COMPETITION, CLADE REPLACEMENT, AND A HISTORY OF CYCLOSTOME AND CHEILOSTOME BRYOZOAN DIVERSITY

One of the striking yet scarcely documented episodes of clade replacem ent in the post-Paleozoic fossil record is the decline of cyclostome B ryozoa and the corresponding, rapid diversification of cheilostome Bry ozoa. These clades are closely associated morphologically and phylogen etically, and their ecological similarities have previously led to the inference that competition was a primary cause of the overt pattern o f replacement. Alternatively, previous compilations of bryozoan famili es and genera have implied that extinctions at the Cretaceous/Tertiary boundary differentially affected cyclostomes, and thus were also an i mportant factor in the transition. We first evaluated the ecological c ontext for competition between the two clades, then updated and reexam ined the history of absolute family diversity for bryozoans in consecu tive geologic stages from Jurassic to Recent. The resulting trends ech o the patterns shown in earlier family level compilations, but indicat e a slight shift in the frequency of cheilostome family originations f rom Late Cretaceous to early Paleogene. The relative fall in cyclostom e family diversity at the Cretaceous/Tertiary boundary is significantl y less than shown in earlier genus level compilations. We then assesse d these various compilations of absolute diversity by analyzing specie s counts and percentages in 728 fossil assemblages, primarily from Nor th America and Europe, over the same time interval. Cyclostome species overwhelmingly dominate assemblages from Jurassic through Cenomanian, then decline significantly in average percentage dominance through th e Campanian. Cheilostomes are predominant in Campanian and later assem blages. Cyclostome species percentages do decrease overall through the Tertiary, but this decrease is small and non-uniform, varying around 30%, with a sharp drop in the Late Neogene. Our within-assemblage resu lts indicate that as cheilostomes radiate, their mean species diversit y, maximum diversity, and variance all increase, thereby accounting fo r much of the decline in average percentage of cyclostomes within asse mblages. While this result does not exclude a role for competition, an hypothesis of relative decline in cyclostome species richness based o n competitive extinction alone seems unlikely. Further, despite decrea ses in absolute species counts following end-Cretaceous extinctions, w ithin-assemblage percentages of cheilostome or cyclostome species show only slight change relative to one another. Comparison of these and e arlier diversity compilations indicates that the dynamics of bryozoan clade replacement may be perceived differently at different ecologic s cales or taxonomic ranks.