THE AMPHIBIAN FAUNA OF THE FOOTHILLS AND ADJACENT HIGHLANDS

To amphibians inhabiting the foothills and mountains of southern México and northern Central America, the isthmus presents a great barrier to dispersal. For example, salamanders of the genus Thorius, the mexicanus and augusti groups of the genus Eleutherodactylus, the bistincta group of the genus Hyla, and the genus Tomodactylus occur on the Mexican Plateau and southward into the mountains of Oaxaca. Nevertheless, no members of these groups are present in the Guatemalan-Chiapan Highlands. The genera Chiropterotriton, Magnadigita, Pseudoeurycea, and Ptychohyla, as well as the eximia group of Hyla are represented by different species in the Guatemalan-Chiapan Highlands than in the mountains of México on the other side of the isthmus. Several species of Plectrohyla occur in the Guatemalan-Chiapan Highlands, but none is known from the Mexican Highlands, although one species occurs in the Tuxtlas.

Living in the humid forests of the foothills are salamanders of the genus Lineatriton, frogs of the spatulatus group of Eleutherodactylus, Anotheca coronata, Hyla miotympanum, and Phyllomedusa moreleti. All of these occur in the foothills of the Sierra Madre Oriental in eastern México and in Los Tuxtlas. Lineatriton, Hyla miotympanum, and the spatulatus group of Eleutherodactylus do not occur in the foothills of the Guatemalan-Chiapan Highlands; those amphibians reach the end of their ranges at the isthmus. Phyllomedusa moreleti and Anotheca coronata are found in the northern foothills of the Guatemalan-Chiapan Highlands, and Phyllomedusa moreleti is found in the foothills on the Pacific slopes of the Chiapan Highlands.

Although the above analysis is not so detailed as that of the lowland inhabitants, it does show that all of the genera and species of amphibians known to inhabit the foothills and highlands adjacent to the isthmus, only two species of amphibians cross the isthmus from one highland mass to the other. Thus, it is evident that the Isthmus of Tehuantepec presents a great barrier to dispersal of these groups of amphibians.