Over the past century, the Santa Cruz Formation of coastal Argentina (late Early Miocene) has yielded a remarkable collection of platyrrhine primates. With few notable exceptions, most of the specimens have been included in Homunculus patagonicus Ameghino, 1891, a stem platyrrhine. Homunculus patagonicus was approximately 1.5 to 2.5 kg in body mass, about the size of a living saki monkey (Pithecia) or a female Cebus. Molar structure indicates that the diet consisted of a mixture of fruit and leaves. A deep jaw, large postcanine tooth roots, large postglenoid processes and moderately large chewing muscle attachments (i.e. massive zygomatic arches, sculpted temporalis origins) suggest that physically resistant foods were key components of the diet. Heavy tooth wear suggests large amounts of ingested silica or exogenous abrasives. Incisor morphology suggests that exudate harvesting may have been part of the behavioral repertoire, although not a specialization. The canines were small, providing no evidence of sclerocarpic foraging. Canines were sexually dimorphic, suggesting that the taxon experienced some intrasexual competition rather than being solitary or pair-bonded. Brain size was small and the frontal cortical region was proportionately small. From the small size and structure of the orbits, the structure of the organ of hearing, the reduced olfactory fossae and the relatively large infraorbital foramina, we infer that Homunculus was probably diurnal, with acute vision and hearing, but with a poor sense of smell and little reliance on tactile vibrissae. Homunculus was an above-branch arboreal quadruped with leaping abilities. The semicircular canals show evidence of considerable agility, reinforcing the inference of leaping behavior. The overall locomotor repertoire is not unlike that of the forest-dwelling extant saki monkey Pithecia. Considered together, the mosaic of dietary and locomotor morphology in Homunculus suggests that Homunculus inhabited an environment – as compared with earlier Colhuehuapian and Pinturan primate habitats – shifting towards greater seasonality in patchy forests near river courses.

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.