Extinction in the Fossil Record

Abstract

Extinction in the fossil record provides a rich source of history and data useful for understanding extinction in current times. Extinction rates in the fossil record can be estimated by loss of living species as we go back into the fossil record, but usually is determined by loss of numbers of taxa (level usually above that of species) past known geological time boundaries. Large-scale extinction events are characteristic of the fossil record, with 5 major mass extinctions recognized but nearly 20 have been recognized as of unusual magnitude. Recognized mass extinctions have been related to major environmental change, including large scale igneous activity, changes in climate, relation to environmental indicators such as isotopic ratios, and extraterrestrial impacts. The presence of Large Igneous Provinces are conspicuously associated with many major mass extinctions, including the end-Cretaceous extinction, also thought to be caused by an extraterrestrial impact. Controversy remains as to relating ultimate effects (e.g., large scale igneous eruptions) and proximate effects (e.g., ocean warming, anoxia, sulfide poisoning) of mass extinctions. Lower rates of extinction are more difficult to explain but might represent a background probability of extinction related to population processes and typical environmental fluctuation. Speciation and extinction rate appears to have declined during the Phanerozoic although this could be an artifact of measuring extinction using higher taxonomic units.