Paleoenvironments and paleoecology of the Santa Cruz Formation (early-middle Miocene) along the Río Santa Cruz, Patagonia (Argentina)

The continental early-middle Miocene Santa Cruz Formation (SCF) in Austral Patagonia contains the best record of South American mammalian faunas prior to Great Biotic Interchange (GABI) and is particular interest because it preserved high-latitude biotic Southern Hemisphere spanning mid-Miocene Climatic Optimum. Through intensive fieldwork we recovered numerous fossil vertebrates, mostly mammals, from SCF along Río (RSC), type area for formation its fauna. We examine whether fauna differed among three distinct temporal intervals spanning, oldest youngest, Atlantic coastal suite localities Fossil Levels (FL) 1–7, at about 17.4 Ma, through RSC Barrancas Blancas (BB), between ~17.2 ~16.3 Segundas (SBB), ~16.5 ~15.6 Ma. With objective reconstructing paleoenvironmental community structure these faunas, compared them with 55 extant lowland across America 8° N 55° S latitude representing a wide range seasonality and, annual rainfall temperature, as well canopy height net primary productivity, sampling communities ranging tropical rainforest semi-arid steppe. Extant nonvolant genera each locality were assigned body size interval niche parameters reflecting diet substrate use, behavioral data literature. Extinct similar metrics on basis their morphology. From generic parameters, compiled indices ratios that express vectors fauna, including total number genera, pervasiveness arboreality, frugivory, browsing, relative richness predators prey. variables used model based uniformitarian principles. sample includes 44 mammals FL 38 BB, SBB. Simpson Coefficients faunal similarity are no greater than expected geographic distances them, do not suggest any apparent climatic differences. Based models obtained significant differences MAP (Mean Annual Precipitation) BB SBB, mean estimates 1635 mm, 1451 1504 confidence overlapping widely. MAT Temperature) ~21 °C ~22 1–7 possibly lower 16 but estimates. Temperature modest (3 4 °C) all localities. Canopy heights exceed 20 m sites. Despite inferred similarities, presence certain key taxa (e.g., caviomorph rodent Prolagostomus typothere Pachyrukhos) together an increased overall abundance rodents ever-growing cheek teeth suggests trend aridification upper part SBB BB. Taken together, propose paleoenvironment consisted largely semi-deciduous forests into savannas gallery-forest components. This habitats occurs today where mesic inland Brazil, northeastern Argentina eastern Paraguay give way northwestward more xeric Paraguayan Gran Chaco. These interpretations general agreement other sources evidence sedimentology, paleosols, isotopes, paleobotany elements. highlight value focusing paleocological studies SFC stratigraphically geographically confined samples instead entire distribution historical collections limited provenance. Santacrucian can be considered study after arrival hystricomorph anthropoid primates before GABI.