MORPHOMETRIC AND TAPHONOMIC ANALYSES OF TANYTRACHELOS AHYNIS FROM THE LATE TRIASSIC COW BRANCH FORMATION, SOLITE QUARRY, VIRGINIA

The Solite Quarry, Virginia (Late Triassic Cow Branch Formation, Dan River Basin) is known for its soft-bodied preservation of insects and abundant, small (<50 cm) aquatic reptiles (Tanytrachelos ahynis). The presence of two distinct morphotypes – one with and one without paired heterotopic bones associated with the proximal caudal region – suggests a sexually dimorphic population. The numerous, partly-to-completely articulated specimens occur in two distinct, cyclic lacustrine horizons separated by ~350 ky; thus, providing a unique opportunity for a thorough morphometric evaluation of allometric patterns, sexual dimorphism, taphonomic overprints on morphology and morphological trends through time.

A preliminary morphometric analysis of 30 specimens was conducted by targeting more than 100 linear measurements of which, the following 11 skeletal elements could be measured consistently for a large percentage of specimens: lengths, widths at proximal end, and widths at distal end of femurs, tibias and fibulae, respectively (9 variables total) as well as widths of the second and third caudal vertebral processes. Datasets for the right and left portions of each specimen were combined (taking the arithmetic mean for elements present in both) to augment database completeness. Principle component analysis suggests that Morphotype A (heterotopic bone present) and Morphotype B (heterotopic bone absent) may differ in size (A < B). Repeated measurements of the same specimens indicate that the variation due to operator errors (imprecision) is small when compared to variation among different specimens. Heterotopic bones are present in ~40% of the population supporting the claim that they may reflect sexual dimorphism. Taphonomic classificatory variables reveal a size-dependent preservational bias, with primary bone material preferentially lost among smaller specimens. Further analysis should provide rigorous insights into allometric and temporal trends within this species. This study should yield an optimized protocol for morphometric and taphonomic studies of Tanytrachelos and its relatives.