DISPARITY-DEPENDENT DIVERSIFICATION IN PALEOZOIC CRINOIDS

Several lines of evidence have shown that taxa exhibit diversity-dependence throughout the Phanerozoic. Ecological crowding is one of the leading hypotheses for the causes of diversity-dependence. Under ecological crowding, we also expect that taxa with more similar ecomorphologies should undergo stronger diversity-dependence. Thus, taxa may also be under disparity-dependence of diversification rates, but this pattern has not been as well-studied as diversity-dependence. In this study, I test for disparity-dependence in Paleozoic Eucladid crinoids using two approaches. First, I correlate standing disparity at the start of each time bin with origination and extinction within those time bins. I find positive correlations for origination and negative correlations for extinction, which provides support for ecological crowding. In the second approach, adding the assumption that lineages can vary in rates, I test whether lineages that are farther apart from others in morphospace also have higher probabilities of giving rise to descendants and lower probabilities of going extinct. Again, there is support for ecological crowding. Furthermore, given that these approaches have not been applied before, I also use a forward simulation approach to show that they are capable of detecting disparity-dependence. The simulations also show that the second approach of testing for disparity-dependence has an advantage over the first approach, in that the second approach is not sensitive to long-term trends in disparity and rates. Overall, the results suggest that the effects of competition between morphologically similar crinoids may extend over macroevolutionary timescales to suppress their diversification.