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Abstract Reconstructing the behavior of extinct species is challenging, particularly for those with no living analogues. However, damage preserved as paleopathologies on bone can record how an animal moved in life, potentially reflecting behavioral patterns. Here, we assess hypothesized etiologies of pathology in a pelvis and associated right femur of aSmilodon fatalissaber-toothed cat, one of the best-studied species from the Pleistocene-age Rancho La Brea asphalt seeps, California, USA, using visualization by computed tomography (CT). The pelvis exhibits massive destruction of the right hip socket that was interpreted, for nearly a century, to have developed from trauma and infection. CT imaging reveals instead that the pathological distortions characterize chronic remodeling that began at birth and led to degeneration of the joint over the animal’s life. These results suggest that this individual suffered from hip dysplasia, a congenital condition common in domestic dogs and cats. This individual reached adulthood but could not have hunted properly nor defended territory on its own, likely relying on a social group for feeding and protection. While extant social felids are rare, these fossils and others with similar pathologies are consistent with a spectrum of social strategies inSmilodonsupported by a predominance of previous studies. 

Collaborative hunting by complex social groups is a hallmark of large dogs (Mammalia: Carnivora: Canidae), whose teeth also tend to be hypercarnivorous, specialized toward increased cutting edges for meat consumption and robust p4-m1 complex for cracking bone. The deep history of canid pack hunting is, however, obscure because behavioral evidence is rarely preserved in fossils. Dated to the Early Pleistocene (>1.2 Ma), Canis chihliensis from the Nihewan Basin of northern China is one of the earliest canines to feature a large body size and hypercarnivorous dentition. We present the first known record of dental infection in C. chihliensis , likely inflicted by processing hard food, such as bone. Another individual also suffered a displaced fracture of its tibia and, despite such an incapacitating injury, survived the trauma to heal. The long period required for healing the compound fracture is consistent with social hunting and family care (food-sharing) although alternative explanations exist. Comparison with abundant paleopathological records of the putatively pack-hunting Late Pleistocene dire wolf, Canis dirus , at the Rancho La Brea asphalt seeps in southern California, U.S.A., suggests similarity in feeding behavior and sociality between Chinese and American Canis across space and time. Pack hunting in Canis may be traced back to the Early Pleistocene, well before the appearance of modern wolves, but additional evidence is needed for confirmation. 

Living hyenas are infamous for crushing the bones of their prey to extract the nutritious marrow inside. This feeding ability is rare today, and African and Asian hyenas, particularly the spotted hyena, are the only true ‘bone-crackers’ in our modern ecosystems. Yet, between 16 to 2 million years ago, the common, but now extinct North American dogs also crushed bone. Their skeletal features – such as highly robust skulls and jaws, teeth to withstand high stress, and large muscle-attachment areas for a powerful bite –share many similarities with the spotted hyena. It is therefore likely that these extinct North American dogs played a similar role in the ecosystem as living hyenas do now. The last of these bone-cracking dogs, Borophagus, vanished approximately 2 million years ago. In a recent study in 2018, researchers discovered fossilized feces, also known as coprolites, which presumably belong to Borophagus parvus that lived in central California between 5 to 6 million years. These coprolites preserve ingested bone and so provide more evidence of what this species of dogs ate. Now, Wang et al. – including some of the researchers involved in the previous study – analyzed the fossil coprolites and their ingredients in great detail using computer tomography, measurements and comparisons with living predators and their prey. The results show that Borophagus parvus weighed around 24 kg and hunted large prey of 35 kg up to 100 kg: the size of a living mule deer. Its skull structure was similar to the spotted hyena, but its digestive system resembled that of striped and brown hyenas. Spotted hyenas have chalk white feces containing digested bone matter, presumably due to a highly acidic digestive system, but the coprolites of Borophagus contained undissolved bones (which they ate regularly). Wang et al. also discovered that these dogs dropped feces in clusters, which is how the spotted hyena and wolves mark territory. This suggests that Borophagus were also social animals. Bone-crackers (modern and extinct) act as apex predators and providers of free organic material needed for decomposition, which are essential roles for maintaining a healthy ecosystem. The extinction of Borophagus likely modified the dynamics of the food web over the past few million years. It remains unclear why this way of feeding is absent in all living animals of North America. Future studies could investigate how the disappearance of Borophagus may have influenced the establishment of modern environments, eventually setting the scene for human habitation of the continent.