Extreme morphological and ecological homoplasy in tropical salamanders

Fossorial salamanders typically have elongate and attenuated heads and bodi es, diminutive limbs, hands and feet, and extremely elongate tails. Batrach oseps from California, Lineatriton from eastern Mexico, and Oedipina from s outhern Mexico to Ecuador, all members of the family Plethodontidae, tribe Bolitoglossini, resemble one another in external morphology, which has evol ved independently. Whereas Oedipina and Batrachoseps are elongate because t here are more trunk vertebrae, a widespread homoplasy (parallelism) in sala manders, the genus Lineatriton is unique in having evolved convergently by an alternate "giraffe-neck" developmental program. Lineatriton has the same number of trunk vertebrae as related, nonelongated taxa, but individual tr unk vertebrae are elongated. A robust phylogenetic hypothesis, based on seq uences of three mtDNA genes, finds Lineatriton to be deeply nested within a clade characterized by generalized ecology and morphology. Lineatriton lin eolus, the only currently recognized taxon in the genus, shows unanticipate d genetic diversity. Surprisingly, geographically separated populations of L. lineolus are not monophyletic, but are sister taxa of different species of the morphologically generalized genus Pseudoeurycea. Lineatriton, long t hought to be a unique monospecific lineage, is polyphyletic. Accordingly, t he specialized morphology of Lineatriton displays homoplasy at two hierarch ical levels: (i) with respect to other elongate lineages in the family (con vergence), and (ii) within what is currently recognized as a single taxon ( parallelism). These evolutionary events are of adaptive significance becaus e to invade the lowland tropics salamanders must be either arboreal or foss orial; the repeated evolution of elongation and attenuation has led to mult iple lowland invasions.