Table 1 summarises the data on cheilostome and cyclostome generic diversity changes.
Following the end-Palaeozoic extinction, cyclostome bryozoans reappear in the fossil record by the Carnian Stage of the Upper Triassic (Bizzarini and Braga 1981). They diversified slowly through the Late Triassic and Early Jurassic but more rapidly during the Mid Jurassic and Early Cretaceous. Sixty-three genera are known from the latest Early Cretaceous (Albian), the oldest stage included in this study. During the Late Cretaceous, cyclostomes steadily increased in diversity to the 170 genera known in Maastrichtian rocks (Figure 2). They declined drastically from the Maastrichtian to the Thanetian and then remained at an approximately constant diversity level through the Cenozoic. (Note that Figure 2 plots only fossil diversity; Holocene data are not plotted.)
Cheilostomes first appeared in the Late Jurassic (Taylor 1994) but diversified very slowly during the Early Cretaceous, with only 10 genera known from the Albian. During the Late Cretaceous, cheilostomes diversified rapidly to reach a level of 185 known genera in the Maastrichtian (Figure 2). Subsequently, they declined through the Palaeocene to a Thanetian diversity of 125 genera. After the Thanetian, they again diversified through much of the remaining Cenozoic, interrupted by a modest Oligocene reversal, apparently reaching a plateau of 240 to 250 genera during the Neogene.
The absolute number of extinctions for both clades is highest for the Maastrichtian (Figure 3). Sixty-one cyclostome genera that ranged through two or more stages have their last record in the Maastrichtian, and 18 are known only from the Maastrichtian, making a total of 79 Maastrichtian cyclostome genera that did not survive beyond the K-T boundary. Fifty-five long-ranged cheilostome genera have their last record in the Maastrichtian, and 26 are known only from that stage, totalling 81 Maastrichtian cheilostomes that did not cross the K-T boundary. The second-highest number of extinctions occurred during the Danian for both clades: 26 long-ranged plus 3 stage-only cyclostomes, and 33 long-ranged plus 13 stage-only cheilostomes became extinct in the Danian. These Maastrichtian and Danian extinctions are much higher than extinctions in any other Late Cretaceous or Cenozoic stage. The next highest number of extinctions for cyclostome genera occurred during the Campanian (14 long-ranged plus 3 stage-only), and for cheilostomes during the Priabonian (24 long-ranged plus 4 stage-only).
Extinctions per million years (E/myr) were also high during the Maastrichtian and Danian both when the two clades are considered together (Figure 4A, B) and individually (Figure 5A, B;Figure 6A, B). Based on genera not confined to a single stage, cyclostomes extinction rates were 9.7 genera/myr in the Maastrichtian, and 6.3 genera/myr in the Danian; cheilostomes extinction rate were 8.7 genera/ myr in the Maastrichtian, and 8.0 genera/myr in the Danian. Including stage-only genera, cyclostome extinction rates were 12.5 genera/myr and 7.1 genera/myr and cheilostome rates were 9 genera/myr and 11.2 genera/myr, respectively. As with absolute numbers of extinctions, these rate values are well above background levels (Figure 7A, B). The next highest rates for cyclostomes are 3.3 genera/myr and 4.4 genera/myr (including stage-only genera) in the Piacenzian, and for cheilostomes 7.3 genera/myr and 8.5 genera/myr (including stage-only genera) in the Priabonian.
Number of extinctions per standing diversity (E/D) also peaked during the Maastrichtian and Danian (Figure 4C, D;Figure 5C, D;Figure 6C, D). Cyclostome extinctions were 0.38 genera/D (0.46 genera/D including stage-only) for the Maastrichtian, and 0.26 genera/D (0.36 genera/D including stage-only) for the Danian. Values for cheilostomes are 0.30 genera/D (0.44 genera/D including stage-only) for the Maastrichtian, and 0.22 genera/D (0.33 genera/D including stage-only) for the Danian. Again, these are well above background levels (Figure 7C, D). Next highest values for cyclostomes are 0.13 genera/D (0.16 genera/D including stage-only) for the Priabonian, and for cheilostomes 0.17 genera/D (0.21 genera/D including stage-only) for the Campanian.
