Vertebral evolution and ontogenetic allometry: the developmental basis of extreme body shape divergence in microcephalic sea snakes

Snakes exhibit a diverse array of body shapes despite their characteristically simplified morphology. The most extreme are seen in within the radiation of fully aquatic sea snakes (Hydrophiinae), where ‘microcephalic’ sea snakes have tiny heads and dramatically reduced forebody girths that can be less than a third of their hindbody. This morphology has convergently evolved in species that specialise on hunting eels in burrows, but its developmental basis has not previously been examined. Here, we infer the developmental patterns underlying extreme body shape changes in sea snakes by testing evolutionary patterns of vertebral number and postnatal ontogenetic growth. Our results show that microcephalic species develop their characteristic shape via changes in both the embryo and postnatal stages: ontogenetic changes cause the hindbodies to reach greater sizes relative to their forebodies in adulthood, suggesting heterochronic shifts in evolution; they also have greater numbers of vertebrae, especially in their forebodies, which contributes to the elongation of the forebody and may compensate for its reduced overall growth. Our findings highlight sea snakes as an excellent system for studying the regulation of segment number and identity in the snake pre-cloacal axial skeleton, and the roles of these developmental traits in snake body shape diversification.Emma Sherratt, Felicity J. Coutts, Arne R. Rasmussen, Kate L. Sander