For more than 120 years, the coastal exposures of the Santa Cruz Formation have been fertile ground for recovery of vertebrates from the late Early Miocene (~18 to 16 million years ago, Ma). As long ago as the 1840s, Captain Bartholomew Sulivan collected fossils from this region and sent them to Charles Darwin, who passed them to Richard Owen. Carlos Ameghino undertook several explorations of the region starting in the late 1880s. Carlos' specimens were described by his brother Florentino, who believed that many of the species were more ancient than now understood and represented the ancestors of many Holarctic mammalian orders. Ameghino's novel claims prompted William B. Scott to organize fossil collecting expeditions in the Santa Cruz beds led by John B. Hatcher. The fossils were described in a series of exhaustive monographs with the conclusion that the fauna was much younger than Ameghino thought. Several brief expeditions took place during the twentieth century, led by researchers from different institutions. Since 2003, we have undertaken the collection of over 1600 specimens, including large series of relatively complete skeletons. In this edited volume we have gathered together a group of researchers to study the coastal Santa Cruz Formation and its associated flora and fauna to provide a paleobiological reconstruction of the Santacrucian vertebrate community and to place it in its biotic and physical environment.

The paleoenviroment and paleoecology of the Santa Cruz Formation (SCF) is summarized, combining the data from the chapters of this book and new examination of the community structure of the vertebrate fauna using modern analogs. Emphasis is placed on the SCF outcrops along the coastal Atlantic between about 50.3° and 51.6° S and their faunas (~17.9 to 16.2 Ma; Santacrucian SALMA). New data on the sedimentology, the ichnology, and the flora and fauna of the SCF is particularly strong for the lower parts of the SCF south of the Río Coyle (FL 1–7). FL 1–7 (~17.4 to 17.5 Ma) is analogous to a single modern fauna of limited geographic and temporal scope. As paleolatitude during Santacrucian times was the same as that of today, FL 1–7 was extratropical and had highly seasonal day lengths. The Andes had not risen to a sufficient altitude to block westerly winds and moisture from reaching the Atlantic coast. New dates for FL 1–7 indicate that the mid-Miocene global climatic cooling had not yet begun. Several taxa recovered at FL 1–7 or in nearby penecontemporaneous levels (e.g. palm trees, the frog Calyptocephalella, the lizard Tupinambis, the anteater Protamandua, and the primate Homunculus) strongly indicate that the climate of FL 1–7 was much warmer and wetter than today. The overall mammalian species richness and niche composition, expressed as percentages of arboreal or scansorial, frugivorous, and grazing, suggest that overall rainfall was in the range of 1000 to 1500 mm per annum. Occurrence of trees and forest-dwelling birds and mammals (porcupines, spiny rats, sloths, scansorial marsupials, and monkeys) supports this conclusion. The occurrence of calcareous root casts in paleosols indicates high seasonality in rainfall with cool wet winters and dry warm summers. Grasses were also present, and a number of vertebrate taxa (giant terrestrial birds, many notoungulates, glyptodonts, and armadillos) appear to have been adapted to open environments. Consideration of sedimentologic, ichnologic, floral, and faunal elements taken together suggests a landscape for FL 1–7 consisting of a mosaic of open temperate humid and semi-arid forests, with ponds in some areas and seasonal flooding in others, no doubt promoting the formation of marshlands with a mixture of grasses and forbes.

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.

This chapter reviews the paleobiology of pilosans (anteaters and sloths) from the Santa Cruz Formation, which comprise at least one genus of vermilinguan and 11 genera of sloths. Paleobiological studies performed on these xenarthrans include: estimation of body mass (through multivariate regression); analysis of the limbs so as to infer the locomotor habits and substrate use (through morphometric and qualitative-comparative morphofunctional analyses), and studies of the masticatory apparatus to infer probable feeding habits (mainly through analysis of tooth morphology and wear facets, plus the anatomy of the masticatory apparatus as a whole). Santacrucian anteaters were small animals, about 6 kg, well suited for climbing and for scratch-digging the substrate in searching for their preferred food, social insects. Various sloths were moderately large-sized forms, the largest reaching about 100 kg, with a locomotor pattern distinct from that of living sloths, resembling more that of vermilinguans and pangolins. The results suggest well-developed digging capabilities, but semi-arboreal habits cannot be ruled out. Megatherioid sloths were most likely leaf eaters, and the primary method of food reduction must have been by shearing or cutting (with a predominance of orthal movements). Mylodontid masticatory movements included a larger transverse component, and food reduction must have been by crushing and grinding, which suggests they fed on more compact, three-dimensional, and fibrous food items such as the underground storage organs of plants. The semi-arboreal habits suggested for anteaters and, probably, sloths indicate they lived in forests or that forested areas were present nearby. The specialized feeding habits of vermilinguans are indicative of subtropical and warm temperate environments because they would have depended on a year-round availability of social insects.