SUBSTRATE AFFINITY AND DIVERSITY DYNAMICS IN BENTHIC MARINE ANIMALS

Many features of evolution in marine animals changed between the Paleozoic and post-Paleozoic. One such change concerns the importance of origination vs. extinction in determining biodiversity trends. During the Paleozoic, short-term variation in genus richness was more strongly correlated with variation in extinction rate than origination rate. During the Mesozoic and Cenozoic, the relative roles of the two processes were reversed. The reasons for this shift may relate to major geographic and environmental differences between the Paleozoic and post Paleozoic. The continents were more concentrated at low latitudes in the Paleozoic, and cratonic flooding was more extensive. Reflecting these and other factors, shallow, carbonate environments were more predominant. If taxa with affinities for carbonate substrates are characterized by different diversity dynamics than terrigenous-loving taxa, the secular shift in diversity dynamics may parallel the major environmental shift. To test this possibility, I determined the substrate affinities of Ordovician-Devonian genera with data from the Paleobiology Database (www.paleodb.org). I classified each genus as carbonate-loving, terrigenous-loving, or mixed, based on the broad lithologic category within which the genus most frequently occurred. I analyzed these three sets of genera separately, as well as in aggregate. In each analysis, I tallied the change in origination rate, extinction rate, and diversity between adjacent stages and calculated correlations between diversity change on the one hand, and origination or extinction change on the other. For these analyses the relevant quantities are the short-term rate changes, not the magnitudes of rates. The diversity-extinction correlation for genera in aggregate is stronger than the diversity origination correlation, consistent with previous findings for the Paleozoic. This difference is more pronounced for carbonate loving than for terrigenous-loving genera, however. Carbonate loving genera show a style of diversity dynamics that is more "Paleozoic-like", i.e. more clearly determined by short-term variation in extinction. Thus, as shown by other kinds of analyses, substrate affinity is an important factor in macroevolution.