Fossil vertebrates, biostratigraphy, biochronology and chronostratigraphy

Fossil vertebrates have been used to establish geological ages and correlations since the beginning of their scientific study by Georges Cuvier in the early 1800s. This work began with straightforward biostratigraphic analysis of vertebrate fossil distributions, and such analysis continues today. The North American land-mammal “ages,” first defined in 1941, represented the first explicit vertebrate biochronology. Since then, a biochronological approach has created much Phanerozoic biochronology in the form of land-mammal”ages,” land-vertebrate “ages” and land-vertebrate faunachrons. In marine settings, fossil fishes (especially Paleozoic ichthyoliths) have been employed in biostratigraphy. Paleozoic tetrapod fossils provide little useful chronology and correlation until the Middle Permian, when Pangea-wide tetrapod assemblages can be correlated based on some relatively cosmopolitan taxa and some locally abundant tetrapod assemblages. This continues through most of the Triassic, but, in Jurassic time provincialization of the tetrapod fauna and other factors have confounded attempts to develop useful vertebrate biostratigraphy and biochronology. The situation improves in the Cretaceous, when in some regions (especially the North American Western Interior basin) tetrapod fossils provide relatively detailed biostratigraphy and biochronology. Biochronological schemes using fossil mammals have proven to be particularly robust concepts used to divide Cenozoic time by land-mammal”ages.” Indeed, Cenozoic mammalian biochronology works so well in some regions (western USA) that little or no reference to the standard global chronostratigraphic scale below the level of epoch is made in age assignments and correlations. Land-mammal “ages” resolve time to about 1–3 million year intervals. They exemplify what can be achieved with vertebrate biochronology in terms of age determinations, correlations and placing vertebrate history into a broader framework of physical and biotic events. I thus advocate further development of such vertebrate biochronology for the entire fossil record of vertebrates. There also continues to be a need for more detailed stratigraphic data on vertebrate fossil distribution in order to refine current biochronological schemes, and I make some recommendations for future research.