Tongue Evolution in the Lungless Salamanders, Family Plethodontidae IV. Phylogeny of Plethodontid Salamanders and the Evolution of Feeding Dynamics

Abstract

-Plethodontids, a monophyletic group, comprise about 60% of living salamanders. Many have a tongue that may be projected rapidly and for a distance up to half the body length. Previous studies have focused on the functional morphology of the projectile tongue and the comparative morphology of the feeding system in the family. Here we present a working hypothesis for the phylogeny of plethodontids and a scenario for the evolution of tongue feeding based on the phylogenetic hypothesis. Primitive and derived states are identified and transformation series proposed for 30 characters. Four alternative phylogenetic hypotheses are presented. Our choice of a preferred phylogenetic hypothesis is based on consideration of conflicting characters and tongue projection mechanics. The preferred hypothesis is not the most parsimonious, in a strict technical sense, because several homoplasious characters appear (on functional grounds) to be closely associated with attainment of tongue projectility, which itself has evolved several times. In our hypothesis the bolitoglossines and plethodontines are sister taxa. They in turn are the sister taxon of the hemidactylines, and this grouping is the sister taxon of the desmognathines. Within the bolitoglossines, Batrachoseps is the sister taxon of the Neotropical genera (supergenus Bolitoglossa). In our scenario for the evolution of tongue feeding, the ancestral plethodontid had an protrusible tongue. From this condition projectile have arisen at least three times within the family: in the ancestors of the hemidactylines; in the ancestors of Ensatina; and in the ancestors of the bolitoglossines. In turn, the projectile condition has given rise to projectile at least once within the hemidactylines and twice within the bolitoglossines. This scenario tends to maximize the number of times projectile and projectile have evolved in the family. [Plethodontidae; phylogeny; tongue; evolutionary morphology.] The lungless salamanders, family Plethodontidae, are distributed in North and Central America and northern South America in the New World, and two species of one genus occur in Europe. The family includes over 200 species, currently placed in 27 genera, which comprise about 60% of all salamander species. The family is characterized by a relatively high degree of terrestriality (Dunn, 1926; Wake, 1966) and, in addition, most plethodontids have a highly specialized feeding mechanism, usually extensive tongue projection. Plethodontids are a monophyletic group based on the presence of an opercularis muscle formed entirely or in part from the cucullaris and, in transformed adults, the presence of a nasolabial groove and the absence of an ossified pterygoid bone. In addition, all are lungless-a condition shared with a few derived species of other families. In this paper we add further synapomorphies from the structure of the feeding apparatus. For the past 50 years, the hyobranchial skeleton and musculature of the feeding system have been the major structural attributes used in considering plethodontid phylogeny. Thus, it is not surprising that most authors who have considered the phylogeny of plethodontids have also made at least passing reference to feeding evolution, if not considering it in detail. Of particular importance has been the perceived dichotomy attached tongues versus free tongues. Previous authors either