New hupehsuchian specimen from the Lower Triassic of Yuan’an, China, confirming the validity of Hupehsuchus nanchangensis and the implications for hupehsuchian taxonomy and ontogeny

Hupehsuchia is an Early Triassic marine reptile lineage from South China that occupies the most basal position within Ichthyosauromorpha. Potential synonymy between Hupehsuchus nanchangensis Young & Dong, 1972 and Nanchangosaurus suni Wang, 1959 remains contentious, with N. suni interpreted as the juvenile of H. nanchangensis, thereby challenging the taxonomic validity of H. nanchangensis. Here, we describe a new small specimen of H. nanchangensis (YGM17-1) from the uppermost Spathian (Upper Olenekian) of Yuan’an, Hubei Province, representing a juvenile. Although comparable in size to N. suni, this new specimen exhibits the diagnostic features of H. nanchangensis that are lacking in N. suni, including exclusion of a frontal from the orbital margin, and three layers of dermal armour. It also exhibits immature characteristics, such as a sub-round humerus, weakly developed posterior rib flanges, and unossified carpals. In contrast, N. suni shows a participation of the frontal in the orbital margin, only one layer of dermal armour, and more mature features, including a humerus with a concave anterior flange, well-developed posterior rib flanges, and well-ossified carpal elements. These differences support the specific independence of H. nanchangensis from N. suni. The revised taxonomic identifications of the specimen clarify the ontogenetic patterns of H. nanchangensis, e.g. allometric limb growth and a gradual increase of neural spine height. Multiple quantitative analyses, including Chow tests, preliminarily support the independence of Hupehsuchus from Nanchangosaurus, but are limited by the small sample size. These findings provide new insights into biodiversity and evolutionary relationships among early ichthyosauromorphs.