Determining the role that ecological and developmental constraints play in controlling disparity: examples from the crinoid and blastozoan fossil record

Abstract

It is widely believed that morphological constraints are responsible for the observed pattern of decreasing major 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. 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 was measured in the Crinoidea and Blastozoa. Mass extinction intervals provided a means in which to remove ecospace limiting constraints and allow the developmental constraint hypothesis to be thoroughly tested. Within the Crinoidea, disparity was measured before and after three mass extinctions. Within the Blastozoa, disparity was measured before and after two mass extinctions. For each taxon, three suites of characters were analyzed: ecological, nonecological, or "developmental" and a combination of the two previous suites plus 50 additional characters. Four different measures of disparity were used to analyze each character suite. In the majority of the cases investigated, disparity rebounds to comparable levels or in some cases higher levels in both the Crinoidea and Blastozoa. 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.

Evolutionary patterns in early tetrapods. II. Differing constraints on available character space among clades

“…Similar patterns have been observed for both echinoderms (Foote 1996;Ciampaglio 2002) and brachiopods (Ciampaglio 2004) after mass extinctions. Contrasting shifts in character space accompanying major shifts in ecology with those seen within clades during 'background' times (sensu Jablonski 1986) might offer means of estimating general rates at which characters are 'gained' and 'lost' to clades as well as offer some clarification of the circumstances needed for rapid morphological diversification.…”

Anuran radiations and the evolution of tadpole morphospace

“…Such slowdown in other taxa has been perceived as the result of growing niche competition and saturation of the available ecological space, increased stabilizing selection on functionally integrated traits, or establishment of intrinsic (genetic or developmental) constraints (43)(44)(45)(46)(47). The Late Cretaceous/Early Tertiary interval is known to be a period of global biotic turnover, and previous studies have suggested that amphibians took opportunistic advantage of contemporary ecosystem remodeling (15,48).…”

Extinction as the loss of evolutionary history

“…Limited phylogenetic analyses have been produced, mostly for graptolites and gastropods, although some broader studies do span the boundary. The loss of morphologic disparity during this event appears to have been high, whether as measured by the major losses among graptolites, conodonts, brachiopods, and possibly nautiloids or by more quantitative studies of disparity within major clades (44,45,92,93). Using reefs as our measure of architectural complexity, there is a major loss of both reef types and carbonate production although there is little ecological impact (78,94), hence the medium ranking in Table 1.…”