ALBERT

Description: 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.

Description: Zooids of cheilostome bryozoans are on average substantially more robust than are zooids of cyclostome bryozoans. The differences include greater number, length, and cross-sectional area of tentacles, plus a more extensively developed funiculus. Median values for mouth size and cilia-generated feeding current velocity are greater for cheilostomes than for cyclostomes so that cheilostomes have the potential for greater intake of nutrient energy per unit time, which may explain their apparently higher growth rates. For unit area of substrate occupied, the Cheilostomata (Class Gymnolaemata; members of the post-Paleozoic fauna) contain greater biomass and apparently generate greater energy flow than do the Cyclostomata, which are the only extant order of the Class Stenolaemata (characteristic of the Paleozoic fauna).

Description: Reticulate Hornera reteramae (Bryozoa, order Cyclostomata) have been discovered as free-lying, lyre-shaped colonies in the Castle Hayne Limestone (Eocene) in North Carolina. These specimens represent a striking instance of homeomorphy in colony form with the Late Mississippian genera Lyropora and Lyroporella of the order Fenestrata.Bryozoans with reticulate growth habit most commonly develop upright colonies. Living representatives generally grow in quiet environments or microenvironments, and fossils are generally found in situ in quiet-water deposits. However, lyre-shaped Hornera reteramae occur in skeletal packstones inferred to have been deposited in a moderate-energy, shallow, open shelf environment near normal wave base. Upper Mississippian lyre-shaped colonies are preserved in skeletal packstones and grainstones in localized channels or on upper surfaces of low-angle cross-bedded sheets interpreted as high-energy, marine shoal deposits. Modification of the reticulate growth habit to low, peripherally weighted, free-lying structures apparently allowed invasion of higher energy sand bottoms, both in Mississippian and Eocene times.