Phanerozoic Diversity Patterns: Tests for Randomness

Patterns of origination within taxa, and patterns of dominance diversity among taxa are examined to distinguish nonrandom from random components. Numbers of first appearances per stage are sporadic and tend to be distributed nonrandomly through time. However, many taxa show a randomly distributed order of stages containing first appearances. A stochastic model of origination only partially reflects the patterns seen in the fossil record. Levels of dominance diversity among many taxa vary synchronously and at the same rates. The Phanerozoic is characterized by a succession of groups of taxa. Such associations are not predicted by a random model of diversity change. The fossil record is examined for evidence of an evolutionary equilibrium with constant taxonomic turnover. Fossil extinction and origination rates do not support the assumption of a static evolutionary equilibrium. Taxonomic and morphological evidence is consistent with a model of progressive specialization through the Phanerozoic.