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South American native ungulates include extinct taxa that evolved within the geographical context given by the isolation of South America during most of the Cenozoic. The ungulates (orders Notoungulata, Litopterna and Astrapotheria) of the Santa Cruz Formation (late Early Miocene) are particularly interesting for paleobiological studies due to their diversity, richness and quality of preservation of the specimens. The body mass estimation of extinct species is one of the basic biological attributes for paleobiological reconstructions. The most common way to estimate body mass from fossils is using linear regression. Here, we used geometric morphometric techniques in order to estimate their body mass. We used regressions based on centroid size of 3D craniomandibular landmark configurations, including extant ungulates (their size and ecological relatives). Cases were weighted to maximize the taxonomic evenness. A broad body size range was recorded. The highest predictive power is obtained with those functions derived from the highest taxonomic and ecological diversity. The highest taxonomic richness corresponds to masses below 100 kg. Among Notoungulata, typotheres (Hegetotheriidae + Interatheriidae) vary from 1 to less than 10 kg, while the smaller toxodontid reached 100 kg and the larger 500 kg. Litoptern proterotheriid body masses vary from 10 to 50 kg, and macraucheniids surpass 100 kg. The astrapotheres (Astrapotheria) reached (or even surpassed) 1000 kg, being the only megamammal in the Santacrucian ungulate assemblage.

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Early Miocene sloths are represented by a diversity of forms ranging from 38 to 95 kg. Their forelimb bones differ in shape from those of their closest living relatives (less than 10 kg), Bradypus and Choloepus. Such differences in shape could be related to differences in substrate preference (arboreal, semiarboreal, or ground-dwelling) or substrate use (climbing, digging, etc.). In order to detect putative patterns related to substrate preference, 21 linear measurements were defined and taken on the forelimb bones. The sample was composed of 22 specimens of fossil sloths and 134 specimens of extant mammals (marsupials, xenarthrans, pangolins, rodents, primates, and carnivorans), including arboreal, semiarboreal, and ground-dwelling taxa. Principal Components Analyses were performed on logarithms of original measurements, while functional indexes (Index of Fossorial Ability, Brachial Index, and Distal Epiphyseal Index) were calculated on raw data. The first three PCs accounted for 93.8% of the cumulative variability. PC1 roughly represented size, while positive values of PC2 represented mechanical advantage for features related to digging habits. Fossil sloths were clearly separated from living ones, sharing a common morphospace with anteaters and other good diggers. Conversely, living sloths shared a morphospace with primates. Similar results were obtained for DEI and IFA, with fossil sloths showing similar values to extant digging mammals. These results suggest that fossil sloths have a different functional pattern of forelimb use than that of extant ones, probably more similar to vermilinguas and pangolins, including putative good digging capabilities and/or semiarboreal habits. Substrate use seems to be interfering in the analysis of substrate preference based on forelimb morphology.

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Body mass estimations for extinct taxa are fundamental in palaeobiological reconstructions, but little work has been done on this topic for non-mammaliaform cynodonts (NMC), the diverse and abundant Permo-Cretaceous forerunners of mammals. Here, we estimated the body mass of five species of NMC cynognathians by linear measurements and circumferences of postcranial elements (humeri and femora) from 14 specimens from Triassic units of the Ischigualasto-Villa Union Basin located between San Juan and La Rioja provinces, and the Cuyo and San Rafael basins, both in Mendoza province, Argentina. For this purpose, we used predictive formulas available in the literature based on variables on appendicular skeleton of different extant groups of mammals and reptiles. Geometric similarity using skull length was applied to provide an estimation of adult masses for species with only samples of juvenile and subadult limb bones. A broad body mass range was recorded. Small traversodontids such as Andescynodon mendozensis and Pascualgnathus polanskii were between 1 to 3.5 kg. Medium-sized traversodontids include adult Massetognathus pascuali with masses from 20 to 40 kg, and the adult forms of large-sized cynognathians like Cynognathus crateronotus and Exaeretodon argentinus reached or surpassed 100 kg. The morphological variations in the skull and the different body sizes observed between traversodontids are interpreted as reflecting different types of diets where small-sized traversodontids had a generalist diet, and the medium/large-sized traversodontids were herbivorous. Finally, palaeoecological working hypotheses regarding cranial and dental morphology, body mass, and their possible relation with diet in non-mammaliaform cynodonts of South America are offered.