Sand pseudomorphs of dinosaur bones: Implications for (non-) preservation of tetrapod skeletal material in the Hartford Basin, USA

Abstract

Early Jurassic tetrapod tracks are common in the Hartford and adjacent Deerfield Basins (Newark Supergroup) of Connecticut and Massachusetts, USA, but skeletal material is rare. Among the few examples is a set of several bones preserved as natural casts on the base of a slab of arkosic sandstone that probably derived from the Portland Formation of Middletown, Connecticut. This mode of preservation is otherwise unknown in the Hartford Basin, and examination of this specimen suggests a complex taphonomic history. The bones were fluvially transported and arranged in a v-shaped pattern as they were deposited on a muddy substrate. Longitudinal cracks, gouging, breakage, rounding, and invertebrate boring indicate significant alteration prior to preservation as casts. Furthermore, the boring represents the first record of osteophagy in the Hartford Basin. Details of the cast preservation, such as sediment rims that wrapped up and over some of the bones, suggest that the bones were dissolved in the subsurface prior to lithification of the overlying sand bed. The decrease in pH that led to bone dissolution probably resulted from the infiltration of naturally acidic rainwater. Root respiration and the decomposition of plants under oxidative conditions may have contributed as well. The depauperate skeletal fossil record of the Hartford Basin is attributable to dissolution under acidic and oxidative sediment conditions. These natural casts offer a unique view of the actions of bone dissolution as a taphonomic process. The lack of body fossils in similar aged deposits elsewhere may also be due to groundwater acidity.