The Coahuilan box turtle (Terrapene coahuila) spends most of its time underwater whereas other box turtles evolved as land organisms. (Photo from National Geographic October 1995). |
The biota of the basin also provides evidence of continuity of the aquatic habitats since ancient times. Many species found in the basin are greatly differentiated from their living relatives. For example, mollusks and crustaceans in the basin exhibit a high degree of differentiation from other members of their taxon, scarcely resembling known mollusks of eastern Mexico (Minckley 1969). However, other groups of organisms are only slightly changed or not changed at all. Nonendemic fishes, for example, are weakly differentiated from other populations. Nearly 50 percent of the fishes occur as similar morphotypes in and outside the basin (Minckley 1969). This indicates that the basin was invaded at differing times, most likely during pluvial periods.
The herpetofauna provides evidence of a continuum of a long history of successive faunal invasions. A good example of the patterns of differentiation is found in a study of amphibians and reptiles conducted by C.J. McCoy (1984). On one end there are highly differentiated species such as the Coahuilan box turtle (Terrapene coahuila), whose phyletic relationships are subject to controversy. In the middle are intermediate levels of differentiation illustrated through species such as the Cuatro Ciénegas soft-shell (Trionyx ater), a species distinct morphologically yet hybridizes freely with invading populations of the Spiney soft-shell (Trionyx spiniferus) (Webb 1973). And on the extreme end of the spectrum are there are relict species which are undifferentiated from main range populations such as the Racer (Coluber constrictor) (McCoy 1984).
Populations of relict species provide evidence of the Cuatro Ciénegas basin as modern refuge. Milstead (1960) studied 14 relict species of the Chihuahuan desert, distributed in five disjunct populations removed from the main portions of their ranges. The populations are restricted to mesic areas in the desert such as damp canyons, springs, and permanent streams. Five of the relict species are found within the Cuatro Ciénegas basin and show little divergence from the main populations to the east. These species are: Gulf Coast Toad, (Bufo valliceps), Barking frog (Eleutherodactylus latrans), Four-lined skink (Eumeces tetragrammus), Blotched water snake (Natrix erythrogaster), and Coahuilan boxturtle (Terrapene coahuilae).With its abundant surficial water, Cuatro Ciénegas acts as a modern refuge for these mesic-species in the midst of the arid desert.