One hundred six species of Bryozoa collected from the northern Adriatic in the vicinity of Rovinj, Croatia, are distributed among the orders Ctenostomata (8 species), Cheilostomata (79 species), and Cyclostomata (19 species). Ctenostomes are underrepresented in the collections relative to the two orders with calcified colonies. Five of the cheilostome species are new: Hagiosynodos hadros n. sp., Schizomavella subsolana n. sp., Cellepora adriatica n. sp., Celleporina siphuncula n. sp., and Rhynchozoon revelatus n. sp. (previously referred to as Rhynchozoon sp. II Hayward). Seven species named by Heller (1867) are stabilized by selection of lectotypes (Beania hirtissima, Adeonella pallasii, Hagiosynodos kirchenpaueri, Exidmonea triforis, Crisia recurva) and neotypes (Mollia circumcincta, Schizomavella cornuta) from Heller's collection in the University of Innsbruck Institute of Zoology. Lectotypes are designated for the Adriatic species Hippoporina lineolifera (Hincks, 1886) and for Schizomavella mamillata (Hincks, 1880). Beania cylindrica (Hincks, 1886) and Schizoporella asymetrica (Calvet, 1927) are recognized as species rather than as subspecific units. The species-rich cheilostome genus Schizoporella Hincks, 1877, which contains some of the most widely known fouling bryozoans, is designated a nomen protectum. The species name Smittina cheilostoma (Manzoni, 1869) is preserved as established usage.

One of the striking yet scarcely documented episodes of clade replacement in the post-Paleozoic fossil record is the decline of cyclostome Bryozoa and the corresponding, rapid diversification of cheilostome Bryozoa. These clades are closely associated morphologically and phylogenetically, and their ecological similarities have previously led to the inference that competition was a primary cause of the overt pattern of replacement. Alternatively, previous compilations of bryozoan families and genera have implied that extinctions at the Cretaceous/Tertiary boundary differentially affected cyclostomes, and thus were also an important factor in the transition.We first evaluated the ecological context for competition between the two clades, then updated and reexamined the history of absolute family diversity for bryozoans in consecutive geologic stages from Jurassic to Recent. The resulting trends echo the patterns shown in earlier family level compilations, but indicate a slight shift in the frequency of cheilostome family originations from Late Cretaceous to early Paleogene. The relative fall in cyclostome family diversity at the Cretaceous/Tertiary boundary is significantly less than shown in earlier genus level compilations. We then assessed these various compilations of absolute diversity by analyzing species counts and percentages in 728 fossil assemblages, primarily from North 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 the Campanian. Cheilostomes are predominant in Campanian and later assemblages. 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 results indicate that as cheilostomes radiate, their mean species diversity, maximum diversity, and variance all increase, thereby accounting for much of the decline in average percentage of cyclostomes within assemblages. While this result does not exclude a role for competition, an hypothesis of relative decline in cyclostome species richness based on competitive extinction alone seems unlikely. Further, despite decreases in absolute species counts following end-Cretaceous extinctions, within-assemblage percentages of cheilostome or cyclostome species show only slight change relative to one another. Comparison of these and earlier diversity compilations indicates that the dynamics of bryozoan clade replacement may be perceived differently at different ecologic scales or taxonomic ranks.

Encrusting bryozoans provide one of the few systems in the fossil record in which ecological competition can be observed directly at local scales. The macroevolutionary history of diversity of cyclostome and cheilostome bryozoans is consistent with a coupled-logistic model of clade displacement predicated on species within clades interacting competitively. The model matches observed diversity history if the model is perturbed by a mass extinction with a position and magnitude analogous to the Cretaceous/Tertiary boundary event, Although it is difficult to measure all parameters in the model from fossil data, critical factors are intrinsic rates of extinction, which can be measured. Cyclostomes maintained a rather low rate of extinction, and the model solutions predict that they would lose diversity only slowly as competitively superior species of cheilostomes diversified into their environment. Thus, the microecological record of preserved competitive interactions between cyclostome and cheilostome bryozoans and the macroevolutionary record of global diversity are consistent in regard to competition as a significant influence on diversity histories of post-Paleozoic bryozoans.

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