ONTOGENETIC AND EVOLUTIONARY DYNAMICS OF SHAPE DISPARITY IN THE TRILOBITE PHACOPS

Patterns of morphological diversity ("disparity") are traditionally described from adult morphologies. Adding an ontogenetic dimension to such analyses can take us beyond describing patterns to explaining them. Such explanations require comparative studies of ontogeny. These comparisons tell us which ontogenetic parameters differ, but they lack explicit information about disparity. In particular, these studies fail to identify the ways in which ontogenetic differences affect disparity patterns. By bringing together studies of disparity and of comparative ontogeny, we hope to explore more substantively the contribution of ontogenetic change to patterns of disparity. Here we examine the ontogenetic dynamics of shape disparity in light of evolutionary changes in ontogeny of shape in the trilobite Phacops.

To analyze the ontogenetic dynamics of disparity, we compare the magnitude and structure of shape disparity at two points in holaspide ontogeny. These observed ontogenetic changes in disparity are then compared to expectations under hypothetical models, in which particular sets of developmental variables are constrained to be equal among taxa. Preliminary analyses of five Phacops species and subspecies suggests that disparity increases dramatically over holaspide ontogeny. Disparity of adults appears to be due largely to two types of modifications of ontogeny: allometric repatterning and changes in developmental rate. Analyzing the two component subclades reveals strikingly different patterns. In one, disparity is relatively modest, and does not increase significantly over growth; in the other, disparity is substantial at both stages, and increases markedly through ontogeny. Differential behavior of these two subclades suggests that the dynamics of one subclade drive the dynamics of the larger clade. Such differences provide evidence for a strong role for development in structuring changes in disparity in evolving lineages.