Drivers of tail evolution in squamates and their implications for the fossorial origin of snakes

The axial skeleton serves as the primary structural support in all vertebrates and is subdivided into five distinct regions: cervical, thoracic, lumbar, sacral, and caudal. Relaxation of constraints acting on the terminal end of the axial skeleton has led to remarkable variation in caudal vertebrae number across Squamata. Despite the tail's critical role in locomotion, reproduction, communication, resource storage, and defense, drivers of this variation remain poorly understood. We tested three alternate hypotheses to identify whether caudal vertebrae counts are influenced by habitat, morphology, or phylogeny. We identify ecology as the most significant driver of caudal diversity, recovering the greatest contrast in caudal vertebrae number between short-tailed fossorial species and long-tailed arboreal species. This indicates that the tail plays a key role in above-ground balance and locomotion. Further, our comparisons of phylogenetic signal in trunk and tail vertebrae provide support for the hypothesis that the tail acts as an independent evolutionary module. Ancestral estimation of caudal vertebral counts indicates that the tail of the ancestor of crown-group snakes had many fewer elements than any other major squamate clade and when compared to total-group Squamata. This represents compelling new evidence for a fossorial origin of snakes. Our findings expand current understanding of squamate caudal diversity and shed new light on the evolutionary drivers of extreme variation in vertebral anatomy across the clade.