Contrasting Evolutionary Flexibility in Sister Groups: Disparity and Diversity in Mesozoic Heart Urchins

The general theme of temporal asymmetries in clade histories has often been studied with particular attention to the lower Paleozoic radiations. Post-Paleozoic patterns, however, are less well understood. In this paper, disparity and diversity changes in Mesozoic heart urchins were analyzed in light of an ordinal-level phylogeny, with particular reference to contrasts among the sister groups Holasteroida and Spatangoida, their paraphyletic stem group Disasteroida, and the more inclusive clade, the superorder Atelostomata. A 38-dimensional landmark-based morphospace representing test architecture was used to describe morphological evolution in terms of total variance and total range. Total variance was also partitioned into oral and aboral contributions reflecting different degrees of developmental entrenchment and functionality. Discordances between disparity and diversity were evident, and were expressed as deceleration in morphological diversification in all groups, as well as disproportionately higher disparity early in the histories of the Atelostomata, Holasteroida and Spatangoida. The finding that the early Atelostomatan disparity peak coincides with the origin of the orders Holasteroida and Spatangoida lends support to the perception of higher taxa as semi-independent entities in the biological hierarchy and as meaningful proxies for morphological distinctness. Morphological selectivity was apparent in a comparison of holasteroid and spatangoid responses to the end-Cretaceous mass extinction. Paleocene spatangoid survivors showed no change in disparity relative to the Campanian-Maastrichtian sample, suggesting nonselectivity, while holasteroids suffered more pronounced loss in disparity (despite a rather high Late Cretaceous level of disparity), indicating morphological selectivity. Oral and aboral patterns suggested particular lines of causation behind changes in disparity. The origin of holasteroids and spatangoids is more consistent with an exploration of the developmental flexibility inbuilt in the ontogeny of aboral constructions than with a uniform functional drive towards ecospace occupation. Within groups, several patterns were also most consistent with intrinsic controls. An increase in the developmental modularity of and decrease in developmental constraint on oral landmarks were evident from disasteroids to holasteroids and spatangoids. For aboral landmarks, no substantial change in disparity was observed from disasteroids to holasteroids and spatangoids, suggesting the maintenance of a developmental constraint despite the passage of time and ecological differentiation. More generally, this study suggests that particular topologies of disparity and evolutionary mechanisms potentially characteristic of the lower Paleozoic radiations of higher taxa (e.g., developmental flexibility) need not be confined to any particular time period or hierarchical level. Published as Contrasting Evolutionary Flexibility in Sister Groups: Disparity and Diversity in Mesozoic Atelostomate Echinoids in Paleobiology 26(1) 2000: 56-79.