Snake Diets and Deep Time

It has been long suspected that scoleocophdian snakes are ancient, and many authors have considered them models for the ancestral snake: small, burrowing serpents with remnant pelvic girdles that feed mostly on social insects. Recent molecular studies place the scoleocophidians as diverging from the rest of the snakes somewhere between 97 and 166 MYA. The rest of the snakes, the Alethinophida, tend to feed on larger prey and relatively few of them are fossorial,  most are terrestrial, arboreal or aquatic. Two hypotheses have been suggested as the answer to the ecological differences between species that live together in existing ecological communities. One suggests that relatively recent living conditions have caused snakes to adjust their feeding behavior and diet to accommodate competitors in the environment  by partitioning resources.The other proposes that the diet was established in deep time and that diet can be predicted on the basis of phylogeny. T. J. Colston and colleagues have tested these hypotheses using the diets of 196 species of snakes across all of the major snake clades. They found more than 20% of alethinophidian snakes ate vertebrates with more than half of them feeding on lizards. 70% of the variation in diets was associated with 7 divergence events. None of the scolecophidians ate vertebrates. They ranked the snake clades by the percent of variation each contributed and found Leptotyphlopidae, Homalopsidae, and Natricidae accounted for a combined variation of 29.8%. The results of Colston et al. suggested major shits in snake diets occurred  early in snake history and that many present day forms eat the same food their ancestors did. However, the authors note the results don't rule out the impact of competition on predation in community structure at present.

The Rainbow Mud Snake, Enhydris enhydris, a homalopsid, feeding on a Betta splendens
In support of this idea, there is a clade of homalopsid snakes (Alfaro et al's clade C) that includes Cantoria violacea, Gerarda prevostiana, and Fordonia leucobalia that all feed on crustaceans, and while a few other species in other clades may feed on them on occasion, these snakes are specialists. They shared an ancestor with a fish-eater (Bitia hydroides) about 17 MYA (12-22 MYA), and the three crustacean-eaters last shared an ancestor about 14.9 MYA (10.2-19.5). The three crustaceans-eaters are all coastal species with distributions that overlap and they avoid competition by feeding on different groups, Cantoria violacea feeds on Alpheus shrimp, Fordonia feeds on hard-shelled crabs and mud lobsters, and Gerarda, the smallest species, feeds on crabs that have recently shed their exoskeleton.

Literature

Alfaro, M. A. et al. 2007. Phylogeny, evolutionary history, and biogeography of Oriental-Australian rear-fanged water snakes (Colubroidae: Homalopsidae) inferred from mitochondrial and nuclear DNA sequences. Molecular Phylogenetics and Evolution 46:576-593.

Colston, T. J., G. C. Costa, and L. J. Vitt. 2010. Snake diets and the deep history hypothesis. Biological Journal of the Linnean Society 101(2):476–486.

Murphy, J. C. 2007. Homalopsid Snakes, Evolution in the Mud. Malabar, Krieger Publishing Co.