Heads and Tails: Comparative Osteology of Nearctic Dipsadid Snakes.

Although numerous studies have addressed some aspects of the cranial osteology of Nearctic dipsadid species, only the species within the genera Heterodon and Carphophis have a formal published description of their skull. Similarly, vertebral data on such species are extremely scarce, and most of the available literature is focused on fossils. Such group has a complex phylogenetic history, being recovered as monophyletic or nonmonophyletic depending on the approach. In this paper, we provide detailed and comparative descriptions of the osteology of dipsadid species distributed in the Nearctic region based on 69 specimens of dry material and high-resolution computed tomography (CT) scans. Additionally, we explore the morphological variation of the skull and cervical vertebrae within the context of distinct phylogenetic hypotheses previously proposed. Only two suprageneric groups previously proposed shared exclusive morphological traits: (Carphophis amoenus + Contia tenuis), proposed by three studies, and (Diadophis punctatus (Ca. amoenus + Co. tenuis)), proposed by one study. Large and detailed studies on the skull, mandible, and vertebrae represent an important step toward the understanding of the evolution of species, especially when they also show intraspecific variation.