Paedomorphosis and retention of juvenile diet lead speciation in a group of Neotropical snakes (Colubroides-Philodryadini).

Dipsadidae is one of the largest clades of extant reptiles, showing an impressive morphological and ecological diversity. Despite this fact, the developmental processes behind its diversity are still largely unknown. In this study, we used 3D reconstructions based on micro-CT data and geometric morphometrics to evaluate the skull morphology of Philodryas agassizii, a small, surface-dwelling dipsadid that consume spiders. Adult individuals of P. agassizii exhibit a cranial morphology frequently observed in juveniles of other surface-dwelling colubroideans, represented in our analysis by its close relative Philodryas patagoniensis. Large orbits, gibbous neurocranial roof and a relatively short jaw complex are features present in juveniles of the latter species. Furthermore, we performed an extensive survey about diet of P. patagoniensis in which we detected an ontogenetic dietary shift, indicating that arthropods are more frequently consumed by juveniles of this dietary generalist. Thus, we infer that P. agassizzii retained not only the ancestral juvenile skull morphology but also dietary preferences. This study reveals that morphological changes driven by heterochronic changes, specifically paedomorphosis, influenced the retention of ancestral life history traits in P. agassizii, and therefore promoted cladogenesis. In this way, we obtained first evidence that heterochronic processes lead speciation in the snake megadiverse clade Dipsadidae.