A PALEOBIOLOGICAL LABORATORY: SYSTEMATICS OF THE CARBONIFEROUS DENDROID BRYOZOAN FAUNA OF THE WARSAW FORMATION

In recent years, bryozoans have proven important to the study of organic evolution from the paleobiological perspective because of their complexity and abundance. Recent research has focused on the younger cheilostomes with Paleozoic bryozoans receiving limited emphasis and that largely on faunas of the earlier Paleozoic. It is important to further treat these epifaunal suspension-feeding faunas that dominated before the Mesozoic Marine Revolution.

The Middle Mississippian (Valmeyeran) Warsaw Formation of the upper Mississippi Valley contains a Paleozoic bryozoan "laboratory." About 64 bryozoan species are recognized, of which only the fenestrates (37 species) have received modern systematic treatment. The fauna also includes 20 branching (or dendroid) species assignable to the orders Cryptostomata and Trepostomata. The remainder includes about four cystoporates and one cyclostome. Fifteen of the dendroid species belong to five rhabdomesine families (representing the same phylogenetic branch as the fenestrates) whereas only five species of Trepostomata occur, perhaps all belonging to the family Stenoporidae. Limited trepostome presence in a rich Carboniferous fauna illustrates changing dominance through the Paleozoic. Although the fauna was carefully studied by E.O. Ulrich, one new genus and ten new species of dendroid bryozoans are recognized. Ongoing new taxon discovery in an intensively studied fauna suggests that even further additions can be expected.

Taxonomic analyses of Paleozoic bryozoans require use of both exterior and interior characters. Quantitative analyses document subtle but crucial aspects of morphology, variation, and species groupings comparable to that demonstrated earlier for Warsaw fenestrates. Zooecial characters (e.g. three-dimensional chamber form and dimensions; apertural shape and dimensions; septal and hemiseptal development and placement) and extrazooecial structures (e.g. stylet type, shape, and development; and metapore size, shape, and development) are quite consistent within species whereas zoarial characters (e.g. endozonal and exozonal thicknesses, branch width, branching frequency) are more variable, presumably reflecting astogenetic state and microenvironmental conditions.