Trace fossils such as bite marks provide rare, direct evidence of animal behavior, including predator-prey interactions. We present an osteoderm of the aetosaur Typothorax coccinarum from the Late Triassic Chinle Formation of Arizona with several punctures and scores, interpreted here as bite marks, preserved as evidence of predation/scavenging by a large carnivore. The marks include a single bite producing four subparallel fusiform pits on the ventral surface and several additional marks, including striated scores, on the dorsal surface. These traces are described and compared with known contemporaneous carnivorous taxa to determine the source of the bite marks. Some Triassic carnivores, including theropod dinosaurs can be ruled out because of tooth shape and serration densities. Phytosaurs and large paracrocodylomorphs remain as likely candidates based on tooth morphology. Although some phytosaur teeth are too rounded to produce the marks seen in this specimen, we demonstrate that the more lingually flattened teeth typically found in the posterior section of the snout are sufficiently mediolaterally compressed to produce a fusiform pit. A protective function for aetosaur osteoderms cannot be confirmed presently, but the extensive carapace these bones formed would have been a major barrier to both scavengers and active predators and may preserve more feeding/predation traces than previously thought. The bite marks described herein support the hypothesis that aetosaurs were prey items of large archosauromorphs, expanding our understanding of the complex, and seemingly carnivore dominated Late Triassic terrestrial ecosystems of North America.

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