Emma Sherratt

Recent research into the snake vertebral column has highlighted the importance of considering regionalization and its implications for the life history of snakes. Our research delves into the distinctions in vertebral column morphology and regionalization within the snake family Colubridae, comparing arboreal and terrestrial species. Our results provide significant support for dissociation between two pre-cloacal regions: the ‘pre-atrial’ (anterior to heart) and the ‘post-atrial’ (posterior to heart). Furthermore, the two ecological groups display distinct proportional compositions between the two pre-cloacal regions and the one post-cloacal region (caudal); arboreal species tend to possess a higher proportion of caudal vertebrae, whereas terrestrial species exhibit a higher proportion of post-atrial vertebrae. Our study reveals that the axial morphology of arboreal species is distinct from that of terrestrial species; however, this was not attributable to a convergent evolution process. By estimating the evolutionary history of vertebral number in each region between the ecological groups, we find each region has a unique pattern, further highlighting evolutionary disassociation between the regions. This study adds to the growing evidence on regionalization of the pre-cloacal snake vertebral column and underscores the importance in continuing to develop our understanding of snake evolution.

Sea snakes in the Hydrophis-Microcephalophis clade (Elapidae) show exceptional body shape variation along a continuum from similar forebody and hindbody girths, to dramatically reduced girths of the forebody relative to hindbody. The latter is associated with specializations on burrowing prey. This variation underpins high sympatric diversity and species richness and is not shared by other marine (or terrestrial) snakes. Here, we examined a hypothesis that macroevolutionary changes in axial development contribute to the propensity, at clade level, for body shape change. We quantified variation in the number and size of vertebrae in two body regions (pre- and post-apex of the heart) for approximately 94 terrestrial and marine elapids. We found Hydrophis-Microcephalophis exhibit increased rates of vertebral evolution in the pre- versus post-apex regions compared to all other Australasian elapids. Unlike other marine and terrestrial elapids, axial elongation in Hydrophis-Microcephalophis occurs via the preferential addition of vertebrae pre-heart apex, which is the region that undergoes concomitant shifts in vertebral number and size during transitions along the relative fore- to hindbody girth axis. We suggest that this macroevolutionary developmental change has potentially acted as a key innovation in Hydrophis-Microcephalophis by facilitating novel (especially burrowing) prey specializations that are not shared with other marine snakes.