Comparative Osteology and Evolution of the Lungless Salamanders, Family Plethodontidae David B
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COMPARATIVE OSTEOLOGY AND EVOLUTION OF THE LUNGLESS SALAMANDERS, FAMILY PLETHODONTIDAE DAVID B. WAKE1 ABSTRACT: Lungless salamanders of the family Plethodontidae comprise the largest and most diverse group of tailed amphibians. An evolutionary morphological approach has been employed to elucidate evolutionary rela tionships, patterns and trends within the family. Comparative osteology has been emphasized and skeletons of all twenty-three genera and three-fourths of the one hundred eighty-three species have been studied. A detailed osteological analysis includes consideration of the evolution of each element as well as the functional unit of which it is a part. Functional and developmental aspects are stressed. A new classification is suggested, based on osteological and other char acters. The subfamily Desmognathinae includes the genera Desmognathus, Leurognathus, and Phaeognathus. Members of the subfamily Plethodontinae are placed in three tribes. The tribe Hemidactyliini includes the genera Gyri nophilus, Pseudotriton, Stereochilus, Eurycea, Typhlomolge, and Hemidac tylium. The genera Plethodon, Aneides, and Ensatina comprise the tribe Pleth odontini. The highly diversified tribe Bolitoglossini includes three super genera. The supergenera Hydromantes and Batrachoseps include the nominal genera only. The supergenus Bolitoglossa includes Bolitoglossa, Oedipina, Pseudoeurycea, Chiropterotriton, Parvimolge, Lineatriton, and Thorius. Manculus is considered to be congeneric with Eurycea, and Magnadig ita is congeneric with Bolitoglossa. Two species are assigned to Typhlomolge, which is recognized as a genus distinct from Eurycea. No. new information is available concerning Haptoglossa. Recognition of a family Desmognathidae is rejected. All genera are defined and suprageneric groupings are defined and char acterized. Range maps are presented for all genera. Relationships of all genera are discussed. Plethodontid salamanders have undergone an adaptive radiation char acterized by structural, functional, and ecological parallelism. The relatively primitive desmognathines and hemidactyliines have remained close to the an cestral habitat, mountain brooks and streams in eastern North America. Evo lutionary trends in the direction of abandonment of the aquatic larval stages and attainment of direct development are evident in desmognathines. The tribes Bolitoglossini and Plethodontini have direct development in terrestrial habitats. Terrestriality has stimulated evolution in these two successful groups and both have extended far beyond the ancestral range. Paedomorphosis has played a significant role in influencing evolutionary patterns within the family. In hemidactyliines, paedomorphosis has led to paedogenesis in four genera. Paedogenesis is a highly specialized but regressive adaptation in these groups. Paedomorphosis has profoundly influenced ter restrial groups by allowing them to achieve reproductive maturity at early structural stages. The phenomenon has led to evolutionary progress and may have been the key to the success of the terrestrial groups. Ancestral plethodontids probably arose from a stock close to that which gave rise to ambystomatids. The subfamily Desmognathinae is a specialized early derivative of this stock. Probable desmognathine remains from Creta ceous formations suggest that subfamilial differentiation was a Mesozoic event. The tribe Hemidactyliini is the most generalized group, both in struc ture and ecology, and is probably close to the ancestral familial stock. The tribe Bolitoglossini may have been the earliest derivative of the plethodontine stock. It ranges to western North America, Europe, and South America, and is the only salamander group that penetrates the tropics. In addition it is the only group of plethodontids that no longer occurs in eastern North America. The final derivative, the tribe Plethodontini, probably differentiated in early Tertiary times. The group has extended its range to western North America but still has relatively primitive representatives in Appalachia. lDepartment of Anatomy and The College, The University of Chicago, 102S East S7th Street, Chicago, Illinois 60637 1 2 MEMOIR OF THE SOUTHERN CALIFORNIA ACADEMY OF SCIENCES VOL.4 INTRODUCTION Primitive species of plethodontids occur in east Despite considerable progress in studies of popula ern North America, primarily in Appalachia. Ad tion genetics and the processes of speciation, biolo vanced species have spread to western North gists have made little headway in understanding the America, Europe, and Central and South America. mechanisms .of macro- and megaevolution. The Because of the variety of adaptive types among