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

南巴塔哥尼亚的中新世早期中新世克鲁斯组（SCF）包含美国大生物圈闭（GABI）之前南美哺乳动物动物群的最佳记录，并且由于它是保存最完好的高纬度大陆生物而特别受关注在南半球，记录了中新世中期最佳气候。通过深入的野外考察，我们沿着圣克鲁斯河（RSC）沿SCF恢复了众多化石脊椎动物，其中大部分是哺乳动物，这是构造及其动物区系。我们检查了SCF动物群是否在SCF的三个不同的时间间隔之间变化，从最老到最年轻的大西洋沿海地区化石水平（FL）1-7，大约17.4 Ma，通过RSC Barrancas Blancas的地区（BB），介于〜17.2和〜16.3 Ma之间，和Segundas Barrancas Blancas（SBB），介于16.5和〜15.6 Ma之间。为了重建这些RSC动物群的古环境和群落结构，我们将它们与南美洲从北纬8°至北纬55°的55个现存的低地哺乳动物地区进行了比较，这些地区代表了广泛的季节性以及年度降雨量和温度作为冠层高度和净初级生产力，从热带雨林到半干旱草原采样社区。根据文献中的行为数据，为每个地方的现有非挥发性哺乳动物属分配了一个体型区间和反映饮食和底物使用的利基参数。灭绝属根据其形态被分配了相似的生态位指标。根据一般的利基参数，我们编制了索引和比率，以表达每种动物的群落结构的向量，包括属的总数，树木的繁盛，节俭和浏览以及捕食者对其猎物的相对丰富度。社区结构变量用于基于统一原则对化石局部社区结构进行建模。化石样本包括来自FL 1-7的44属哺乳动物，来自BB的38属和来自SBB的44属。化石地区之间动物区系相似性的辛普森系数不超过根据它们之间的地理距离所期望的值，并且没有暗示任何明显的气候差异。根据这些模型，我们获得的FL 1–7，BB和SBB的MAP（平均年降水量）没有显着差异，平均估计值为1635毫米，1451毫米，和1504毫米，估计的置信区间有很大的重叠。FL 1–7和SBB的MAT（平均年平均温度）估计值介于〜21°C和〜22°C之间，BB的MAT估计值较低，为16°C，但估计值范围广且重叠。温度季节性适中（3°C至4°C），并且在所有地区都相似。所有场地的树冠高度均超过20 m。尽管存在这些地理上和推断的气候相似之处，但仍存在某些关键的分类单元（例如，啮齿类啮齿动物 所有场地的树冠高度均超过20 m。尽管存在这些地理上和推断的气候相似之处，但仍存在某些关键的分类单元（例如，啮齿类啮齿动物 所有场地的树冠高度均超过20 m。尽管存在这些地理上和推断的气候相似之处，但仍存在某些关键的分类单元（例如，啮齿类啮齿动物腹泻和Typhoe Pachyrukhos），加上颊齿不断增长的啮齿动物的总体丰度和丰富度提高，表明与FL 1-7和BB相比，SBB的SCF上部发生了干旱化的趋势。综上所述，我们认为南卡罗莱纳州的古环境主要由半落叶林组成，范围包括具有廊林成分的热带稀树草原。今天，这种栖息地发生在这里，巴西南部，阿根廷东北部和巴拉圭东部的内陆大西洋内陆森林逐渐向西北进入更干燥的巴拉圭大查科。这些解释与沉积学，古土壤，同位素，古植物学和其他动物元素中的其他证据来源普遍相符。我们着重强调将SFC的古环境和古生物学研究的重点放在地层和地理上受限的样本上，而不是基于有限来源的历史资料收集SCF的整个时间和地理分布。在葫芦科啮齿动物和类人猿灵长类动物到达之后但在GABI之前，圣塔克鲁斯人可以被认为是南美动物研究的模型。