HOW DEVELOPMENTAL DIVERSITY GENERATES MORPHOLOGICAL DISPARITY

The recent quantification and operationalization of morphological disparity has informed our ideas about adaptation, diversification, extinction, biodiversity, and constraint. Unfortunately, patterns in disparity are rarely presented in the context of mechanistic hypotheses ("How is disparity generated?"). Although among-taxon differences in ecology, function, history, construction, and development may all play important roles in generating disparity, I focus here on developmental differences. This focus is justified because developmental differences can be (1) directly observed from fossil ontogenetic series; (2) straightforwardly and unambiguously quantified without reference to hypotheses at lower levels of analysis; (3) parameterized to yield substantive insights about the particular aspects of development that generate the most disparity.

Here I analyze the ontogeny of morphological disparity exhibited by a clade of phacopid trilobites. Disparity is measured among the juveniles of all taxa, and this value is compared to the disparity among adults. These comparisons suggest that disparity often decreases through ontogeny -- a result contrary to classical paradigms. Finally, I construct developmental models in which among-taxon developmental differences are ignored. By comparing the observed among-adult disparity to that predicted by the models, I assess the contributions of various developmental parameters to adult disparity. For these trilobites, developmental diversity clearly drives morphological disparity; among developmental parameters, allometric repatterning appears to play the strongest role.