This chapter describes the paleobiology of Santacrucian cingulates (armadillos and glyptodonts). At least five genera of armadillos and four genera of glyptodonts were sympatric in the Santa Cruz Formation. Body masses were calculated based on a variety of scaling models, allometric equations, multiple regressions, and geometric similarity. The locomotor habits were inferred from indices previously modeled in living dasypodids, using morphogeometric analyses and the application of a strength indicator. Feeding habits were inferred from jaw biomechanics, and the shape, arrangement, and wear patterns on teeth, and from ecomorphological analyses. All armadillos fall in the range of medium-sized living armadillos, and all glyptodonts are larger than any living armadillo, slightly greater than 100 kg, but are smaller than Middle Miocene–Pleistocene glyptodonts. All Santacrucian armadillos were good diggers but none reached the degree of fossoriality found in some specialized living taxa; all glyptodonts were ambulatory. The variation in the masticatory apparatus of the armadillos exceeds that in the living species, denoting a broader range of specializations and strong niche partitioning among the fossil species. The degree of variation in the masticatory apparatus of glyptodonts suggests differences in the selectivity of feeding and habitat preference. The taxonomic richness of armadillos is similar to that recorded today in the Chaqueña biogeographic province, supporting the environmental interpretation of a mixture of open and relatively closed vegetation in relatively dry conditions.