The paleoenviroment and paleoecology of the Santa Cruz Formation (SCF) issummarized, combining the data from the chapters of this book and newexamination of the community structure of the vertebrate fauna using modernanalogs. Emphasis is placed on the SCF outcrops along the coastal Atlanticbetween about 50.3° and 51.6° S and their faunas (~17.9 to16.2 Ma; Santacrucian SALMA). New data on the sedimentology, the ichnology, andthe flora and fauna of the SCF is particularly strong for the lower parts of theSCF south of the Río Coyle (FL 1–7). FL 1–7 (~17.4to 17.5 Ma) is analogous to a single modern fauna of limited geographic andtemporal scope. As paleolatitude during Santacrucian times was the same as thatof 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 andmoisture from reaching the Atlantic coast. New dates for FL 1–7 indicatethat the mid-Miocene global climatic cooling had not yet begun. Several taxarecovered at FL 1–7 or in nearby penecontemporaneous levels (e.g. palmtrees, the frog Calyptocephalella, the lizardTupinambis, the anteater Protamandua, and the primate Homunculus) strongly indicate that the climate ofFL 1–7 was much warmer and wetter than today. The overall mammalianspecies richness and niche composition, expressed as percentages of arboreal orscansorial, frugivorous, and grazing, suggest that overall rainfall was in therange of 1000 to 1500 mm per annum. Occurrence of trees and forest-dwellingbirds and mammals (porcupines, spiny rats, sloths, scansorial marsupials, andmonkeys) supports this conclusion. The occurrence of calcareous root casts inpaleosols indicates high seasonality in rainfall with cool wet winters and drywarm summers. Grasses were also present, and a number of vertebrate taxa (giantterrestrial birds, many notoungulates, glyptodonts, and armadillos) appear tohave been adapted to open environments. Consideration of sedimentologic,ichnologic, floral, and faunal elements taken together suggests a landscape forFL 1–7 consisting of a mosaic of open temperate humid and semi-aridforests, with ponds in some areas and seasonal flooding in others, no doubtpromoting the formation of marshlands with a mixture of grasses and forbes.