The Toroweap Formation of the Permian has both an open-marine fauna and
a molluscan fauna. The open-marine fauna in the west includes
brachiopods, bryozoans, crinoids, and horn corals. The molluscan fauna
in the east includes bivalves and gastropods, with scattered scaphopods
and cephalopods. This formation reflects transgressions and regressions
of an eastward advancing and west-ward retreating sea.