Cambrian and Recent disparity: The picture from priapulids

An understanding of several macroevolutionary trends has been greatly advanced in recent years by a focus on disparity (morphological variety) rather than taxic diversity. A seminal issue has been the nature of the Cambrian Radiation, and the question of whether problematical Cambrian fossils embody a range of anatomical design far exceeding that observed thereafter. Arthropods have hitherto furnished the only case study, revealing comparable levels of Cambrian and Recent disparity. The generality of this observation needs to be tested in other groups, and the priapulid worms provide a well-documented example. Cladistic analysis of morphological characters for priapulids reveals a paraphyletic series of Cambrian taxa below a crown-group of post-Cambrian genera. However, one extant family (the Tubiluchidae) may be more closely related to Cambrian forms or resolve basally. Character-based morphospace analysis demonstrates greater disparity amongst Recent taxa than amongst their Cambrian counterparts. There is relatively little overlap between the regions of morphospace occupied by Cambrian and Recent genera (contrasting with the situation in arthropods). The Tubiluchidae are morphologically intermediate between Cambrian and other Recent families using several measures of phenetic proximity, and they inhabit environments more comparable with their Cambrian cousins. This work confirms the extensive morphological diversification of major clades by the Cambrian but lends no support to models of a post-Cambrian "decimation" of disparity.