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 shape changes along the precloacal axis are seen in fully aquatic sea snakes (Hydrophiinae): "microcephalic" sea snakes have tiny heads and dramatically reduced forebody girths that can be less than a third of the hindbody girth. This morphology has evolved repeatedly in sea snakes that specialize in hunting eels in burrows, but its developmental basis has not previously been examined. Here, we infer the developmental mechanisms underlying body shape changes in sea snakes by examining evolutionary patterns of changes in vertebral number and postnatal ontogenetic growth. Our results show that microcephalic species develop their characteristic shape via changes in both the embryonic and postnatal stages. Ontogenetic changes cause the hindbodies of microcephalic species to reach greater sizes relative to their forebodies in adulthood, suggesting heterochronic shifts that may be linked to homeotic effects (axial regionalization). However, microcephalic species also have greater numbers of vertebrae, especially in their forebodies, indicating that somitogenetic effects also contribute to evolutionary changes in body shape. Our findings highlight sea snakes as an excellent system for studying the development of segment number and regional identity in the snake precloacal axial skeleton.