We constructed two phenotypic spaces: a static, adult one (adult morphospace) and a dynamic, developmental one (allometric space). Comparative disparity analyses show a strikingly stable occupation in both spaces, despite extensive change in taxonomic composition. In contrast, disparity analyses of subclades reveal clearly distinct morphological and allometric disparity dynamics.Allometric approaches allow developmental insights into morphological diversification otherwise intractable from the analysis of adult morphospace alone.

Temporal asymmetries in clade histories have often been studied in lower Paleozoic radiations. Post-Paleozoic patterns, however, are less well understood. In this paper, disparity and diversity changes in Mesozoic heart urchins were analyzed at the ordinal level, with contrasts among the sister groups Holasteroida and Spatangoida, their paraphyletic stem group Disasteroida 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. Discordances between disparity and diversity were evident and were expressed both as deceleration in morphological diversification in all groups and as disproportionately higher disparity early in the histories of the Atelostomata, Holasteroida Spatangoida. The finding that the early atelostomate disparity peak coincides with the origin of the orders Holasteroida and Spatangoida lends support to the perception of orders as semi-independent entities in the biological hierarchy and as meaningful proxies for morphological distinctness.A comparison of holasteroid and spatangoid responses to the end-Cretaceous mass extinction revealed morphological selectivity. Paleocene spatangoid survivors showed no change in disparity relative to the Campanian-Maastrichtian sample, suggesting nonselectivity. Holasteroids suffered a pronounced loss in disparity (despite a rather high Late Cretaceous level of disparity), indicating morphological selectivity of extinction.Partitioning of disparity into plastral and nonplastral components, reflecting different degrees of developmental entrenchment and functionality, suggests that the origin of holasteroids and spatangoids is more consistent with an exploration of the developmental flexibility of nonplastral constructions than with uniform ecospace occupation. Within groups, several patterns were also most consistent with intrinsic controls. For plastral landmarks, there is an apparent increase in developmental modularity and decrease in developmental constraint from disasteroids to holasteroids and spatangoids. For nonplastral 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 certain 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 given time period or hierarchical level.

The quantification of disparity is an important aspect of recent macroevolutionary stud ies, and it is usually motivated by theoretical considerations about the pace of innovation and the filling of morphospace. In practice, varying protocols of data collection and analysis have rendered comparisons among studies difficult. The basic question remains, How sensitive is any given dis parity signal to different aspects of sampling and data analysis? Here we explore this issue in the context of the radiation of the echinoid order Spatangoida during the Cretaceous. We compare pat terns at the genus and species levels, with time subdivision into subepochs and into stages, and with morphological sampling based on landmarks, traditional morphometries, and discrete char acters. In terms of temporal scale, similarity of disparity pattern accrues despite a change in tem poral resolution, and a general deceleration in morphological diversification is apparent. Different morphometric methods also produce similar signals. Both the landmark analysis and the discrete character analysis suggest relatively high early disparity, whereas the analysis based on traditional morphometries records a much lower value. This difference appears to reflect primarily the mea surement of different aspects of overall morphology. Disparity patterns are similar at both the ge nus and species levels. Moreover, inclusion or exclusion of the sister order Holasteroida and the stem group Disasteroida in the sampled morphospace did not affect proportional changes in spa tangoid disparity. Similar results were found for spatangoid subclades visa -vis spatangoids as a whole. The relative robustness of these patterns implies that the choice of temporal scale, morpho metric scheme, and taxonomic level may not affect broad trends in disparity and the representation of large-scale morphospace structure.