San Angelo Formation

The San Angelo Formation is a geologic formation in Texas. It preserves fossils dating back to the Permian period. Along with the Chickasha Formation is one of the two geologically youngest formations in North America to preserve fossils of caseids, and it is the youngest one to preserve remains of undoubted sphenacodontids, namely, Dimetrodon angelensis.

Stratigraphy and age
Some studies argued that the San Angelo Formation belongs to the Kungurian stage of the Cisuralian series because it underlies the Blaine Formation, which is, according to the same studies, either upper Kungurian or lower Guadalupian. However, a recent study concluded that Olson was correct in regarding the San Angelo Formation as belonging to the Roadian, and that the Blaine Formation also dates from the Roadian.

Fossil content
Everett C. Olson regarded the San Angelo Formation as preserving some of the oldest known therapsids, several of which he classified in a taxon he called Eotheriodonta. These taxa are now interpreted as caseids and sphenacodontids, not therapsids.