Functional Morphology and Biofacies Distribution of Cheilostome Bryozoa in the Danian Stage (Paleocene) of Southern Scandinavia

Abstract

Highly diversified assemblages of cheilostome Bryozoa in the Danian Stage of southern Sweden and Denmark represent the culmination of primarily divergent evolutionary trends originating in the first appearance of the group in Early Cretaceous time. Functional relationships between colony and zooid morphology are less likely to have been obscured by vestigial structures and convergent and parallel evolution in these assemblages than in later Cenozoic faunas. The Danian assemblages, then, provide a test of the hypothesis that, in the early evolution of cheilostomes, environmentally correlated variation in the form of colonies depended functionally upon the structure of their component zooids. Theoretically, the rigidly erect growth form should have an adaptive advantage over the presumed ancestral encrusting form, by virtue of a vastly increased potential zooid density relative to substrate occupied. A rigidly erect colony must be able to resist stresses induced by vertical loading, bending, and twisting and thus appears to require calcified walls, especially on the frontal sides of its zooids. Given the constraints imposed by the cheilostome mode of growing and calcifying zooid walls and of operating the hydrostatic system, zooid morphotypes can be relatively graded for efficiency in structural support of the colony by the degree to which their joint calcification approaches a laterally merging, continuously thickening, distally tapering skeletal mass analogous to the outer walls of an enlarging cantilever beam. These hypothetical relationships are generally consistent with biofacies distributions of more than 50 species associated with a single middle Danian mound in southern Sweden. This mound is typical of many which accumulated, probably at depths approximating the shelf-edge, in southern Scandinavia during Danian time. It includes three biofacies: (1) the flanks, dominated by bryozoans; (2) the core, rich in octocorals with less abundant colonial scleractinians and bryozoans; and (3) transitional areas, between the two, dominated by octocorals but with abundant bryozoans. Sediments of the three biofacies contain distinctive assemblages of cheilostome species which differ in abundance rather than by presence or absence. The flanks are dominated by species inferred to have had erect colonies and the more complex zooid morphotypes. This group of species constitutes the bulk of the total fauna in weight-abundance but fewer than half the species. Species dominant in the core facies make up about half the total number of species and are inferred to have had mostly encrusting colonies with zooids of all morphotypes recognized, including the simplest. The transitional facies includes a mixture in subequal proportions of the two groups of species dominant in the other facies; however, this facies has other distinctive species in abundance and thus may represent an ecotone. Morphologically, the cheilostomes abundant in the transitional facies are intermediate in inferred zooid morphotypes and colony forms. The relation between abundance and morphology of Danian cheilostomes suggests that attainment of the more advantageous rigidly erect colony form was functionally more probable for zooid morphotypes susceptible of heavy frontal calcification than for others. If a minimum amount of frontal calcification must have been present before the rigidly erect mode of growth could be assumed, then frontal calcification was associated originally with some other function, such as protection of the lophophore. It is possible that the various further advances in zooid morphotype could also have been made as separate prospective adaptations, but it seems more likely that some or all of them represent direct adaptive improvements for the structural support of rigidly erect colonies.