BIASES IN THE CRETACEOUS-PALEOGENE POSTCRANIAL RECORD OF MAMMALS: IMPLICATIONS FOR INTERPRETING PALEOECOLOGY

Mammalian paleontology is largely based on dental data, however research suggests postcranial data can capture more diversity than dentition alone, and postcrania allow for analysis of ecological properties that cannot be determined directly from teeth, including locomotion and habitat. Study of ecological change in mammalian assemblages through time however requires temporally and stratigraphically well-constrained data, and must account for changes in depositional environment. The Hell Creek and Tullock formations in northeastern Montana preserve some of the only terrestrial strata spanning the Cretaceous-Paleogene (K-Pg, ~66 Mya) mass extinction boundary in the world, and provide an ideal location to test hypotheses of extinction and recovery. Though nearly all K-Pg microvertebrate fossils are found as isolated elements in time-averaged outwash deposits, the hypothesized paleoenvironment of the Tullock Formation differs from the Hell Creek Formation in having more swamps, ponds, and stagnant water, thus potentially affecting paleoecological interpretations across the mass extinction boundary.

I visited 37 localities that have historically produced more than one postcranial element. I collected surface and in situ microvertebrate material, and described the stratigraphy and sedimentology, when possible. I combined this data with field notes and published descriptions, resulting in lithology and depositional environment designations for each locality. Results suggest postcranial elements are overwhelmingly found in sandstone and channel deposits, and are often associated with mud “rip-up” clasts and organics. Even excluding the time-averaged and re-worked Bug Creek Anthill Assemblages (a rich sandstone channel deposit), proportionally more postcrania are found in sandstone and channel deposits in the Paleogene than the Cretaceous. Indicative of large river systems, sandstone channel deposits are more common in the Paleogene, as are larger assemblages per locality. Excluding the three largest Paleogene channels from this analysis still preserves the preponderance of sandstone productive horizons compared to siltstone and mudstone deposits. Remaining postcrania are more abundant in Cretaceous mudstones than siltstones, a pattern that is reversed for Paleogene postcrania.