The independent variable in the early origin of higher taxa.

Taxa of high rank tend to originate at earlier times in the fossil record than do taxa of lower rank. Statistical analysis of published stratigraphic ranges shows that the early origin of higher taxa is a secondary consequence of taxonomic structure. For instance, the probability that the origination of an order also begins a new class is a decreasing function of the number of orders that have already originated in the same phylum. Similar decreasing functions also apply to families and orders within classes, and to genera and families within orders, indicating that the pattern of origination is self-similar across at least three taxonomic ranks. These functions are sufficient to predict the extent to which higher taxa originate earlier, but the converse is not true. These results are more consistent with genetic constraints or taxonomic hindsight than with ecological saturation as explanations for the early origin of higher taxa.