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, environ mentally 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 distri butions 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