CONSTRAINTS ON MORPHOLOGY: DEVELOPMENTAL OR ECOLOGICAL? LESSONS FROM THE FOSSIL RECORD

It is widely believed that morphological constraints are responsible for the observed pattern of decreasing morphological innovation in both the Metazoa and Metaphytes over geological time. This is readily seen as the decreasing trend of origination of higher taxa; phyla, classes, and orders. At present, there are two competing evolutionary hypotheses that have been proposed to explain this phenomenon, (1) the empty ecospace hypothesis, and (2) the developmental constraint hypothesis. To empirically distinguish between hypotheses (1) and (2), the change in disparity before and after several mass extinction events were measured in crinoids, blastozoans and articulate brachiopods. The assumption is that ecospace-limiting constraints are removed following mass extinctions, revealing the effect of developmental constraints.

Within the Crinoidea disparity was measured before and after three mass extinctions. Within the Blastozoa disparity was measured before and after two mass extinction. For each taxon three suites of characters were analyzed: ecological, non-ecological or "developmental", and combination of the two previous suites plus fifty additional characters. Within the articulate brachiopods, disparity was measured before and after five mass extinctions. For each taxon within the group both ordered (size-related) and unordered (valve shape and ornament) character sets were analyzed. Additionally, a separate analysis was performed on a subset of the articulates, the rhynchonellids and terebratulids.

In the majority of the cases investigated, disparity rebounded to comparable levels or, in many cases higher levels. The results indicate that developmental constraints are not responsible for the decrease in disparity throughout the geologic range of the taxa. The more likely scenario is that increasingly structured ecological guilds have made it much more difficult to allow large increases in disparity.