Hydrodynamic Transport Potential of Modeled Turtle Shell in a Controlled Fluvial Setting: a Case Study in Experimental Taphonomy
NOTO, Christopher R; Department of Biological Sciences, University of Wisconsin Parkside, Kenosha, WI. PETERSON, Joseph E; Department of Geology, University of Wisconsin Oshkosh, Oshkosh, WI. ACOSTA, Melissa; Department of Biological Sciences, University of Wisconsin Parkside, Kenosha, WI.
The Arlington Archosaur Site (AAS) is a fossil locality located in an urban area between Fort Worth and Dallas, Texas. The AAS contains a diverse array of Woodbine Group (~96 mya) fossils of many different vertebrate, invertebrate, and plant species. Evidence suggests most AAS fossils were deposited in a low-energy freshwater or brackish environment, such as a tidal coastal wetland. However, questions remain as to the nature of the depositional environment that originally created the site. Among the most abundant remains scattered throughout the site are whole and fragmented pieces of turtle shell (both carapace and plastron) of a variety of sizes. Shell pieces exhibit a diversity of preservation states from complete pieces to small, eroded fragments. To test the hypothesized processes of turtle shell deposition at the AAS, a series of taphonomic experiments were conducted to explore 1) potential entrainment velocities and settling orientations of shell pieces, and 2) abrasion of shell pieces during transport and entrainment. To test potential entrainment velocities and settling orientations of shell pieces, representative models of common shell elements were designed using CAD software and 3D printed. Molds made from the 3D prints were used to create Alumalite resin casts. Models were placed into a flume with manual velocity control. Flow velocity was increased to induce transport, and transport distance and settling orientations were recorded. To test the potential taphonomic modification to shell pieces during fluvial transport, modern turtle elements were placed in a rock tumbler with uniform sized sediment (silt, fine sand, coarse sand) for one week to observe patterns of wear. Preliminary results suggest element length and degree of curvature affect transport potential. Curved elements require lower entrainment velocities while smaller, flatter elements require greater entrainment velocities but, once mobile, are transported further. Surface wear was observed on all shell pieces, though the nature and location differed. In silt and fine sand wear was concentrated around the edges, while in coarse sand the majority of the surface was abraded. Studying turtle shell preservation patterns may be useful for understanding processes driving vertebrate accumulation at fossil sites such as the AAS, where experimental systems such as those outlined above may further elucidate the roles of water transport and/or surface exposure in bone preservation.