Roger Conant

Amphibians and Reptiles of the Chicago Area

Amphibians and reptiles of the Chicago Area , Amphibians and reptiles of the Chicago Area , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Semiaquatic snakes of the genus Thamnophis from the isolated drainage system of the Río Nazas and adjacent areas in Mexico

Several streams traversing portions of the Chihuahuan Desert in northern Mexico were once part of a master river system, the Pleistocene or postPleistocene Rio Nazas, which emptied into the Rio Grande during one or more pluvial periods of the past. Several species of fishes, the turtle, Pseudemys scripta, and the water snake, Natrix erythrogaster, which occur in one or more of the now isolated drainage systems, are identical with or only subspecifically distinct from their counterparts in the Rio Grande. It is assumed that these aquatic and semiaquatic vertebrates achieved their present relict distributions in the desert by working headward from the Rio Grande. Three species of garter snakes, Thamnophis eques, T. melanogaster, and T. rufipunctatus, are also closely associated with the now disrupted streams. A detailed study of scutellation and pattern variations among the Thamnophis populations from the several rivers and adjacent areas suggests that migration along the streams contributed to present-day distribution patterns, but that a more important factor was probably overland movement during the pluvials. All three species of Thamnophis are widespread upon the Mexican plateau, and none is known to occur in the Rio Grande system downstream from the Rio Conchos in Chihuahua. A new race of Thamnophis eques from high elevations in the Sierra Madre Occidental of Durango and Chihuahua is described.