DISPARITY ESTIMATES, SCALE, AND TAXONOMIC LEVEL: COMPARISONS BETWEEN GENUS- AND SPECIES-LEVEL ANALYSES IN SPATANGOID ECHINOIDS

Comparisons between disparity and diversity are an important aspect of many evolutionary studies, but are usually justified on theoretical grounds. In practice, varying protocols in data collection and analysis have made comparisons among studies difficult. The basic question remains, how sensitive is any given disparity signal to different aspects of sampling and data analysis?

Here we address this question in the context of the radiation of the echinoid order Spatangoida during the Cretaceous. Previous work documented its pattern of disparity at the genus-level using landmarks, and with time intervals of an average duration of 15.8 My. Deceleration in morphological diversification was suggested, a pattern that has been also found in other groups. A new, independent study focuses on the species-level, using traditional morphometrics and discrete characters, and with time intervals of shorter duration. It provides an opportunity to reassess spatangoid disparity patterns.

Different morphometric methods produce similar signals. Initial concordance between disparity and diversity later followed by smaller disparity increases is also suggested at the species level. However, implied disparity in the Neocomian differs considerably with choice of variables. Both the landmark analysis and the discrete character analysis indicate relatively high disparity relative to diversity, while the analysis based on traditional morphometric variables documents a much lower disparity. This difference might reflect not only the measurement of different aspects of overall morphology, but also the tendency for traditional morphometrics to emphasize global over local differentiation. In terms of temporal scale, the similarity of disparity pattern despite a change in stratigraphic resolution supports previous studies. The concordance between genera and species in this study implies that the choice of taxonomic level may not substantially affect the representation of the structure of morphological space.