Digging Past the Dinosaurs?: Locomotor Trends and Mammalian Survivorship at the K–Pg (Cretaceous/Paleogene) Boundary

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The Ohio State University

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Sixty-five million years ago a bolide approximately 10 km in diameter collided with the Earth. This event triggered global devastation and a mass-extinction event, the Cretaceous-Paleogene (K-Pg) extinction, signaling the end of the “Age of Dinosaurs.” Non-avian dinosaurs are the most famous victims of the impact, but other groups such as ammonite cephalopods, angiosperms and mammals were severely affected by the ecological consequences of this catastrophic event. However, there were survivors. In 2004 Douglas S. Robertson et al. described the geophysical scenario surrounding the impact event including a global thermal pulse lasting 2 to 20 hours caused by infrared radiation (IR) from falling impact ejecta. The authors suggested all terrestrial survivors of the impact event were capable of buffering the resulting 10 kW.m-2 of power (the thermal equivalent of an oven on broil) by insulating themselves below ground or underwater. Called the “Sheltering Hypothesis” the hypothesis suggests the survivors of the impact event were morphologically equipped for fossorial or semi-aquatic locomotion. To test the hypothesis I turned to the mammalian fossil record. Other groups such as arthropods and birds exhibit differential survival patterns, but poor sampling and conservative morphologies make it difficult to test the Robertson hypothesis. Regardless of clade, specific locomotor demands produce predictable post-cranial skeletal structures in mammals that indicate ecological roles in ancient and modern contexts. In order to conduct this study, indicators of locomotor behavior were based on isolated post-crania as the mammalian fossil record near the K-P does not preserve articulated specimens or complete disarticulated specimens. One of the most reliable indicators of fossorial (digging) behavior is the relative length of the olecranon process of the ulna. Utilizing Ohio State’s Higher Vertebrates collections, I first demonstrate extant mammals can be grouped into locomotor guilds based on this feature using linear measurements and linear regression. Unfortunately, complete fossil ulnae are usually not preserved, so a novel proxy based on the length of the semi-lunar notch is described. This proxy is based on correlations to ulnar length based on linear regression analysis. This proxy allows us to understand the locomotor guilds of mammals before and after the K-P impact. Using fossils from the Pioneer Trails Regional Museum in North Dakota and the University of California Museum of Paleontology at Berkeley I compare the locomotor diversity of mammals before and after the impact event. Preliminary results refute the Robertson hypothesis and suggest mammalian survivorship was more closely related to body size than locomotor guild. Derived adaptations for fossorial or semi-aquatic locomotion are apparent before the boundary including the likely swimmer Didelphodon vorax, a large marsupial that went extinct at the boundary. Survivors appear to be scansorial generalists. Future areas of inquiry include incorporating more post-cranial structures and material into the sample, and examining the consequences of a “Lilliputian” mammalian community on extinction recovery patterns throughout the ecosystem.


Winner Denman Undergraduate Research Forum - Biological Sciences
Winner Math and Physical Sciences Undergraduate Research Forum - Geological Science


Post-crania, Sheltering Hypothesis, Locomotor Guilds, Ulna morphology, Ecomorphology, Mesozoic Diversity, Didelphodon