The transition between the neck and trunk regions of the vertebral column (the cervicodorsal transition) is challenging to pinpoint in archosaurs, a group of amniotes containing pterosaurs, crocodylians, non-avian dinosaurs, and birds. The challenge results from the variable number of cervical vertebrae in the neck. The cervicodorsal transition is traditionally determined by taking the first vertebra connected to the sternum via rib segments to be the first dorsal. However, the traditional definition is inapplicable to most fossil archosaurs, because the typically cartilaginous sternum and sternal ribs were rarely preserved in the fossil record. Various alternative criteria for recognizing the first dorsal vertebra have been adopted in biological studies, but no clear consensus on the best approach has been reached.
We examined eight transitional features across the transition from unequivocal cervical to dorsal vertebrae in 50 taxa. Among the eight features, four show the most consistent patterns of change along the vertebral column in our sample, including (1) parapophyseal position relative to the neurocentral suture, (2) diapophyseal orientation, (3) progressive disappearance of the foramen transversarium, and (4) progressive disappearance of the hypapophysis. Any one of the four transitional features can provide an accurate estimation on the position of the cervicodorsal transition in extant crocodylians and birds. We constructed three different models to capture the cervicodorsal transition, each based on two transitional features and defining an intermediate cervicodorsal region. Among the three, the Diapophysis-Parapophysis model is most widely applicable to the taxa in our sample. The transitional models do not pinpoint the cervicodorsal transition as precisely as does the first connection between the vertebrae and sternum, but instead delineate a cervicodorsal region adjacent to or encompassing the exact point of transition. A consistent anatomical framework can therefore be established for studies of the mechanics, functions, and evolution of the archosaur axial skeleton.