THE ROLE OF BACKGROUND AND MASS EXTINCTIONS IN THE DECIMATION OF MORPHOLOGIC DISPARITY

Although extinction in the fossil record is often assessed at the level of families and genera, documenting the loss of higher taxa through geologic time may provide insight into the evolution of morphologic disparity. Because disparity represents the range of anatomical designs, phyla, classes, and orders often serve as taxonomic proxies for disparity. An analysis of 231 extinct Phanerozoic marine metazoan orders reveals a temporally asymmetrical pattern of ordinal extinction, with most orders becoming extinct during the Paleozoic Era (n=180; 77.9%), some during the Mesozoic Era (n=45; 19.5%), and only a handful during the Cenozoic Era (n=6; 2.6%). While many orders (n=102; 44.2%) died out during times of background extinction, a large proportion (n=54; 23.4%) went extinct during the traditional Big Five mass extinctions. The single most devastating of these in terms of numbers of orders lost was the Late Devonian (Frasnian) mass extinction (n=16; 6.9%), followed by the end-Permian (n=12; 5.2%), end-Triassic (n=10; 4.3%), end-Ordovician (n=9; 3.9%), and end-Cretaceous (n=7; 3.0%) mass extinctions. In contrast, perhaps less than 5% of Phanerozoic species were lost during these five major crises. The remaining thirteen mass extinctions of the Phanerozoic Eon extinguished 57 orders (24.7%), with the Famennian (n=8; 3.5%), mid-Carboniferous (n=9; 3.9%), and Guadalupian (n=10; 4.3%) events comparable in severity to the end-Cretaceous, end-Ordovician, and end-Triassic mass extinctions, respectively. Surprisingly, relatively few orders died out during the immediate aftermaths of the Big Five (n=4; 1.7%) and other mass extinctions (n=14; 6.1%). Preliminary calculations also indicate that the disparity lost during the Cambrian Period (n=42; 18.2%) actually exceeded that lost during the three largest Big Five mass extinctions combined. Given the unsurpassed morphologic experimentation operating in the Cambrian explosion, however, the disparity lost by the end of the Cambrian may have been greater than that lost during all of subsequent Phanerozoic history.