PHENOTYPIC VARIATION IN AN EPIBIOTIC BRYOZOAN (LEIOCLEMA SP.) FROM CHESTERIAN EPHEMERAL HOST SUBSTRATES

The morphologic consequences of microenvironmental variation have been poorly documented from both ancient and modern ecosystems. However, documentation of environmental sources of variation in the phenotype is essential for meaningful studies of microevolution and speciation. A fossil assemblage from the Chesterian Warsaw Fm. near St. Louis, MO provides an opportunity to evaluate phenotypic variation in the Paleozoic trepostome bryozoan Leioclema sp. Samples were collected from the surface of a recently excavated road cut bench. The shale outcrop had weathered to form a lag of calcareous fossils. Specimens of Leioclema sp. were found as fragments in discrete piles. Twenty-two piles, separated by at least one meter, were collected in their entirety. Each pile contained 20-200+ branch fragments of Leioclema sp., along with rare fragments of brachiopods, crinoids and more common encrusting cystoporate bryozoans. The morphology of the attachment bases of Leioclema sp. colonies demonstrate that they were attached to a large, soft-bodied host, such as a sponge (an excellent Paleozoic example of carbonate sediment production from epibionts growing on ephemeral host substrates). Multiple attachment bases were usually found for each discrete pile, therefore, (1) multiple Leioclema colonies (genotypes?) are represented in each pile, and (2) each pile represents a near contemporaneous, relatively short lived micro-community. Morphological characters were measured from one specimen (5-8 mm diameter) from each of the separate piles. Eight characters measured from shallow tangential sections reflect the size and spacing of autozooids and mesozooids and their wall thickness. Four characters from transverse sections represent the spacing of diaphragms and endozonal size and wall thickness. Results from one-way ANOVA indicate that none of the fourteen characters exhibit significant difference among the colony fragments/piles. This suggests that neither genetic differences among the colonies or microenvironmental effects among these microcommunities have a significant influence on the hard part morphology for this species.