  Understanding the relationship between taxonomic and morphological changes is
important in identifying the reasons for accelerated morphological
diversification early in the history of animal phyla. Here, a simple general
model describing the joint dynamics of taxonomic diversity and morphological
disparity is presented and applied to the data on the diversification of
blastozoans. I show that the observed patterns of deceleration in clade
diversification can be explicable in terms of the geometric structure of the
morphospace and the effects of extinction and speciation on morphological
disparity without invoking major declines in the size of morphological
transitions or taxonomic turnover rates. The model allows testing of hypotheses
about patterns of diversification and estimation of rates of morphological
evolution. 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 expansion.
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.
