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 variet y) rather than taxic diversity. A seminal issue has been the nature of the Cambrian Radiation, and the question of whether problematical Cam brian fossils embody a range of anatomical design far exceeding that o bserved thereafter. Arthropods have hitherto furnished the only case s tudy, revealing comparable levels of Cambrian and Recent disparity. Th e generality of this observation needs to be tested in other groups, a nd the priapulid worms provide a well-documented example. Cladistic an alysis of morphological characters for priapulids reveals a paraphylet ic series of Cambrian taxa below a crown-group of post-Cambrian genera . However, one extant family (the Tubiluchidae) may be more closely re lated to Cambrian forms or resolve basally. Character-based morphospac e analysis demonstrates greater disparity amongst Recent taxa than amo ngst their Cambrian counterparts. There is relatively little overlap b etween the regions of morphospace occupied by Cambrian and Recent gene ra (contrasting with the situation in arthropods). The Tubiluchidae ar e morphologically intermediate between Cambrian and other Recent famil ies using several measures of phenetic proximity, and they inhabit env ironments more comparable with their Cambrian cousins. This work confi rms the extensive morphological diversification of major clades by the Cambrian but lends no support to models of a post-Cambrian ''decimati on'' of disparity.