690 Geologic Time, History of Biodiversity in There have been a number of other mass extinctions (five major events in all) and numerous regional extinctions that were severe locally The most famous of the mass extinctions, the K/T, occurred about 65 million years ago at the close of the Mesozoic era, although it did not have as profound an effect on the marine environment as the P/T event The K/T extinction strongly affected terrestrial as well as marine life It probably resulted from the impact of an extraterrestrial object, such as a meteor, with the earth The ‘‘background’’ extinctions that account for the general turnover of taxa are accompanied by ‘‘background’’ speciations, so that species richness levels not change much during normal turnover and tend to be significantly affected by background effects only during trends that last tens of millions of years or more However, the more abrupt, severe regional and mass extinctions are not immediately compensated by correspondingly massive speciations, and indeed the recovery of species richness usually requires millions of years following a mass extinction For phyla, classes, or orders that are lost to extinction, there is no recovery at all, and the unique gene regulatory systems involved in their architectures are gone forever Clearly, biodiversity data from the fossil record support the notion that species richness is largely associated with environmental heterogeneity, and thus the preservation of species diversity can be promoted by preserving habitat diversity There are entire animal phyla that are represented today by very few species: the phylum Phoronida by only 12 or so, and the phylum Priapulida by perhaps 17, for example These phyla harbor uniquely organized genomes that have been present for over half a billion years of earth history, and that are largely unstudied Their preservation should be a priority, for they represent major elements in the disparity of the present biosphere See also: Archaea, Origin of Biodiversity, Origin of Biogeography, Overview Cladogenesis Eukaryotes, Origin of Genes, Description of Microbial Biodiversity Taxonomy, Methods of References Bengtson S (ed.) (1994) Early Life on Earth New York: Columbia University Press Foote M and Sepkoski Jr JJ (1999) Absolute measures of the completeness of the fossil record Nature 398: 415 Pace NR (1997) A molecular view of microbial diversity and the biosphere Science 276: 734 Schopf JW and Klein C (eds.) (1992) The Proterozoic Biosphere Cambridge, United Kingdom: Cambridge University Press Valentine JW (ed.) (1985) Phanerozoic Diversity Patterns Princeton, New Jersey: Princeton University Press Valentine JW, Jablonski D, and Erwin DH (1999) Fossils, molecules and embryos: New perspectives on the Cambrian explosion Development 126: 851