The Colubrid Snake Genus Thamnophis: A Revision of the Sauritus Group

Abstract

The Sauritus group of the gartersnakes is composed of two polytypic species, Thamnophis sauritus (Linnaeus) and Thamnophis proximus (Say), commonly called the ribbonsnakes. Both species vary geographically in number of ventrals, number of subcaudals, relative tail length, and several features of the color pattern. Supralabial number is subject to geographic variation in T. sauritus but not in T. proximus. Most of the geographic variation is discordant, transition zones in one character seldom corresponding with those of other characters. As color pattern fluctuates less locally than do the meristic characters, it is more reliable in defining geographic races. Four subspecies of T. sauritus are recognized, two of them previously undescribed ( T. s. septentrionalis of the Great Lakes region, northern New England, and Nova Scotia; T. s. nitae of northwestern peninsular Florida). Of six races of T. proximus four are new (T. p. orarius of coastal Louisiana, Texas, and northern Tamaulipas; T. p. rubrilineatus of the Edwards Plateau region of central Texas; T. p. diabolicus of New Mexico, trans-Pecos Texas, Coahuila, and Nuevo Leon; T. p. alpinus of the Chiapas Highlands in southern Mexico). The evolutionary history of this species group and the trends within each species are discussed. Members of the group are highly specialized and occupy an advanced position within the genus.