A surprise at the tail's end: The bizarre tail vertebrae of reed snakes (Serpentes, Calamariidae) reveal an extraordinary caudal vertebral morphology for Caenophidia

Abstract

Caudal vertebrae of caenophidian snakes are characterized by the presence of paired haemapophyses–a pattern observed from the earliest-diverging acrochordid lineage to the most derived viperids, and documented in the fossil record from the Late Cretaceous onward, including the basalmost known extinct caenophidian forms. Using micro-computed tomography scanning, we document an extraordinary deviation from this pattern. We examined five species across three genera of the Asian family Calamariidae (Reed snakes) and demonstrate that they, uniquely among Colubroidea, lack haemapophyses throughout the caudal vertebral series, possessing instead prominent haemal keels–a condition that represents a novel diagnostic feature for Calamariidae. This simplification of vertebral morphology, coupled with an extremely reduced number of caudal vertebrae, likely correlates with the fossorial lifestyle characteristic of this snake family. Furthermore, we identify and illustrate unexpected modifications of subcentral structures in certain other caenophidian taxa, that is, a pareid and several distantly related elapoid species. Our findings underscore the significant taxonomic and diagnostic value of caudal vertebral morphology, a skeletal element that has been largely overlooked.