Populations of the Upper Ordovician trepostome bryozoan Batostoma jamesi were collected from two different paleoenvironmental settings in the Kope Formation of southeastern Indiana. Within each colony and population, morphologic changes were analyzed during colony growth, or astogeny. Morphological measurements of zooecia, mesozooecia, and acanthostyles display similar patterns of change during colony growth in both populations but magnitudes are generally larger in the high diversity population.
Canonical variates analyses provided multivariate confirmation of univariate character differences found within each population. Statistically significant multivariate morphological differences between growth stages persist even if assignments of colonies to populations are ignored. Results suggest different potentials for altering growth trajectories in different environments with early growth stage flexibility in colonies from lower diversity settings and later-stage flexibility in colonies from higher diversity settings.
Heterochronic changes occur between species populations. Relative to the high-diversity population, the low-diversity population displays the following: 1) progenesis and hypermorphosis for zooecia, reflecting the ability to exist over a broader range of areal densities and surface areas than in populations from high-diversity associations; 2) postdisplacement and progenesis for mesozooecia, producing mature mesozooecial densities earlier in growth and at smaller sizes while the onset of mesozooecial development is delayed; and 3) acceleration, predisplacement, and progenesis for acanthostyles, resulting in a more rapid rate of development, an earlier onset of style development and more styles, and an earlier time of maturation, respectively.
The estimated level of morphological integration is higher in the high diversity population regardless of stage of colony growth. Within populations, integration is stronger during early growth stages in colonies from high diversity settings and during later growth stages in colonies from low diversity settings. Character heritabilities are high in both diversity-level populations, suggesting that these patterns of morphological integration were not the result of non-heritable phenotypic plasticity. Mean heritability is greater in the high diversity population and differs statistically only between the late growth stages of populations. Patterns of morphological integration may result from differing levels of stabilizing selection in different environments. Depending on the timing of selection, these different levels of integration are capable of affecting the outcome of selection on species populations.