SAND PSEUDOMORPHS OF DINOSAUR BONES: IMPLICATIONS FOR (NON-) PRESERVATION OF SKELETAL MATERIAL IN THE HARTFORD BASIN, USA

Tracks of Early Jurassic vertebrates are common in the Hartford Basin (Newark Supergroup) of Connecticut and Massachusetts, but their skeletons are rare. Among the few examples of skeletal material is a set of several bones attributed to a small theropod dinosaur (Colbert and Baird 1958). The specimen is probably derived from the Upper Portland Formation of Middletown, Connecticut, and the bones are preserved as natural casts on the base of an arkosic sandstone slab. This mode of preservation is unique for the Hartford Basin and is unusual in general. Examination of this specimen reveals a complex taphonomic history. Longitudinal cracks, missing flakes, breakage, rounding, and an invertebrate boring indicate significant alteration prior to burial; the bones were also hydrodynamically sorted and oriented. Previously, the natural casts were inferred to have formed when currents removed the bones from a muddy substrate and the resulting molds were infilled with sand. However, details of the casts suggest that the original bone material was dissolved in the subsurface prior to lithification of the overlying sand bed. For example, the bones were emplaced deeply in the underlying substrate, and sharp-edged sediment rims wrapped around and slightly over them. The bones were removed without disturbing these rims, which were not abraded by the overlying coarse-grained, high-energy deposit. The depauperate skeletal fossil record of the Hartford Basin is attributable to bone dissolution due to acidic and/or oxidative soil and groundwater conditions; these natural casts offer a unique view of the actions of this taphonomic process. Furthermore, the boring on one of the bones represents the first record of osteophagy in the Hartford Basin.