goal of this comparative osteological study of the plethodontid species, the family is veritably a "liv salamander family Plethodontidae is to elucidate ing laboratory." It is an ideal group in which to problems relating to evolution above the species study patterns of vertebrate evolution, particularly level. Salamanders are generalized, evolutionarily speciation, mechanisms and patterns of adaptive conservative tetrapods. They were chosen for study radiation, and the origins of higher taxa. because available evidence indicates that evolution This investigation has several objectives. Skeletal ary rates have been relatively slow in the group. variation is analyzed, particularly to elucidate The family Plethodontidae includes a large variety evolutionary relationships on generic and higher of adaptive types among its diverse species. Many levels. Implications for taxonomy and phylogeny: intermediate adaptive stages are extant, thus facili are emphasized and the biogeographic history of tating analysis of major evolutionary events. Hope the family is reanalyzed on the basis of these data. fully, this study will provide groundwork for A functional and evolutionary approach is em comprehensive analyses of evolutionary patterns in ployed to focus attention on factors involved in the the family Plethodontidae, as well as in other invasion of new adaptive zones and subsequent groups. adaptive radiations, that is, on major features of The amphibian order Caudata includes nearly evolution in the family Plethodontidae. 300 species of salamanders. Almost two-thirds of The following brief sketches of the plethodontid these comprise the family Plethodontidae. The genera are presented to provide general background family is distinguished from all others by absence information for the reader. of lungs combined with presence of a cutaneous Salamanders of the genus Desmognathus (eight depression (the nasolabial groove) extending frorri species), the dusky salamanders, are found in the each nostril to the upper lip of transformed indi eastern portion of North America from Quebec and viduals. The most comprehensive treatment of the Nova Scotia to the Gulf Coastal Plain. The stocky, family is Dunn's (1926) pioneer monograph, agile members of this genus occur in a variety of which is now, unfortunately, badly out of date. habitats, but most are aquatic to semiaquatic in While papers by Piatt (1935), Taylor (1944), habits. The most aquatic species are the largest, and Soler (1950), Tanner (1952) and others have pro the smallest species are terrestrial and even semi vided additional information, the introduction to arboreal. All are quick, alert forms, adept at leap "The Salamanders of the Family Plethodontidae" ing and climbing. (Dunn, 1926) has been the best source of informa An aquatic genus, Leurognathus (one species), tion concerning structure and phylogeny. The most resembles the larger species of Desmognathus in recent general reviews of theories of familial evolu habitus. It occurs in mountain brooks in the south tion are given by von Wahlert (1957) and Martof ern Appalachian Mountains. (1962). A recently discovered genus, Phaeognathus (one A major adaptive radiation has been the out species), is a primarily terrestrial form apparently standing feature of the phylogeny of the family. restricted to a small area on the Coastal Plain of Primitive and generalized plethodontids live in Alabama. The single species, an elongate form with semiaquatic habitats and pass through extended very short legs, lives in burrows. larval periods during their ontogeny. Some species Gyrinophilus (three species) is a group of large, are completely aquatic, but advanced species range stout salamanders with elongated trunks and short, from semiaquatic to terrestrial, with many of the stocky tails. The species occur in springs, seepages, intermediate stages still represented by living rivulets, and cave waters from Quebec to northern species. Many advanced species have abandoned Alabama and Georgia. One species is permanently the aquatic larval stage and undergo direct terres larval. trial development; they occupy a large variety of The strikingly colored (brownish to bright red, terrestrial habitats, from cold and barren highlands with dark spots) members of the genus Pseudo to heavily forested tropical lowlands. Some of the triton (two species) are very stout, short-legged terrestrial species are arboreal, and a few are semi species which occur in springs and seepages in fossorial, but most are surface dwellers that take eastern North American from New York to Flor daytime refuge in burrows or under surface cover. ida, and west to Ontario, Ohio, and Louisiana. 1966 EVOLUTION OF LUNGLESS SALAMANDERS 3 The nearly aquatic salamanders of the genus species have flattened bodies and relatively long Stereochilus