fused or interlocking mandibular symphyses, large masseteric fossae, microfractures in their tooth enamel, and high estimated bite forces. Australohyaena antiquua shows particularly pronounced adaptations for bone-cracking, with a very deep jaw and strongly arched nasals similar to what is seen in modern hyaenids. Based on studies of the postcranial skeleton, it appears as though most sparassodonts were scansorial (adapted for climbing), although terrestrial adaptations evolved in Lycopsis longirostrus, borhyaenids, proborhyaenids, and thylacosmilids. Most sparassodonts were plantigrade, Borhyaena has been suggested to have been digitigrade but this has been questioned. The one exception was Thylacosmilus, which has been interpreted as having a digitigrade forefoot and a semiplantigrade hindfoot, this has been supported by fossil tracks. One unusual aspect of sparassodont paleoecology is that at most fossil localities their remains are nearly ten times rarer than would be expected based on comparisons with carnivorous mammals at fossil sites in other parts of the world. The exact reasons for this are not clear, though this appears to be a broader pattern applicable to other groups of Cenozoic South American terrestrial carnivores (i.e., terror birds). Sociality Little is known of the behavior and biology of sparassodonts outside of general locomotor and dietary habits. Argot (2004) proposed that Thylacosmilus atrox may have exhibited protracted parental care after weaning of the offspring, given that saber teeth in general have been suggested to require long juvenile periods for the young to gain the skill necessary to use them effectively. However, this has not been tested further. Sparassodonts have relatively large and complex brains for metatherians, comparable to those of some Australian marsupials like Australian possums, though the body masses used to produce these estimates of relative brain size are low compared later studies suggesting these values could be overestimated. Wounds have been documented on the face of specimens of Borhyaena tuberata and Sipalocyon gracilis, potentially suggesting aggressive habits similar to the modern Tasmanian devil (Sarcophilus harrisii). Senses Sparassodonts appear to have had very little binocular vision, with borhyaenids having the greatest degree of depth perception (but still lower than modern carnivorans) and the eyes of Thylacosmilus facing almost completely to the sides. However, later studies have found that Thylacosmilus likely held its head in a downward-facing position, which would have allowed for more binocular vision than previously thought. Pathology Several specimens of hathliacynids (Sipalocyon and Cladosictis) show a pathological disorder characterized by the presence of growths on