The San Angelo Formation, Permian of Texas, and Its Vertebrates

Abstract

A vertebrate fauna has been obtained from the San Angelo formation, Permian, of Hardeman and Foard counties, Texas. This formation, which is composed of sandstones, red shales, and conglomerates, is considered to be of earliest middle Permian age. It appears to have been deposited by a series of major streams with three different source areas. The environment of deposition was deltaic, and beds seem to have been laid down under alternating wet and dry conditions. Eight genera of vertebrates, a xenacanth shark, a captorhinomorph reptile, and six synapsid reptiles have been determined. Six of the genera and seven species are new. The vertebrate fauna occurs in large part in floodplain deposits of the delta. It is believed to have been derived from an upland chronofauna that developed concurrently with the deltaic chronofauna of the Wichita and Clear Fork rather than from this deltaic assemblage. In evolutionary position the San Angelo fauna is notably advanced over that known from the Clear Fork. It includes adaptive equivalents of various middle Permian vertebrates of Russia and South Africa, but none of the San Angelo forms is ancestral to any of the Old World types. There is no indication that there were any connections between the Old and New World faunas at this time.