Species diversity among fossil invertebrates of the Phanerozoic is highly correlated with volume and area of sedimentary rocks. The correlations are statistically significant at the 1% level. The relationship holds even in regions (such as Canada) where the area and volume of rock do not increase through time. These results are interpreted as indicating that the apparent number of species is strongly dependent on sampling and that many of the changes in diversity seen in the Phanerozoic are artifactual. Consequently, there is no compelling evidence for a general increase in the number of invertebrate species from Paleozoic to Recent. This conclusion applies primarily to marine organisms. Diversity may have been in dynamic equilibrium throughout much of this time.