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Morphological integration and covariance during astogeny of an Ordovician trepostome bryozoan from communities of different diversities

Correlations exist between diversity and colonial growth pattern and variability in Paleozoic stenolaemate bryozoans. Habitats occupied by diverse communities contained colonies whose growth trajectories were different and were more highly constrained than those of low-diversity habitats. Because previous studies have not examined variability during colony growth, it was unclear whether observed variabilities were constant during growth (astogeny) or whether variability changed with age. Astogenetic trajectories and growth variability are here reexamined in populations of colonies of Heterotrypa ulrichi from the same high-diversity and low-diversity communities examined by Pachut (1989). Growth trajectories of populations differ in both intermonticular and monticular regions of colonies for fundamental stereological characters. Total zooidal surface areas generally decrease during astogeny whereas zooidal densities increase. Surface areas per zooid also decrease with increasing age, but are consistently larger in monticular zooecia and in populations from low-diversity communities irrespective of the colony region sampled. These patterns of declining values are caused by increasing wall thicknesses, larger and more abundant mesozooecia and acanthostyles, and the development of maculae during colony growth. Larger values in low-diversity settings may have provided reproductive and feeding benefits necessary for survival. Covariances reflect levels of morphological integration and are larger in populations from high-diversity communities in both intermonticular and monticular regions. Values differ more during early stages of growth (1–3) than later in astogeny (stages 4–5), and are consistently higher in intermonitulcar than in monticular zooecia. Lower levels of integration in monticules may be caused by spatial adjustments as the number of monticules increases across the expanding colony surface during growth. In general, stabilizing selection may have been weaker and less effective in establishing character covariance patterns in populations from low-diversity communities, perhaps because of greater environmental stress. These findings suggest that evaluations of astogenetic patterns and age-related changes in levels of morphological integration within a paleoenvironmental context are important in measuring species richness, in determining the direction of character evolution, and in assessing tempo and mode of evolutionary changes.