic; T_2b, Badong Formation, Middle Triassic; T₃-J, Upper Triassic-Jurassic; K-Q, Cretaceous-Quaternary.

Fig. 2

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

Fig. 3

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

Fig. 4

Phylogenetic position of the new taxon and phenograms of relative neck length in selected Triassic eosauropterygians. (A) Strict consensus of four most parsimonious trees (TL = 534, CI = 0.352, RI = 0.613). Bootstrap values ≥ 50% are labelled. Taxa are marked by different colours. (B) Phenograms of neck-trunk length ratio (Chusaurus as sister group to the clade including Panzhousaurus). (C) Phenograms of neck-trunk length ratio (Chusaurus as sister group to the clade including Keichousaurus). Clades: a. Sauropterygia; b. Placodontia; (c) Eosauropterygia; (d) Pachypleurosauridae; (e) Nothosauridae; (f) Pistosauroidea