A BRACHIOPOD FAUNULE FROM THE ORDOVICIAN MARTINSBURG SHALE, SHAWANGUNK MOUNTAINS, NEW YORK

An exposure of the Middle to Upper Ordovician Martinsburg Formation in a shale pit on the Mohonk Preserve, southeastern New York, consists of predominantly dark gray shales and siltstones interbedded with fine grained graywacke beds, occasional prominent pyrite layers and disseminated sphalerite, chalcopyrite and galena. Carbonaceous material occurs mostly as fine-grained patches throughout the matrix. The studied section is tectonically stressed with shiny quartz slickensided surfaces, parallel cross-laminated strata and ripple marks. Crinoid stems, some slightly disarticulated, and free columnals occur on different bedding surfaces indicating a possible change in current regime. Scattered linear to sinusoidal horizontal burrow structures ranging in diameter from .5-3 cm are found on the silty beds. Some of the burrows are infilled with coarse quartz grains. The faunal constituents include brachiopods (93%), crinoids (3%), bivalves (3%), ostracodes (<1%), corals (<1%) and unidentified burrowers (<1%). The brachiopods are represented by a low diversity assemblage of dalmanellids and what appears to be a new species of Sowerbyella. The fauna can be classified into distinct trophic groups: (1) high-level suspensions feeders (crinoids, corals); (2) low-level suspension feeders (brachiopods, bivalves); (3) animals that collect food from the sediment surface (ostracodes); and (4) animals that feed within the sediment (burrowers). This partition of feeding niches leads to a reduced competitive trophic structure and therefore increased community stability.