CORAL ABUNDANCE, DIVERSITY, AND BODY SIZE DECREASE WITH BATHYMETRY IN THE EARLY MISSISSIPPIAN LODGEPOLE FORMATION, MONTANA
Although low in diversity, rugose and tabulate corals comprise a large portion of the macrofossil fauna. Solitary rugose corals are by far the most common coral taxa in the Lodgepole Formation and are found in all three depositional environments. In contrast, colonial rugose and tabulate corals (Lithostrotionella, Aulopora, Syringopora, and Cleistopora) are comparatively rare and mostly occur in outer ramp and proximal slope environments.
Generally, coral abundance, diversity, and body size (based on corallite and colony diameter) decrease from proximal to distal slope environments. Higher levels of dissolved oxygen, fluid flow velocities, irradiance, and greater availability of firm substrates in shallower water environments may explain these observations.
Colonial coral preservation is best on the proximal slope, most specimens being preserved in situ. Additionally, colonies on the proximal slope exhibit the largest body sizes, some up to 30 cm across. Many corals found in outer ramp environments exhibit poorer preservation as a consequence of higher energy depositional conditions. Sedimentological observations suggest that the colonial corals may have been limited in abundance and size by frequent turbidite disturbance and by limited availability of hard substrates. The observed size ranges of coral colonies suggest that the frequency of obrution events during the accumulation of Lodgepole sediments was on the order of a few years to decades.