A New Subspecies of Slider Turtle (Pseudemys scripta) from Coahuila, México

by John M. Legler

PLATE 12

Ventral views of four subspecies of Pseudemys scripta: Upper left—P. s. ornata (KU 40131 ♀), Río Playa Vicente, San Andrés Tuxtla, Veracruz, × ⅓; Upper right—P. s. gaigeae (IU 43583 ♀), 1 mi. E La Cruz, Chihuahua, × ⅜; Lower left—P. s. elegans (CNHM 55627 ♂), Múzquiz, Coahuila, × ⅔; Lower right—P. s. taylori new subspecies (KU 46970 juvenile), paratype, 6 mi. W Cuatro Ciénegas, Coahuila, × 11/16.

Natural history.—Specimens of P. s. taylori were caught in hoop nets in clear deep pools and in the Río Chiquito. No specimens were collected or observed in marshy situations where the water was shallow or stagnant. Individuals were seen only near dusk and in early morning when a number floated just below the surface with only their heads showing. They were never seen on land during our short stay in the basin. The few stomachs that were opened contained vegetable material. In terms of number of specimens trapped, P. s. taylori was the most abundant turtle in pools at and near the type locality (Webb and Legler, 1960).

Relationships and phylogeny.—The basin of Cuatro Ciénegas now drains, via the Río Salado, into the lower Río Grande. Brief descriptions of habitats and topography in the basin are given by Gilmore (1947:148-150, fig. 2) and Webb and Legler (1960). In the more northern parts of the Salado drainage (for example, in the Río Sabinas near Múzquiz) slider turtles are typical P. s. elegans. Assuming that conditions which permit genetic exchange between populations of turtles in the Salado drainage system differ in no major respect from conditions in other parts of the range of Pseudemys scripta, it is logical to suppose that the differentiation of P. s. taylori at Cuatro Ciénegas was preceded by the isolation of a population in that basin.

The Río Chiquito drains through a narrow gap in the northeastern end of the basin of Cuatro Ciénegas. Interruption of this stream would effectively isolate aquatic habitats in the basin.

It is here proposed that P. s. taylori is a relict of an earlier, lower Río Grande stock, part of which became isolated in the basin of Cuatro Ciénegas in postpluvial times. The morphological similarity of P. s. taylori and P. s. elegans indicates that both were derived from this parent stock; similarity of both subspecies to populations of P. s. ornata in Tamaulipas suggests that the latter subspecies may also be a derivative of the mentioned stock of the lower Río Grande.

The proposed former isolation of the basin of Cuatro Ciénegas is supported by evidence found in studies of other turtles in the basin. Of the four kinds of turtles known to occur there (Terrapene coahuila, P. s. taylori, Trionyx spinifer emoryi, and Trionyx ater), all but T. spinifer seem to be endemic. These three kinds comprise a graded series, in regard to their degree of differentiation from closest known relatives, as follows: 1) Terrapene coahuila is morphologically the most generalized and primitive of living box turtles; the species is unique in its highly aquatic mode of life (see Legler, 1960:532-534, for brief discussion of relationships within genus Terrapene); 2) Trionyx ater seems to represent a relict population of pre-Trionyx spinifer stock; presumably, spinifer has reinvaded the basin of Cuatro Ciénegas in relatively recent times and, as noted above, spinifer and ater now occur sympatrically (at least in a geographic sense) in the basin (Webb and Legler, op. cit.); and, 3) evidence presented above suggests that P. s. taylori intergrades with P. s. elegans outside the basin.

The three endemic populations of turtles at Cuatro Ciénegas therefore, differ by varying degrees from their closest living relatives. This variation in degree of difference possibly results from varying periods of isolation. Probably the basin of Cuatro Ciénegas has been isolated from, and reconnected to, the lower Río Grande drainage at several times in the past. The relationships of fishes in the basin, now under study by other workers, also suggest that the basin was isolated more than once.

Remarks.—Local names for the above-mentioned localities in the basin of Cuatro Ciénegas are as follows: Anteojo (6 mi. W Cuatro Ciénegas); El Mojarral (8.5 mi. SW); and Ojo de Agua de Tío Candido, on Rancho Orozco (16 km. S). The Río Chiquito is referred to by some natives as "Río Colorado" and by some as "Río Salado." The local name for P. s. taylori is tortuga negra (the name is used also for Terrapene coahuila).