Dynamics of clade diversification on the morphological hypercube

Understanding the relationship between taxonomic and morphological changes is important in identifying the reasons for accelerated morphological diver sification early in the history of animal phyla. Here, a simple general mod el describing the joint dynamics of taxonomic diversity and morphological d isparity is presented and applied to the data on the diversification of bla stozoans. I show that the observed patterns of deceleration in clade divers ification can be explicable in terms of the geometric structure of the morp hospace and the effects of extinction and speciation on morphological dispa rity without invoking major declines in the size of morphological transitio ns or taxonomic turnover rates. The model allows testing of hypotheses abou t patterns of diversification and estimation of rates of morphological evol ution. In the case of blastozoans, I find no evidence that major changes in evolutionary rates and mechanisms are responsible for the deceleration of morphological diversification seen during the period of this clade's expans ion. At the same time, there is evidence for a moderate decline in overall rates of morphological diversification concordant with a major change (from positive to negative values) in the clade's growth rate.