THE UTILITY OF USING FOSSIL FRESHWATER TURTLE SHELLS TO ESTIMATE PALEOFLOW IN FLUVIAL CHANNEL DEPOSITS

The fossilized remains of freshwater turtles are relatively common in many post-Triassic fluvial channel deposits around the globe. Often these remains consist of fragmentary shell material, but in locations that exhibit rapid deposition it is possible to excavate numerous intact turtle shells, either grouped or isolated, belonging to a variety of freshwater taxa. The turtle shell is comprised of the dorsal carapace and ventral plastron, both of which can be subdivided into numerous smaller elements that are typically fused together. There are three general morphologic groups of fossilized turtles that are recovered from fluvial deposits: the robust tortoise-like variety (rare), the nearly flat soft-shelled turtle variety, and the riparian variety (most common). Of these, the riparian varieties are ideally suited as paleoflow indicators due to the shape of their carapace (oblong), structure of their shell (robust but not heavy), and abundance in fluvial deposits.

The Upper Cretaceous (Campanian) Kaiparowits Formation of southern Utah is one of several localities in which numerous intact turtle shells, comprising nearly a dozen different species, have been collected. A taphonomic analysis of the intact riparian turtle shells reveals a trend of shell alignment that is lateral (or perpendicular to anterio-posterior) to current direction, as supported by other paleoflow indicators. It was hypothesized that shell morphology may have caused preferential transport and deposition on the bed of the ancient channel based on the paleoflow direction. Actualistic experiments were conducted in a laboratory using a flume and a variety of common extant turtle shell morphologies to observe the transport and deposition of shells under a variety of flow conditions. Observations of shell transport in the flume verified the influence of carapace shape on the hydrologic properties of the shell as it moved within the flow. Riparian shells frequently rolled and came to rest on the plastron (flat side down) with the shell long axis direction perpendicular to flow (as long as there was no interference from other objects). Results of these experiments compare well with the data collected in the Kaiparowits Formation, which offers a potential utility of using turtle shells, when available, as part of future paleoflow studies.