The diversity histories of cyclostome and cheilostome bryozoans have followed different trajectories during the past 100 million years, although both clades have experienced (apparently) simultaneous periods of anomalously high extinction when data are resolved to stage-level occurrences. The highest, essentially equivalent extinction rates occurred during or at the ends of the latest Cretaceous (Maastrichtian) and earliest Palaeogene (Danian); the latest Eocene (Priabonian) also was a time of high extinction rates for the two clades.
The Maastrichtian and Priabonian extinctions appear to be parts of well-documented global extinctions that affected both terrestrial and marine ecosystems. The Maastrichtian extinction was one of the four or five major extinctions of the Phanerozoic (Raup and Sepkoski 1982), whereas the mid- to Late Eocene extinction was in the second tier (Raup and Sepkoski 1984, 1986). The relative scale of bryozoan extinctions at these two times is proportional to the intensity of the two extinctions for the biosphere as a whole. Of the bryozoan genera alive at the beginning of the Maastrichtian, approximately 46 percent were extinct by its end, whereas only 13 percent went extinct during the Priabonian.
The scale of the Danian extinction of bryozoans has previously been under-appreciated, but it was almost as intense as the Maastrichtian extinction, with 30 percent of genera alive at the beginning of the Danian going extinct. The Danian is not recognised as a time of major global, pan-ecosystem extinction. The demise of so many bryozoan genera appears to be the result of the end-Danian disappearance of the broad carbonate (‘chalk’) sea that had covered northern Europe and parts of Asia since the Cenomanian. The majority of Danian bryozoan genera are known from northern Europe, and those that went extinct likely were endemic to chalk sea environments. The combined Maastrichtian and Danian extinction pattern for bryozoans is generally similar to that for echinoids, which showed 36 percent and 26 percent extinction of genus-level clades in the Maastrichtian and Danian, respectively (Smith and Jeffery 1998). Ending of widespread shallow-water carbonates was suggested by Smith and Jeffery as the probable cause of the high rate of Danian extinctions.
The earlier suggestion (Voigt 1981) that the Danian was a time of low ‘creativity’ for cheilostomes is not supported by our data on originations. The Danian had the highest Cenozoic rate of O/D/myr for cheilostomes. The Cretaceous affinity of its faunas is due to the large proportion of genera that originated during the Cretaceous and ranged into the Danian before going extinct; it is not due to anomalously low origination rate of new genera.
There is some indication of increasing extinction rate of cyclostomes during the Neogene, although extensive overlap of the 95 percent error bars makes the trend equivocal. A simultaneous, similarly equivocal increase in origination rates almost cancels the possible increase in extinction rate, so that total cyclostome diversity declines only slightly during the Neogene.
Cheilostome and cyclostome origination rates correlate in both the Late Cretaceous and Cenozoic, but origination rates correlate only in the Late Cretaceous. During the Cenozoic, cyclostomes maintained low rates of origination while cheilostome rates were much more variable. This pronounced difference in Cenozoic origination rates appears to be the basic cause for the contrasts in diversity histories of the two clades and is consistent with the prediction of the coupled-logistic curves of Sepkoski et al. (2000) that simulated bryozoan diversity based on competitive interaction between members of the two clades. This difference in origination rates during the Cenozoic also bears on
Foote’s (2000a) findings that the post-Palaeozoic diversity history of gymnolaemates (mostly cheilostomes) is weakly driven more by originations than by extinctions, while the post-Palaeozoic history of stenolaemates (mostly cyclostomes) is driven more by extinctions than by originations. Indeed, stenolaemates have the most strongly extinction-driven diversity history of all post-Palaeozoic clades.