Observed phenotypic variation in a Paleozoic bryozoan

Abstract

Documentation of morphologic variation within and among fossil species (and larger clades) provides fundamental data needed for studies of evolution, paleoecology, and the systematic foundation required for most fields of paleobiology. In paleontological (and, frequently, biological) studies, morphologic variation is used as a general proxy for genetic variation. Although the occurrence of ecophenotypic variation is well appreciated in these studies, it is only with the use of colonial (clonal) organisms that the scope and significance of phenotypic variation can be evaluated directly. Systematic evaluation of intracolonial morphologic variation (transects through growth series) can yield insights about ecophenotypic variation in bryozoans and suggest the most appropriate methods for data collection in paleobiologic and taxonomic studies.In this study, morphological conservatism is documented within local segments of bryozoan colonies; each zooid is generally more similar to adjacent zooids than to distant zooids within the same colony. One region of a colony, therefore, can be more similar to a region of a different colony than to a distant region of its own colony. Variation within one colony does not, however, represent the total variation among a group of specimens, indicating a colonial level of morphologic control (genetic or macroenvironmental) over morphogenesis. Directional morphogenetic gradients (associated with successive ontogenetic histories) are not recognized in these specimens, but fluctuating trends within colonies (some cyclic), were observed and are indicative of changing microenvironmental influence during skeletal formation. In order to best document morphologic variation within a population, for any type of paleobiological study, individual measurements should be widely distributed over large colony fragments and (or) a minimal number of measurements collected from each of a large number of smaller fragments.Direct extrapolation of these results to non-colonial organisms is not appropriate at this time. However, additional, related studies with bryozoans and other colonial organisms (e.g., corals, graptolites), should provide a greater, general appreciation of relationships between morphology and genetics.