Computed tomography reveals hip dysplasia in the extinct Pleistocene saber-tooth catSmilodon

Abstract

22 23 24 2 25 Abstract 26Reconstructing the behavior of extinct species is challenging, particularly for 27 those with no living analogues. However, damage preserved as paleopathologies on 28 bone can record how an animal moved in life, potentially reflecting patterns of behavior.29 Here, for the first time, we use computed tomography (CT) to assess hypothesized 30 etiologies of pathology in a pelvis and associated right femur of an adult Smilodon 31 fatalis saber-toothed cat, one of the best-studied mammal species from the Pleistocene-32 age Rancho La Brea asphalt seeps, Los Angeles, California. The pelvis exhibits 33 massive destruction of the right acetabulum that previously was interpreted, for nearly a 34 century, to have resulted from trauma and infection. We evaluated this historical 35 interpretation using CT imaging to supplement gross morphology in identifying 36 symptoms of traumatic, infective, or degenerative arthritis. We found that the pathologic 37 distortions are inconsistent with degenerative changes that started only later in life, as in 38 the case of infective or traumatic arthritis. Rather, they characterize chronic remodeling 39 that began at birth and led to degeneration of the joint over time. These findings suggest 40 that this individual suffered from hip dysplasia, a congenital condition common in 41 domestic dogs and cats. 42The individual examined in this study reached adulthood (at least four to seven 43 years of age) but never could have hunted properly nor defended territory on its own. As 44 such, this individual, and other critically pathologic Smilodon like it, likely survived to 45 adulthood by association with a social group that assisted it with feeding and protection.46 The pathologic specimens examined here in detail are consistent with a spectrum of 47 social strategies in Smilodon supported by a predominance of previous studies. This 3 48 application of a relatively new and interdisciplinary technique to an old question 49 therefore informs the longstanding debate between social and solitary hypotheses for 50 the behavior of an extinct predator. 51 52 Introduction 53 The saber-toothed cat Smilodon fatalis is one of the most well-studied apex 54 predators from the late Pleistocene epoch, if not across the entire history of fossil 55 mammals [1]. Much of our knowledge about this species comes from the Rancho La 56 Brea asphalt seeps in Los Angeles, California, United States, which selectively trapped 57 and preserved thousands of individuals of Smilodon from at least 50,000 years ago until 58 the species' extinction approximately 11,000 years ago [1]. The seeps functioned as a 59 carnivore trap: a large herbivore stuck in the asphalt inadvertently would attract large 60 carnivores and scavengers, which themselves would become entrapped in great 61 numbers [2]. Studies of Smilodon at Rancho La Brea have enabled reconstruction of its 62 feeding behavior as an ambush predator specializing on herbivorous megafauna, 63 inferences derived using independent approaches ranging fro...