A new long‐necked archosauromorph from the Guanling Formation (Anisian, Middle Triassic) of southwestern China and its implications for neck evolution in tanystropheids

Abstract

AbstractA long neck is an evolutionary innovation convergently appearing in multiple tetrapod lineages, including groups of plesiosaurs, non‐archosauriform archosauromorphs, turtles, sauropodomorphs, birds, and mammals. Among all tetrapods both extant and extinct, two Triassic archosauromorphs,TanystropheusandDinocephalosaurus, have necks that are particularly elongated relative to the lengths of their trunks. However, the evolutionary history of such hyper‐elongated necks in these two archosauromorph clades remains unknown, partially because known close relatives such asMacrocnemusandPectodenspossess only moderately elongated necks. Here, we describe a newly discovered early diverging archosauromorph,Gracilicollum latensgen. et sp. nov., based on a specimen comprising a partial neck and an incompletely preserved skull. The long neck is composed of at least 18 cervical vertebrae. The dentition suggests that this new taxon most likely represents an aquatic piscivore, similar toDinocephalosaurusandTanystropheus hydroides. Despite possessing a high number of cervical vertebrae,Gracilicollumgen. nov. is recovered as a tanystropheid in an evolutionary grade betweenMacrocnemusandTanystropheusrather than as a close relative ofDinocephalosaurus, a result that is primarily attributable to the presence of palatal teeth and the anatomy of the cervical vertebrae inGracilicollumgen. nov. Considering the information provided by the new specimen, we provide a detailed discussion of the cervical evolution in dinocephalosaurids and tanystropheids, which is shown to be highly complex and mosaic in nature.