One of the conspicuous large-scale changes in the Mesozoic marine fossil record is the decline of the stenolaemate order Cyclostomata and the rise of cheilostome bryozoans to the same approximate level of dominance in marine faunas that had previously been occupied by stenolaemates. Such large-scale turnovers between taxonomically related and/or ecologically similar groups may be due to competitive replacement, to accumulated "small-scale" chance events that by happenstance favor one group over the other, or to the destruction of the formerly dominant group during a mass extinction, followed by the radiation of the second into the vacated ecospace. Timing of the cyclostome-cheilostome turnovers does not correspond with a major mass extinction. Competitive interactions between recent cyclostomes and cheilostomes favor cheilostomes, suggesting that competitive replace is a viable hypothesis. The hypothesis will be tested through predictions of the effects of competition between the two clades as seen in the Mesozoic to early Cenozoic fossil record. Implications of the study are much larger than better insight into this one pattern. If tests of the predictions support the predicted changes, this should provide strong evidence for the importance of organic interactions in determining macroevolutionary patterns.
