LIFE AFTER IMPACT: A REMARKABLE MAMMAL BURROW FROM THE CHICXULUB AFTERMATH IN THE HELL CREEK FORMATION, NORTH DAKOTA

The terminal-Cretaceous Chicxulub impact is among the most-severe such events in Earth’s history, playing a critical role in shaping Cenozoic evolution. The impact caused the third-largest mass extinction, wiping out 75% of life on earth, including the dinosaurs that had dominated the prior ~175 million years. The ensuing adaptive radiations resulted in a dramatic shift in biomes, one of the most pivotal being the Mammalia, which quickly took over as the dominant terrestrial vertebrates. While survival of mammal lineages across the Cretaceous-Paleogene (KPg) boundary is clearly evidenced by their later successors in the fossil record, the specific attributes, behavior, or adaptations that enabled their survival has been subject to conjecture due to lack of direct evidence. A variety of plausible advantages or coping behaviors have been proposed, including small, more-adaptable body size, generalized diet, endothermy, and burrowing. The latter, in particular, is an effective survival tactic for extant mammals, and likely extended back to the Cretaceous as well. However, few of these strategies have been preserved in the fossil record across the KPg boundary. At Tanis, a new terrestrial fossil bearing locality in the Hell Creek Formation, North Dakota, a fossil burrow provides the first direct evidence of mammalian coping behavior immediately following the KPg impact. The burrow, with a long, narrow entry tunnel and several simple flask-shaped chambers, penetrates through an ejecta-bearing inundation deposit that was laid down immediately after the Chicxulub impact. Sediment filling the burrow’s interior is high in palynotaxa typical for the upper Hell Creek (ie. high percentages of Aquilapollenites and Wodehouseia, low pine or fern), indicating it was excavated and abandoned very shortly after impact, predating the post-impact palynological shift that was strongly recorded in the sedimentary record. Scattered mammal bones within the burrow indicate a mid-sized (~1 to 2 kg) individual that compares favorably with the Cimolestidae. The fossil is unique in that it preserves mammalian behavior shortly after impact, at a time that was critical to their survival, and helps to explain the success of the Mammalia after Chicxulub.