PHANEROZOIC TIMESCALE BASED ON TETRAPOD BIOCHRONOLOGY

A biochronologic timescale based on tetrapod (amphibian, reptile and mammal) biostratigraphy can be constructed for the Late Devonian to Recent. Paleozoic tetrapod biostratigraphy is plagued by the rarity or absence of good index fossils, particularly prior to the Middle Permian. Tetrapods can be used to divide Late Devonian-Carboniferous time into only four intervals, largely because of the limited geographic distribution, low taxonomic diversity and incomplete sampling of the Devonian-Carboniferous tetrapod record. Tetrapods support division of the Permian Period into three epochs, which corresponds well to a threefold division of Permian time based on marine biostratigraphy. Permian time can be divided into 10 land-vertebrate faunachrons (lvfs), but a hiatus in the global tetrapod record is evident between the youngest, pelycosaur-dominated Permian assemblages of North America and the oldest, therapsid-dominated tetrapod assemblages of Russia. Triassic tetrapod biochronology identifies 8 global lvfs comparable in resolution to the marine ages of the Triassic. However, by Jurassic time the fragmentation of Pangea and provincialization of the global tetrapod fauna forces recognition of distinct provincial biochronologies. At present, I recognize only about 4 Jurassic and 7 Cretaceous lvfs, which is much less resolution than provide by Jurassic-Cretaceous marine biochronology. Cenozoic tetrapod biochronology is the scheme of land-mammal “ages” (=lvfs), which are provincial biochronologies that have been proposed for all the continents except Antarctica. The most extensively developed scheme, in western North America, recognizes 19 lvfs and a variety of subdivisions that discriminate time intervals of less than 1 my in many parts of the Cenozoic timescale. In the future, Jurassic-Cretaceous tetrapod biochronology needs much refinement, and the pre-Middle Permian tetrapod record needs further development to provide a better basis for biochronology.