Taphonomic and biostratigraphic analysis of fossil freshwater turtles in the upper Cretaceous (Campanian) Kaiparowits Formation of southern Utah, USA

Abstract

Freshwater turtles comprise a large percentage of many fossil vertebrate assemblages in western North America. The Upper Cretaceous (Campanian) Kaiparowits Formation, a thick sedimentary unit composed primarily of fluvial and floodplain deposits, is well-exposed within Grand Staircase-Escalante National Monument in southern Utah and is known for abundant fossil turtle remains. The abundance of fossil turtles may be attributed to a variety of factors including high preservation potential. The purpose if this project is to analyze the preservation potential of freshwater turtles, particularly in fluvial depositional environments, by examining the morphology of turtles in relation to transportability and the environment of deposition. It is hypothesized, for instance, that turtles with large, robust shells that were deposited in locations offering rapid burial had a greater preservation potential than those with more fragile shells and those that were exposed for longer periods of time prior to burial. Taphonomic data, including preservation quality, taxon identification, and sedimentary context, was collected for over 700 turtle specimens in both museum collections and in the field. Depositional environment was inferred for specimens with sedimentary context data. A comparison of taphonomic data shows a preservation preference towards turtles that were buried within channel deposits, but only for those with robust shells. Remains of larger turtles are common, but typically only as fragmentary remains. Small turtle taxa are all found as fragments, primarily in overbank, pond, and other floodplain deposits. An analysis of transport and deposition trends was conducted by comparing the orientation and alignment data from in situ fossil specimens to the results of a flume study using the shells from a variety of extant taxa. Results show that shells with a more highly domed carapace tend to be deposited in a carapace-up orientation. There is also a trend for oblong shells to align themselves lateral-to-flow in the channel prior to burial. Examination of alignment trends supports previous paleoflow direction estimates using sedimentary structures. An analysis of the biostratigraphic distribution of the various turtle taxa within the Kaiparowits Formation reveals no apparent change in turtle fauna during deposition of the formation.