Hostname: page-component-76d6cb85b7-92wsb Total loading time: 0 Render date: 2026-07-12T22:29:06.508Z Has data issue: false hasContentIssue false

Competition, clade replacement, and a history of cyclostome and cheilostome bryozoan diversity

Published online by Cambridge University Press:  08 April 2016

Scott Lidgard
Affiliation:
Department of Geology, Field Museum of Natural History, Chicago, Illinois 60605
Frank K. McKinney
Affiliation:
Department of Geology, Appalachian State University, Boone, North Carolina 28608
Paul D. Taylor
Affiliation:
Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD

Extract

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.

Information

Type
Research Article
Copyright
The Paleontological Society 

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

Article purchase

Temporarily unavailable