Taxonomic level as a determinant of the shape of the Phanerozoic marine biodiversity curve.

Key aims of recent paleobiological research have been the construction of Phanerozoic global biodiversity patterns and the formulation of models and mechanisms of diversification describing such patterns. Two conflicting theories of global diversification have been equilibrium versus expansionist growth of taxonomic diversity. These models, however, rely on accurate empirical data curves, and it is not clear to what extent the taxonomic level at which the data are analyzed controls the resulting pattern. Global Phanerozoic marine diversity curves are constructed at ordinal, familial, and generic levels using several fossil-range data sets. The fit of a single logistic model reduces from ordinal through familial to generic level, while conversely, that of an exponential growth model increases. Three sequential logistic equations, fitted to three time periods during which diversity appears to approach or reach an equilibrium state, provide the best description of the data at familial and generic levels. However, an exponential growth curve describes the diversification of marine life since the end-Permian extinction equally as well as a logistic. A species-level model of global Phanerozoic marine diversification, constructed by extrapolation of the trends from familial to generic level, suggests growth in numbers of marine species was broadly exponential. When smaller subsets of the data are analyzed, the effect of taxonomic level on the shape of the diversity curve becomes more pronounced. In the absence of species data, a consistent signal at more than one higher taxonomic level is required to predict a species-level pattern.