A review of the paleoenvironment and paleoecology of the Miocene Santa Cruz Formation

Abstract

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 analogues. Emphasis is placed on the SCF outcrops along the coastal Atlantic between about 50.3º and 51.6º S and their faunas (~ 17.9 to 16.2 Ma; Santacrucian SALMA). New data on the sedimentology, 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 Middle 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, ichnology, 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.