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

A 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, Tanystropheus and Dinocephalosaurus, 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 as Macrocnemus and Pectodens possess only moderately elongated necks. Here, we describe a newly discovered early diverging archosauromorph, Gracilicollum latens gen. 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 to Dinocephalosaurus and Tanystropheus hydroides. Despite possessing a high number of cervical vertebrae, Gracilicollum gen. nov. is recovered as a tanystropheid in an evolutionary grade between Macrocnemus and Tanystropheus rather than as a close relative of Dinocephalosaurus, a result that is primarily attributable to the presence of palatal teeth and the anatomy of the cervical vertebrae in Gracilicollum gen. 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.

Differential scaling patterns of vertebrae and the evolution of neck length in mammals.

Almost all mammals have seven vertebrae in their cervical spines. This consistency represents one of the most prominent examples of morphological stasis in vertebrae evolution. Hence, the requirements associated with evolutionary modifications of neck length have to be met with a fixed number of vertebrae. It has not been clear whether body size influences the overall length of the cervical spine and its inner organization (i.e., if the mammalian neck is subject to allometry). Here, we provide the first large-scale analysis of the scaling patterns of the cervical spine and its constituting cervical vertebrae. Our findings reveal that the opposite allometric scaling of C1 and C2-C7 accommodate the increase of neck bending moment with body size. The internal organization of the neck skeleton exhibits surprisingly uniformity in the vast majority of mammals. Deviations from this general pattern only occur under extreme loading regimes associated with particular functional and allometric demands. Our results indicate that the main source of variation in the mammalian neck stems from the disparity of overall cervical spine length. The mammalian neck reveals how evolutionary disparity manifests itself in a structure that is otherwise highly restricted by meristic constraints.

Evolution of the head-trunk interface in tetrapod vertebrates.

Vertebrate neck musculature spans the transition zone between head and trunk. The extent to which the cucullaris muscle is a cranial muscle allied with the gill levators of anamniotes or is instead a trunk muscle is an ongoing debate. Novel computed tomography datasets reveal broad conservation of the cucullaris in gnathostomes, including coelacanth and caecilian, two sarcopterygians previously thought to lack it. In chicken, lateral plate mesoderm (LPM) adjacent to occipital somites is a recently identified embryonic source of cervical musculature. We fate-map this mesoderm in the axolotl (Ambystoma mexicanum), which retains external gills, and demonstrate its contribution to posterior gill-levator muscles and the cucullaris. Accordingly, LPM adjacent to the occipital somites should be regarded as posterior cranial mesoderm. The axial position of the head-trunk border in axolotl is congruent between LPM and somitic mesoderm, unlike in chicken and possibly other amniotes.