INTRODUCING THE TURTLE COMPACTION INDEX – A NEW TOOL FOR UNDERSTANDING BURIAL-DEPTH RELATED TAPHONOMIC PATTERNS IN FOSSIL TURTLE SHELLS

Turtle shells are one of the most abundant macroscopic terrestrial vertebrate fossils in late Mesozoic and younger strata. This is in part due to their innate resistance to mechanical stress, which results in distinct taphonomic patterns of turtle-shell compaction. We address the question whether these distinct patterns of turtle-shell compaction correlate to timing and depth of taphonomic processes. As a result of our analysis, we propose a new proxy, the Turtle Compaction Index (TCI). The TCI uses fossil turtle-shell assemblages to estimate the timing and depth of fossilization and lithification. This new proxy is based on the mechanical failure properties of extant turtle shells and the material properties of sediments that encase fossil turtle shells and enables estimation of burial depths over which turtle shells become compacted. Pilot TCI studies use suites of shallowly buried turtle shells from the early Paleocene strata of the Denver basin and late Cretaceous strata of the Williston basin. These suites of turtle shells suggest that such shallowly buried turtle-shell assemblages are sensitive indicators of the depths (~10–500 m) at which fossils and their encasing sediment become sufficiently lithified to prevent further shell compaction. This depth corresponds to the timing when taphonomic processes wane. Our results also confirm previously hypothesized shallow Cenozoic burial histories for the Denver and Williston basins and the fossils they preserve. TCI-derived depths from mudstone-encased turtle shells can be paired with thicknesses and ages of overlying strata to create geohistorical burial curves that indicate when such post-burial processes were active.