Finally, calibrating extinctions by both standing diversity and time (E/D/myr), Maastrichtian and Danian extinctions still stand above background levels (Figure 4E, F;Figure 5E, F;Figure 6E, F). Cyclostome extinctions were 0.06 genera/D/myr (0.07 genera/D/myr including stage-only) for the Maastrichtian, with the same values for the Danian. Values for cheilostomes are 0.05 genera/D/myr (0.07 genera/D/myr including stage-only) for the Maastrichtian, and 0.05 genera/D/myr (0.08 genera/D/ myr including stage-only) for the Danian. While Maastrichtian and Danian extinctions per standing diversity per million years stand well above background level (Figure 7E, F), they are essentially matched in the Piacenzian for cyclostomes (0.05 genera/D/myr, and 0.07 genera/D/myr including stage-only), and in the Santonian for cheilostomes (0.05 genera/ D/myr and 0.07 genera/D/myr including stage-only).
The absolute number of originations for both clades is highest for the Maastrichtian (Figure 8). Thirty-four cyclostome genera that ranged through two or more stages have their first record in the Maastrichtian. Adding the 18 that are known only from the Maastrichtian brings the total to 52 Maastrichtian cyclostome originations. Sixty long-ranged cheilostome genera have their first record in the Maastrichtian. Adding the 26 that are known only from that stage brings total Maastrichtian cheilostome originations to 86. The second-highest number of cyclostome originations was during the Cenomanian when 28 long-ranged and six stage-only (making 34 in total) cyclostome genera appeared. The second-highest number of cheilostome originations is found in the Lutetian, which has the earliest records of 49 long-ranged and 8 stage-only genera (57 total).
Originations per million years (O/Ma) were also high near the transition from Cretaceous to Cenozoic (Figure 9A, B;Figure 10A, B;Figure 11A, B). Maastrichtian cyclostome originations were 5.4 genera/myr for long-ranged genera and 8.3 genera/myr including stage-only genera. However, cyclostome originations/myr were not above background levels during the Danian (Figure 12A). Instead, the Cenomanian had originations/myr almost equal to the Maastrichtian (5.2 genera/myr without and 6.3 genera/myr with stage-only genera). Cheilostome originations/myr were high during both stages: 9.5 genera/myr without and 13.7 genera/myr including stage-only genera for the Maastrichtian, and 7.6 genera/myr and 10.7 genera/myr, respectively, for the Danian. However, these Maastrichtian and Danian values for cheilostome originations/myr scarcely differ from those for the Coniacian, Santonian, Bartonian, and Priabonian (Figure 11A, B;Figure 12B).
Number of genera originating per standing diversity (O/D) was high for cyclostomes during the Maastrichtian (0.20 genera/D for long-ranged and 0.33 genera/D including stage-only genera), but these values are exceeded by those for the Cenomanian (Figure 10C, D;Figure 12C) and are not much above those for the Coniacian and Campanian. The value of O/D for the Danian is not above background levels. Cheilostome O/D for the Maastrichtian (0.32 genera/D for long-ranged and 0.46 genera/D including stage-only genera) are exceeded by or approximately equal all other Late Cretaceous stages and were almost equalled by Lutetian O/D values. Danian cheilostome generic O/D values are only slightly above the median value for all stages (Figure 11C, D;Figure 12D).
Originations calibrated by both standing diversity and time (O/D/myr) for the Maastrichtian and Danian are even closer to the norm for Late Cretaceous and Cenozoic stages (Figures 9E, F;Figure 10E, F;Figure 11 E, F;Figure 12E, F). Maastrichtian cyclostome values (0.03 genera/D/myr for long-ranged genera and 0.05 genera/D/myr including stage-only) are far exceeded by Cenomanian and Coniacian values, and the Danian values are near the median for all stages. Cheilostome generic originations/D/myr for both Maastrichtian and Danian are only slightly above typical values for the Cenozoic and are exceeded minimally by Turonian and substantially by Cenomanian, Coniacian, and Santonian values; i.e., Maastrichtian and Danian cheilostome O/ D/myr are below the norm of Late Cretaceous stages but high relative to Cenozoic stages.