Taphonomy of the Large Vertebrate Fauna from the Thomas Farm Locality (Miocene, Hemingfordian), Gilchrist County, Florida

Abstract

The results of a taphonomic investigation of the Thomas Farm locality are presented. Sedimentological evidence indicates that deposition took place in a large (about 35 m in diameter), high-walled (30 m) sinkhole. Fossiliferous sediments most likely represent the remains of a large debris cone that formed in the bottom of the sink. Laminar water-lain clays indicate that the sinkhole was water-filled at least periodically throughout the course of deposition. Two massive layers of limestone rubble represent two separate sequences of roof and wall collapse of the sinkhole. A major change in the nature of the deposit occurred following these events, as indicated by the presence of upper massive layers of calcareous sands rich in microvertebrates. Taphonomic evidence suggests that bones of megafaunal vertebrates accumulated attritionally. In the lower clay and sandy clay units of the site, the megavertebrate bones show an orientation pattern consistent with that indicative of moving water. Movement of water through the sink proceeded from northeast to southwest through underground joints in the limestone forming the sinkhole. Current velocity was of sufficient strength to winnow out less dense elements, primarily those belonging to Voorhies Dispersal Group I. Lack of severely weathered, water-worn or carnivore-chewed bone suggests that bones did not lie exposed on the surface for long periods before being deposited on the debris cone, nor was there a strong fluvial or carnivore-mediated influence on the bone assemblage. The faunal composition of the megavertebrate taxa in all but the uppermost layers is similar throughout the site, suggesting that the fauna available for fossilization did not change greatly throughout the course of deposition. For the major part of the depositional history of Thomas Farm, the taphonomic factors responsible for this sampling did not change appreciably. The upper layers contain very little megafauna and are not considered extensively in this study. Faunal evidence indicates that the sink was located in a wooded area, and the climate at the time of deposition was tropical to subtropical.