DISPARITY, CONSTRAINTS, AND THE ARTICULATE BRACHIOPOD FOSSIL RECORD

It is widely believed that morphological constraints are responsible for the observed pattern of decreasing major morphological innovation in both the metazoa and metaphytes throughout the Phanerozoic. This is seen as the decreasing trend of origination of higher taxa. Currently, 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 distinguish between hypotheses (1) and (2) the change of morphological innovation before and after several mass extinction events were measured in the Articulate Brachiopoda. Mass extinction intervals provided a means in which to remove ecospace limiting constraints and allow the developmental constraint to be tested. Within the articulate brachiopods disparity was measured before and after four mass extinctions. For each taxon three suites of characters were analyzed: unordered (size), ordered (valve shape and ornament), and combination of the two character suites. Four different measures of disparity were used to analyze each character suite. Additionally, a separate analysis consisting of rhynchonellids and terebratulids was performed. In the majority of the cases investigated disparity rebounded to comparable levels or, in many cases higher levels, with the rhynchonellids and terebratulids showing the largest increase in disparity after the end-Permian extinction. The results indicate that developmental constraints may not be responsible for the decreasing 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.