TAPHONOMY AND PALEOECOLOGY OF ECHINODERM LAGERSTATTEN FROM THE SILURIAN (WENLOCKIAN) ROCHESTER SHALE

The Lewiston Member of the Rochester Shale (Silurian, Wenlockian) of w estern New York and southern Ontario contains three intervals of echin oderm Lagerstatten (Homocrinus beds). These intervals preserve moderat e diversity level-bottom communities dominated by epibenthic suspensio n. feeders. Taphonomic and sedimentologic evidence indicates that thes e communities, existed in low-energy environments below normal wave ba se but were episodically disrupted and smothered by distal mud tempest ites or gradient current deposits. The most favorable conditions for t he preservation, of these communities existed at the lower end of stor m wave base, where relatively thick distal mud flows were frequently g enerated but where energy conditions precluded later reworking. These tiered communities were strongly dominated by suspension feeders, incl uding brachiopods, bryozoans and pelmatozoans, which together make up approximately 93% of the preserved fauna. Low-level benthic forms such as brachiopods, fistuliporoid bryozoans, short-stemmed crinoids and e drioasteroids were buried intact, directly attached to shell pavements representing the blanketed seafloor. Disturbance was minimal but incl uded toppling and uprooting of echinoderms and shells and minor disart iculation. prior to burial. In contrast, long-stemmed pelmatozoans wer e buried within sparsely fossiliferous mud layers (up to 5 cm thick) c ommonly with columns and splayed arms oblique within the burial Layers . Preservation of vagrant organisms, such as the asteroid Paleaster, t he ophiuroid Protaster and ten, genera of trilobites, provides further evidence of rapid and deep burial. In addition, scanning electron mic roscopy of the mud layers directly overlying smothered assemblages ind icates the presence of micrograding and flocculated microfabrics. The Homocrinus intervals record the short-term succession of communities m ediated by episodic storm-related sedimentation in low-energy offshore shelf environments. Distinct successional stages or seres are recogni zed: a) a suite of infaunal deposit feeders, b) low diversity (25-30 s pecies) pioneer Amphistrophia-Dalmanites sere, c) intermediate diversi ty (35-60 species) Striispirifer sere, and in some cases d) high diver sity (60-100 species) bryozoan-pel matozoan thicket sere. Early seres (a, b) resemble faunal assemblages that typify deeper, dysoxic muddy b ottom communities. With increasing diversity, organism interactions an d density of species within tiers increased significantly in the Strii spirifer association and reached a peah in the bryozoan thickets. Howe ver the absolute vertical height of upper tiers may be as great or gre ater in the Low diversity assemblages. The Homocrinus-bearing Striispi rifer-Amphistrophia community and associated communities tracked east- west trending lithofacies belts that shifted position in response to t ransgressive/regressive pulses during Rochester Shale deposition. Wate r depth and sedimentation rate appear to have exerted major controls o n the level of community development.