PALEOZOIC BRACHIOPOD AUTECOLOGY BASED ON TAPHONOMY - EXAMPLE FROM THE DEVONIAN ROSS FORMATION OF TENNESSEE (USA)

The articulation:disarticulation ratio for shells of the terebratulid Rensselaerina medioplicata (40), rhynchonellid Sphaerirhynchia lindenensis (3.3), atrypid ''Atrypa'' reticularis (3. 1) and athyrididinid Meristella atoka (8.5) indicate that a significantly greater percentage of shells remained articulated relative to the strophomenids Leptostrophia beckii, Schuchertella woolworthana, and Leptaena rhomboidalis, and the orthid Discomyorthis oblata (all < 0.30) from the Birdsong Member of the Early Devonian Ross Formation of Tennessee (USA). The terebratulids, rhynchonellids, atrypids and athyrididinids possess a cyrtomatodont hinge, whereas the strophomenids and orthids have a deltidiodont hinge. However, the spiriferidinids Macropleura macropleura and Kozlowskiellina perlamellosa also possess a cyrtomatodont hinge, yet the percentage of articulated shells is similar to that of the strophomenids. The strophodontid L. beckii is unique in having denticles along its hinge, but this character did not result in a greater percentage of the shells remaining articulated relative to strophomenids with a simpler hinge. Smaller size shells were more commonly disarticulated and fragmented based on the negatively skewed size-frequency distributions of each species. Single, thicker pedicle valves outnumber thinner brachial valves, except for L. rhomboidalis. Fractures are localized anteriorly by rugae and pseudopunctae in L. rhomboidalis and by frilly lamellae in A. reticularis. The atrypids, strophomenids, terebratulids and athyrididinids had hydrodynamically stable shells which commonly show shell deformation resulting from burial in life orientation. The orthids, and to a lesser extent rhynchonellids and spiriferidinids, commonly show shell deformation as a result of burial in non-life orientations.