ABSTRACT: For about a half century, most vertebrate paleontologists have correlated the youngest Permian tetrapod assemblages in North America, which are from the San Angelo, Flowerpot and Chickasha formations in Texas-Oklahoma, to the oldest therapsid-bearing assemblages of the Russian Kazanian. This correlation was not based on shared low-level taxa (genera and species), but on the supposed therapsids in the American faunas and the presence of some �counterparts� (equivalent evolutionary grade) among theAmerican and Russian captorhinids and caseid pelycosaurs.Marine biostratigraphy indicates that the youngest tetrapod assemblages in Texas-Oklahoma are of Kungurian (late Leonardian) age, whereas the base of the Kazanian is no older than late Roadian. Thus, a very real hiatus in the global tetrapod record, previously named Olson�s gap, is evident between the youngest, pelycosaur-dominated assemblages of North America and the oldest, therapsid-dominated tetrapod assemblages of Russia. This hiatus is equivalent to most of Roadian time, and is at least two million years long. A review of the global record of Permian tetrapod body fossils and footprints reveals that this is a hiatus of global extent. Olson�s gap corresponds to a significant remodelling of the Permian tetrapod fauna.
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