A paleobiological study of Santacrucian native ungulates is presented in thischapter. Seven families are recorded: Hegetotheriidae, Interatheriidae,Toxodontidae, and Homalodotheriidae (Notoungulata); Proterotheriidae andMacraucheniidae (Litopterna); and Astrapotheriidae (Astrapotheria); however, adetailed systematic revision is still pending. A broad body size range isrecorded. Typotheres (Hegetotheriidae + Interatheriidae) vary from 2 to10 kg, proterotheriids vary from 20 to 100 kg, toxodonts and macraucheniidssurpass 100 kg, and astrapotheres reach 1000 kg. The highest taxonomic richnesscorresponds to the range between 20 and 100 kg. Locomotor behavior isinterpreted as mostly cursorial. Typotheres might have engaged in occasionaldigging, but this group also includes the most agile and fastest forms, more sothan proterotheriids. Only two taxa in the sample (Interatherium and Astrapotherium) show evidence of swimming capabilities andpotential aquatic habits. In contrast, feeding behavior presents less variation,which agrees with previous interpretations: notoungulates inhabited openhabitats and fed mainly on grass, while litopterns and astrapotheres inhabitedclosed habitats and were mainly browsers. We infer that notoungulates hadexceptional chemical digestion capabilities whereas litopterns may have reliedon long periods of chewing to process their food.