We view the Miocene Smilisca as the prototype described in the preceding section, and suppose that it lived in the mesic tropical environment of the eastern part of the Central American Peninsula (in what is now Costa Rica and western Panamá). Two stocks differentiated, probably in middle Miocene times; one of these, the ancestral stock of the baudini group, was widespread on the Caribbean lowlands from the Nicaraguan Depression to the Bolivar Trough, and the other, the ancestral stock of the sordida group, was restricted to the Pacific lowlands of the same region. In late Miocene time the ancestral stock of the baudini group dispersed northwestward around the deep embayment in the Nicaraguan depression into upper Central America (in what is now Honduras and Guatemala) and thence into southern México. Apparently differentiation took place on each side of the Nicaraguan Depression; the frogs to the south of the depression evolved into S. phaeota, whereas those to the north of the depression represented the stock from which S. baudini and cyanosticta arose. Prior to the uplift of the mountains in the late Miocene and the Pliocene the baudini-cyanosticta stock probably was widespread in northwestern Central America. The elevation of the mountains resulted in notable climatic changes, principally the development of sub-humid environments on the Pacific lowlands. The frogs living on the Pacific lowlands became adapted to sub-humid conditions and developed into S. baudini. The stock on the Caribbean lowlands remained in mesic environments and evolved into S. cyanosticta.
Possibly in the middle Miocene before the Talamanca Range in Costa Rica and western Panamá was greatly uplifted, the ancestral stock of the sordida group invaded the Caribbean lowlands of what is now Costa Rica. The subsequent elevation of the Talamanca Range in the Pliocene effectively isolated the ancestral stock of S. sila on the Pacific lowlands from the puma-sordida stock on the Caribbean lowlands. The former was subjected to the sub-humid conditions which developed on the Pacific lowlands when the Talamanca Range was uplifted. It adapted to the sub-humid environment by living along streams and evolving stream-adapted tadpoles. On the Caribbean side of the Talamanca Range the puma-sordida stock inhabited mesic environments. The stock that evolved into S. puma remained in the lowlands as a pond-breeding frog, whereas those frogs living on the slopes of the newly elevated mountains became adapted for their montane existence by developing stream-adapted tadpoles and thus differentiated into S. sordida.
Probably the six species of Smilisca were in existence by the end of the Pliocene; at that time a continuous land connection existed from Central America to South America. The climatic fluctuations in the Pleistocene, and the post-Wisconsin development of present climatic and vegetational patterns in Middle America, brought about the present patterns of distribution of the species. From its place of origin on the Caribbean lowlands of lower Central America, S. phaeota dispersed northward into Nicaragua and southward along the Pacific slopes of northwestern South America. Perhaps in the late Pleistocene or in post-Wisconsin time when mesic conditions were more widespread than now, S. phaeota moved onto the Pacific lowlands of Costa Rica. Its route could have been through the Arenal Depression. Subsequent aridity restricted its range on the Pacific lowlands to the Golfo Dulce region. Climatic fluctuation in northern Central America restricted the distribution of S. cyanosticta to mesic habitats on the slopes of the Mexican and Guatemalan highlands and to certain humid areas on the lowlands. Smilisca baudini was well adapted to sub-humid conditions, and the species dispersed northward to the Rio Grande Embayment and to the edge of the Sonoran Desert and southward into Costa Rica. In southern México and Central America the species invaded mesic habitats. Consequently, in some areas it is sympatric with S. cyanosticta and phaeota.
Smilisca puma dispersed northward onto the Caribbean lowlands of southern Nicaragua. Its southward movements probably were limited by the ridges of the Talamanca Range that extend to the Caribbean coast in the area of Punta Cahuita in Costa Rica. Smilisca sila dispersed along the Pacific lowlands and slopes of the mountains from eastern Costa Rica and western Panamá through eastern Panamá to northern Colombia. Climatic fluctuation in the Pleistocene evidently provided sufficient altitudinal shifts in environments in the Talamanca Range to permit S. sordida to move onto the Pacific slopes. From its upland distribution the species followed streams down to both the Caribbean and Pacific lowlands, where it is sympatric with S. puma on the Caribbean lowlands and S. sila on the Pacific lowlands.
The evolution of the species-groups of Smilisca was effected through isolation by physical barriers in the Cenozoic; the differentiation of the species was initiated by further isolation of populations by changes in physiography and climate. Present patterns of distribution resulted from Pleistocene and post-Wisconsin climatic changes. Today, sympatric species have different breeding habits and breeding calls which reinforce the differences in morphology.
The genus Smilisca is composed of six species of tree frogs; each species is defined on the basis of adult morphology, larval characters, and breeding behavior. Keys are provided to aid in the identification of adults and of tadpoles.
Analysis of the characters and examination of type specimens indicates that several currently-recognized taxa are synonymous, as follows:
1. Hyla beltrani Taylor, 1942 = Smilisca baudini.
2. Hyla gabbi Cope, 1876 = Smilisca sordida.
3. Hyla manisorum Taylor, 1954 = Smilisca baudini.
4. Hyla nigripes Cope, 1876 = Smilisca sordida.
5. Hyla wellmanorum Taylor, 1952 = Smilisca puma.
Smilisca phaeota cyanosticta Smith, 1953 is elevated to specific rank, and one new species, Smilisca sila, is named and described.