A comparative study of diversification events: the early Paleozoic versus the Mesozoic.

Abstract

We compare 2 major long-term diversifications of marin animal families that began during periods of low diversity but produced strikingly different numbers of phyla, classes, and orders. The 1st is the early-Paleozoic diversification and the other the Mesozoic phase of the post-Paleozoic diversification. The earlier diversification was associated with a great burst of morphological invention producing many phyla, classes, and orders and displaying high per taxon rates of family origination. The later diversification lacked novel morphologies recognized as phyla and classes, produced fewer orders, and displayed lower per taxon rates of family appearances. The chief difference between the diversifications appears to be that the earlier one proceeded from relatively narrow portions of adaptive space, whereas the latter proceeded from species widely scattered among adaptive zones and representing a variety of body plans. Our data support those models that hold that evolutionary opportunity is a major factor in the outcome of evolutionary processes. -from Authors