S7-9 Sunday, Jan. 6 13:30 - 14:00 Tiny heads: the evolution of microcephalic sea snakes SHERRATT, E*; SANDERS, KL; The University of Adelaide, Australia; The University of Adelaide, Australia email@example.com https://researchers.adelaide.edu.au/profile/emma.sherratt
Snakes exhibit a diverse array of body sizes and shapes, contrary to common belief that they have a simplified body plan. Among living snakes, arguably the most extreme shape changes along the pre-cloacal body axis are seen in fully aquatic sea snakes (Hydrophiinae) in the genera Microcephalophis and Hydrophis. These ‘microcephalic’ sea snakes have tiny heads and dramatically reduced forebody girths that can be less than a third of their hindbody girths. Previously we have shown that this morphology has evolved multiple times in species that specialise on hunting eels in burrows. Furthermore, our research has attributed this variation to evolutionary changes in vertebral patterning during embryo development and divergence in postnatal somatic growth patterns. Yet to be examined is what happens to the morphology of the heads of these so-called microcephalic species. Here we use micro-CT reconstructions of skulls and geometric morphometric methods to characterise skull shape variation in sea snakes. Examining neonate, juvenile and adult snakes across ~40 species, we show that microcephalic species not only have a reduced head size, but their skull shape is most similar to young individuals of regular Hydrophis sea snakes. Microcephaly thus appears to have evolved by heterochronic processes, with the skull resembling a neotenic form. Our findings suggest that convergent evolution of microcephalic sea snakes has evolved through external selection pressures acting on developmental pathways, such that through shifts in growth and timing, dramatic morphologies can repeatedly evolve.