A specimen of the aquatic reptile Champsosaurus sp. from the Paleocene Black Peaks Formation in southwestern Texas is the southernmost yet known. The fragmentary specimen exhibits some unusual features, such as a great anterior extent of the quadratojugal on the lower temporal arch, and cannot be attributed with confidence to any of the named species. Champsosaurus appears to have been tolerant of temperate climates and had a northern latitudinal range exceeding that of crocodylians. It seems likely that the brief southward extension in range of Champsosaurus during early Paleocene time resulted from a decrease in mean annual temperature, comparable to over 10° of paleolatitude.