Genera of Leptodactylid Frogs in Mexico.
by John D. Lynch.
INTRODUCTION
According to the most recent review of the Mexican amphibian fauna (Smith and Taylor, 1948), six genera of leptodactylid frogs occur in Mexico. One other genus, _Pleurodema_, occurs in Lower Central America.
Smith and Taylor recognized one species of _Engystomops_, 28 of _Eleutherodactylus_, three of _Leptodactylus_, eight of _Microbatrachylus_, 12 of _Syrrhophus_, and five of _Tomodactylus_.
Subsequent to the publication of their checklist of the Mexican amphibia (1948), numerous taxonomic changes have been proposed. Many species of _Eleutherodactylus_ have been added to the fauna, either through the extension of their recorded ranges into Mexico from Guatemala or by the recognition of species unknown in 1948, whereas some nominal species have been synonymized. _Microbatrachylus_ has been regarded as synonymous with _Eleutherodactylus_ (Lynch, 1965); four species of _Microbatrachylus_ currently are regarded as valid (Duellman, 1961, Lynch, 1965). _Syrrhophus_ was revised in part by Duellman (1958) and Firschein (1954), and a species of _Tomodactylus_ transferred to _Syrrhophus_ by Dixon (1957), who redefined _Tomodactylus_ and added more species to the genus.
Since beginning my studies of the Mexican leptodactylids in 1962, I have become acutely aware of difficulties involved in defining the genera. A revision of _Eleutherodactylus_ and a review of _Syrrhophus_ are nearing completion, but prior to their publication it is desirable to redefine the genera of the Mexican leptodactylids, and in so doing recognize an heretofore unnamed genus. The definitions of _Eleutherodactylus_ and _Leptodactylus_ may need to be altered in the future, since both are widespread in South America and occur in the West Indies. Their definitions as given here are as precise as present knowledge permits. _Syrrhophus_ and _Tomodactylus_ are small assemblages that occur only in southwestern United States, Mexico, and Guatemala.
Taylor (1952) synonymized _Engystomops_ with _Eupemphix_ which, although related, should be regarded as generically distinct (Gallardo, 1965). Perhaps the most conservative classification is that of Myers (1962) who, without published evidence, combined _Eleutherodactylus_, _Syrrhophus_, and the South American _Lithodytes_ in a single genus.
The major problem for students working with the Mexican leptodactylids has not been the separation of _Engystomops_ or _Leptodactylus_ from other genera but the separation and definition of the eleutherodactyline frogs currently placed in three genera, _Eleutherodactylus_, _Syrrhophus_, and _Tomodactylus_. As will be shown in this paper, these are more conveniently placed in four genera. Once a fourth genus is recognized, certain phylogenetic problems disappear and a reasonable zoogeographic interpretation is possible for Middle American leptodactylid distribution.
ANALYSIS OF CHARACTERS
In Mexico and northern Central America approximately 55 species of eleutherodactyline frogs (_Eleutherodactylus_, _Syrrhophus_, and _Tomodactylus_) are known. Four genera can be recognized on the basis of the nature of inguinal glands, morphology of the hands and feet, and certain osteological features.
[Illustration: FIG. 1. _Tomodactylus angustidigitorum_ (UMMZ 114305, 4.5) illustrating the lumbo-inguinal gland typical of members of the genus. From a kodachrome by Wm. E. Duellman.]
Glands
Leptodactylids have a variety of glands that have been used as generic characters. Smith and Taylor (1948) regarded the so-called inguinal gland as a generic character in Mexican eleutherodaycty-lines. Lynch (1965) showed that _Eleutherodactylus_ and _Microbatrachylus_ cannot be separated by the nature of the gland or the condition of the prevomers (dentate or not). _Syrrhophus_ and _Tomodactylus_, as defined by Smith and Taylor (1948), are not generically distinct because of overlap in the condition of the prevomers and in the development of the gland. Firschein (1954) stated that _Syrrhophus_ differed from _Tomodactylus_ by having an axillary gland, but it is now known that one species of _Syrrhophus_ lacks the gland.
The inguinal glands of _Eleutherodactylus_ and _Syrrhophus_, if present, are diffuse, irregular in outline, and generally not prominent; in _Tomodactylus_ the gland is higher on the body (a lumbo-inguinal gland), compact, oval in outline, and prominent (Fig.
1). Axillary glands occur in most _Syrrhophus_ but are not known in _Tomodactylus_ or _Eleutherodactylus_.
Hands and feet
The tips of the digits are laterally expanded in most _Eleutherodactylus_, _Syrrhophus_, and _Tomodactylus_. Two species of _Eleutherodactylus_ (_augusti_ and _tarahumarensis_) and two _Tomodactylus_ (_angustidigitorum_ and _grandis_) lack any expansion of the digital tips. All but two of the species of eleutherodactyline frogs (_E. augusti_ and _E. tarahumarensis_) have a transverse groove across the tips of the digits (Fig. 2).
[Illustration: FIG. 2. Palmar views of the hands and lateral views of the tip of the third digits of _Eleutherodactylus alfredi_ (left, KU 93994, 5) and _Hylactophryne augusti_ (right, KU 102594, 3).]
Supernumerary tubercles rarely are present on the feet of _Eleutherodactylus_, but are present and numerous in every species of _Syrrhophus_, _Tomodactylus_, and in the members of the _augusti_ group of _Eleutherodactylus_ (Fig. 3). The tubercles are small and numerous in _Syrrhophus_ and larger in _Tomodactylus_ and the _Eleutherodactylus augusti_ group. Most species of _Eleutherodactylus_ have no plantar supernumerary tubercles; a few species have such tubercles, which never extend between the metatarsal tubercles as in _Syrrhophus_ and _Tomodactylus_.
