Ecological opportunity, historical biogeography and diversification in a major lineage of salamanders

Aim Spelerpini is a major radiation in the Plethodontidae, the largest family of salamanders. Seventy-five percent of its species richness occurs in Eurycea, one of its six genera. We hypothesized that this was the result of the transgression of the Western Interior Seaway that provided ecological opportunity via ancestral range expansion into novel geographical regions, leading to an adaptive radiation. Location Eastern North America. Methods We sampled all but one species and two subspecies of extant Spelerpini, including several putative species, for four genes using maximum likelihood and Bayesian inference approaches to generate a comprehensive, robust phylogeny. We used five fossil calibrations to generate a chronogram, a likelihood framework to estimate the ancestral ranges/splits of all nodes on the phylogeny, and a Bayesian inference method to estimate diversification rate shifts putting the evolution of this group into a historical biogeographical context. Results A well-resolved, strongly supported phylogeny of the Spelerpini was recovered. Eurycea is among the oldest genera within the Spelerpini, originating c. 42 Ma with an ancestor occurring in four of five physiographical regions, each corresponding to a major Eurycea lineage. There is strong support for a rate shift in the Edwards Plateau neotenic Eurycea. Main conclusions A pattern of niche lability was found in the Spelerpini, as opposed to a pattern of niche conservatism found in other major radiations of plethodontids in eastern North America. The genus Eurycea dispersed widely into novel regions experiencing ecological opportunity as the Western Interior Seaway transgressed. This represents the first fossil calibrated and the most thoroughly sampled phylogeny of the group to date.