An Analysis of the Paleozoic Radiation of Marine Animals

Mathematical models of taxonomic origination, extinction, and fossil preservation can provide temporal insight into the large-scale evolutionary experimentation and ecologic expansion of phyla and classes in the Paleozoic Era. Previous work demonstrated that the Cambrian explosion of marine life was characterized by stratigraphic range extensions (intervals of complete non-preservation) of <10⁷ years, thereby validating the rapidity of that unique episode in Earth history (Rivera 2007). The present analysis examines the origination of twenty higher taxa throughout Paleozoic time. At the 50% confidence level, sampled taxa had estimated range extensions from 3.3 ´ 10⁵ to 1.3 ´ 10⁷ years. However, only three taxa (15%) extend into earlier geologic ages or epochs. Polychaeta (Middle Cambrian) extends into the Early Cambrian, while Chondrichthyes and Osteichthyes (both Late Silurian) extend into the Early and Middle Silurian, respectively. At the 80% confidence level, sampled taxa had estimated range extensions from 7.7 ´ 10⁵ to 3.1 ´ 10⁷ years. Eight taxa (40%) extend into earlier geologic ages or epochs. Subphyla or classes appearing in the Early Cambrian Atdabanian Age (Malacostraca, Ostracoda, Bivalvia, and the extinct Blastozoa) are estimated to extend into the earlier Tommotian Age. Polychaeta may extend even deeper into the middle of the Early Cambrian Manykaian Age (535 Ma), while Asterozoa extends deeper into the Early Ordovician. Finally, Chondrichthyes and Osteichthyes extend into the Late Ordovician and Middle Silurian, respectively. At either confidence level, the following general patterns emerge: 1) Coleoidea still originates in the Early Devonian; 2) the Ordovician origination of Crinoidea, Bryozoa, Echinoidea, and the extinct Ammonoidea is confirmed, as is the Late Cambrian origin of Gastropoda, Cephalopoda, and Nautiloidea; 3) the extinct classes Graptolithina and Conodonta, despite some range extension, still originate in the Middle Cambrian; and 4) Brachiopoda and the extinct class Trilobita still originate in the Early Cambrian.