[Illustration: FIG. 3. Plantar views of feet of _Eleutherodactylus alfredi_ (left, KU 93994, 4.5), _Syrrhophus pipilans nebulosus_ (middle, KU 58900, 7.5), and _Hylactophryne augusti_ (right, KU 102594, 3) showing differences in size and arrangement of supernumerary tubercles.]
Tarsal folds and tubercles are lacking in _Syrrhophus_, _Tomodactylus_, and the _augusti_ group of _Eleutherodactylus_. Several species of _Eleutherodactylus_ lack tarsal folds and tubercles, but in nearly every species group, one or more species possess either an inner tarsal fold, inner tarsal tubercle(s), or outer tarsal tubercles.
The terminal phalanges of _Syrrhophus_, _Tomodactylus_, and all _Eleutherodactylus_ (except the frogs of the _augusti_ group) are distinctly T-shaped. In the latter, the bones are knob-shaped distally (Fig. 4). T-shaped terminal phalanges also are present in _Lithodytes_ and _Trachyphrynus_ but not in other leptodactylid genera. At least one species of _Eupsophus_ (_E. quixensis_) has terminal phalanges that resemble those of the _Eleutherodactylus augusti_ group. Several species of _Eleutherodactylus_, _Syrrhophus_, and _Tomodactylus_ with slender fingers have T-shaped terminal phalanges although the terminal dilations proportionately are only scarcely wider than the finger tips in the _Eleutherodactylus augusti_ group. The presence of a terminal groove at the tip of the finger is an external indicator of the T-shaped terminal phalanges.
[Illustration: FIG. 4. Terminal phalanges of four leptodactylid frogs (all 13.5). (a) _Eleutherodactylus mexicanus_, KU 55593; (b) _Eupsophus roseus_, KU 84731; (c) _Eupsophus quixensis_, UIMNH 59643; and (d) _Hylactophryne augusti_, KU 56192.]
Skull
All Mexican eleutherodactyline frogs have quadratojugal-maxillary articulations, completely roofed skulls in adults, median contact of the nasals, separated occipital condyles, and large prevomers. The premaxillae of all species are visible when the skulls are viewed from directly above. The pterygoid lacks a medioventral flange and does not meet the palatine. In no species is the anterior arm of the squamosal in contact with the maxillary. Of the numerous species examined (30 _Eleutherodactylus_, four _Syrrhophus_, and four _Tomodactylus_), the species in the _Eleutherodactylus augusti_ group are unique in having a sphenethmoid with a blunt anterior edge.
Pectoral Girdle
All species have large cartilaginous plates in the pectoral girdles; none possesses a bony style. No divergent modifications of the clavicle and coracoid bones are known in the family.
GENERIC ACCOUNTS
Genus ~Eleutherodactylus~ Dumeril and Bibron, 1841
_Type-species._--_Hylodes martinicensis_ Tschudi, 1838
_Diagnosis and definition._--Small to large frogs (12 to 110 mm. snout-vent length) having slightly to widely expanded digital pads, each pad bearing a terminal transverse groove; lumbo-inguinal, inguinal, and axillary glands absent, or if present, diffuse, irregular in outline, not compact; plantar supernumerary tubercles absent, or if present, six or fewer, restricted to distal area of plantar surface, and not extending between metatarsal tubercles; tarsus bearing inner or outer tubercles or folds or not; toes free to one-half webbed; terminal phalanges T-shaped; sternum cartilaginous, lacking bony style; sphenethmoid not truncate anteriorly; nasals in contact medially; maxillary and quadratojugal in contact; anterior arm of squamosal not in contact with maxillary; dermal cranial elements not involved in integumentary-cranial co-ossification; prevomers large, dentigerous processes present or not, dentate or not; maxillary and premaxillary bones dentate; occipital condyles separated; development direct.
_Composition._--About 420 names have been applied to frogs of this genus; many of these names are synonyms, and many other species remain undescribed and unnamed. Perhaps the genus contains 350 species. Thirty-one species occur in Mexico and northern Central America.
_Distribution._--From Tamaulipas and Sinaloa, Mexico, exclusive of the Mexican Plateau, to at least Peru and southernmost Brazil and throughout the West Indies.
Introduced into Florida.
_Etymology._--Greek (_eleuthero_ + _dactylus_) meaning free-toed.
Genus ~Engystomops~ Jimenez de la Espada, 1872
_Type species._--_Engystomops petersi_ Jimenez de la Espada, 1872
_Diagnosis and definition._--Small frogs (20 to 40 mm.
snout-vent length) having undilated digital tips lacking transverse grooves; lumbo-inguinal or inguinal glands absent; plantar supernumerary tubercles present, extending between metatarsal tubercles; tarsus bearing spinelike tubercle on inner edge; toes free; terminal phalanges pointed; sternum bearing bony style; spenethmoid not truncate anteriorly; nasals in contact medially; maxillary and quadratojugal in articular contact; anterior arm of squamosal not in contact with maxillary; dermal cranial elements not involved in integumentary-cranial co-ossification; prevomers moderate in size, lacking teeth; maxillary and premaxillary bones edentate; occipital condyles separated; tadpole free living.
_Composition._--Four nominal species (_E. petersi_, _E.
pustulatus_, _E. pustulosus_ and _E. schereri_).
_Distribution._--Central Veracruz and eastern Oaxaca, Mexico, to Trinidad, Bolivia, and Peru, east of the Andes.
_Etymology._--Greek (_engys_ + _stoma_) meaning narrow-mouthed.
Genus ~Hylactophryne~ new genus
_Type-species._--_Hylodes augusti_ Duges, 1879