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QUANTITATIVE TAPHONOMY OF A TRIASSIC REPTILE TANYTRACHELOS AHYNIS FROM THE COW BRANCH FORMATION, DAN RIVER BASIN, SOLITE QUARRY, VIRGINIA

MICHELLE M. CASEY^*,1, NICHOLAS C. FRASER² and MICHA KOWALEWSKI¹

¹ Virginia Polytechnic Institute and State University, Department of Geosciences, Blacksburg, Virginia 24061, USA
² Virginia Museum of Natural History, Martinsville, Virginia 24112, USAmichelle.casey{at}yale.edu

The Virginia Solite Quarry occurrence of exceptionally abundant and uniquely preserved specimens of the tetrapod Tanytrachelos ahynis offers an opportunity to quantify multiple aspects of vertebrate taphonomy. Presence or absence of 128 skeletal elements and of 136 skeletal variables were recorded for 99 specimens from two distinct layers within the quarry—lake cycles 2 and 16. Anatomical completeness, or the percent of bones and variables present in a specimen, is low in spite of protection given by anoxic bottom waters from predators and scavengers; the median specimen preserves 14.5% of bones and 11.8% of measured variables. Specimen size, soft-tissue preservation, and postexhumation weathering have no significant effect on specimen completeness; soft-tissue preservation has a significant impact on degree of articulation. Tanytrachelos specimens with heterotopic bones are not significantly more complete than those lacking such bones; this quantitative pattern reinforces independent qualitative taphonomic evidence supporting a biological rather than taphonomic interpretation of the two morphotypes. Lake cycles 2 and 16 differ significantly in terms of articulation and anatomical completeness when anatomical specimen completeness has been corrected for postdepositional faulting. Preservation frequency of bones and variables, or the percent of specimens in which a bone or variable is present, varies greatly but is low with substantial removal of smaller skeletal elements and preferential preservation of such hindlimb elements as femora. Low anatomical specimen completeness and positive correlation between bone size and frequency of preservation both indicate specimen disturbance by minor hydraulic currents. Taphonomic patterns support a low-to-moderate-depth depositional environment, which is shallower than the original reconstruction.