. The biogeography of the herpetofauna of the subhumid forests of Middle America : (Isthmus of Tehuantepec to northwestern Costa Rica). Herpetology; Reptiles, Fossil. for its relationships to other species (fide Sites et al., 1992). However, Cole (1978) placed this species in the S. variabilis group based on a chromosomal synapomorphy. Our proposed scenario would require 5. chrysostictus to be derived from a variabilis-\{] ancestor. The overall histor- ical biogeography of the genus Sceloporus (see Sites et al., 1992) would suggest a northeastern Mexican origin for the variabilis group. La
. The biogeography of the herpetofauna of the subhumid forests of Middle America : (Isthmus of Tehuantepec to northwestern Costa Rica). Herpetology; Reptiles, Fossil. for its relationships to other species (fide Sites et al., 1992). However, Cole (1978) placed this species in the S. variabilis group based on a chromosomal synapomorphy. Our proposed scenario would require 5. chrysostictus to be derived from a variabilis-\{] ancestor. The overall histor- ical biogeography of the genus Sceloporus (see Sites et al., 1992) would suggest a northeastern Mexican origin for the variabilis group. Larsen and Tanner (1975:14) presented a different scenario for the S. variabilis group, postulating the origin of the members of the group from 5. cozumelae, an insular or coastal form on the mainland, at a time when the present-day range of this species was part of a subma- rine bank. Sceloporus variabilis, also a member of the Eastern Mexican Generalized Track, occurs from southern Texas, USA, to northwestern Costa Rica. Pleistocene dis- persal southward from its region of origin in northeastern Mexico (along the east coast of Mexico to the Isthmus of Tehuantepec, across the Isthmus of Tehuantepec and southward along the Pacific coast to northwestern Costa Rica, and into the subhumid interior valleys of Mexico, Guatemala, and Honduras) is postulated for this species. Another species in this track, Sceloporus serrifer, belongs to a group (S. torquatus species group) postulated to have evolved on the Mexican Plateau (see Fig. 35 in Sites et al., 1992). The present-day range of S. serrifer (includes S. cyanogenys; see Olson, 1987) is from southern Texas, USA, to the Yucatan Peninsula and north-central Guatemala. Another species in this track, Tantilla rubra, probably utilized a north-to-south dispersal along the foothills of eastern Mexico, and then across the Isthmus of Tehuantepec to the Plains of Tehuantepec and then into the Central Depression of Chiapas. Two final members of
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