New information on Late Triassic sauropodomorph dinosaurs provides support for the independent acquisition of postcranial skeletal pneumaticity in avemetatarsalian lineages.

Abstract

Within Avemetatarsalia, postcranial skeletal pneumaticity (PSP) occurs in pterosaurs, as well as theropod (including extant Aves) and sauropod dinosaurs. However, the evolutionary origins of PSP in the latter clade remain largely unknown, with few studies assessing species closely related to, but outside, the sauropod radiation, that is, early-branching sauropodomorphs. Furthermore, most proposed identifications of PSP in early-branching sauropodomorphs relate to external indicators of internal pneumaticity, for example, the presence of vertebral subfossae. To address this deficit, we CT scanned representative elements from the vertebral columns of the early-branching sauropodomorphs Thecodontosaurus antiquus, Pantydraco caducus, Ruehleia bedheimensis and Plateosaurus longiceps, all from the Late Triassic of Europe. These new data were compared with the small number of early-branching sauropodomorphs with published vertebral CT scan data, namely the Late Triassic Brazilian species, Buriolestes schultzi, Pampadromaeus barberenai and Macrocollum itaquii. Based on the sampled vertebrae, PSP is absent in Buriolestes, Pampadromaeus, Pantydraco and probably Thecodontosaurus. It is possible that the neural arches of the posterior cervical vertebrae of Thecodontosaurus possess PSP, but this can only be interpreted from broken transverse cross-sections and not CT scans. The posterior cervical vertebrae of Ruehleia possess PSP in the neural arches; however, their corresponding centra, along with the centra and neural arches of the anterior-middle dorsal vertebrae, are apneumatic. Plateosaurus possesses PSP in the neural arches of the middle cervical vertebrae through to the middle dorsal vertebrae, whereas the presacral centra are apneumatic. Where present, pneumatic internal chambers are neither exclusively camerate nor camellate, nor do they align with the 'protocamerate' bone structure previously described in the posterior cervical and anterior dorsal vertebrae of Macrocollum. From external indicators, PSP might be present in the sacral neural arches of Ruehleia and Plateosaurus but is absent in the caudal vertebrae. However, our results reveal that PSP cannot be unambiguously determined from external indicators; subfossae do not always communicate with internal chambers; and internal chambers sometimes communicate with undivided fossae. PSP in early-branching sauropodomorphs probably evolved first in the neural arches of the posterior cervical vertebrae, expanding anteriorly and posteriorly along the vertebral column. Furthermore, the distribution of PSP in Late Triassic early-branching sauropodomorphs does not appear to be correlated with body size. Finally, our results lend support to the idea that pterosauromorphs, theropods and sauropodomorphs evolved PSP in the Late Triassic independently of each other.