758
Journal of Paleontology 89(5):748–761 To account for paleoenvironmental conditions prevailing in
the area inhabited by the present fossil assemblage, a brief overview of the paleoclimatic progression and paleoenviron- mental evolution that occurred throughout the Early Miocene is required.
According to Compagnucci (2011) changes in land surface
represent one of the processes that affect climate in time scales from millions to hundreds of thousands of years, and the climatic conditions in the region of Patagonia were strongly
affected by the uplift of the Andes. The Andean orogeny is the final consequence of the Andean structural cycle that developed throughout the past ~190 Myr (Early Jurassic to present), which results from the subduction of the eastern plates of the Pacific Ocean (Farallon and Nazca) and the Antarctic plate under Gondwanan basement (South American plate) at the west margin of South America (Armijo et al., 2015). As part of this Andean cycle, the uplift of the southern Patagonian Andes (approximately between 17 and 14 Myr) formed a large topographic barrier to atmospheric circulation in the Southern Hemisphere Westerlies and established a pronounced oro- graphic rain shadow; such a barrier led to a strong aridification in the eastern foreland and increased precipitation rates on the windward western side of the mountains during the Miocene in Patagonia (Blisniuk et al., 2006, cited in Compagnucci, 2011). Besides, ice had begun accumulating at the South Pole
during the Late Eocene (approximately between 37.2 and 33.8 Myr) culminating in the Pleistocene ice age. Oligocene cooling occurred together with changes in continental distribu- tion, plateau uplift and opening of oceanic gateways (Drake Passage and Tasmanian Gateway), implying the late Eocene establishment of the Antarctic Circumpolar Current and development of the Polar Frontal Zone, reducing poleward ocean heat transport and favoring glaciation over Antarctica. The Late Miocene Tortonian period (approximately between 11 and 7 Myr) was characterized by intensive Antarctic glaciations and the beginning of glaciations in the North Atlantic region (Compagnucci, 2011). The Andes orogen interplayed with an increasingly drying
climate driven by the global Cenozoic cooling, resulting in pronounced alterations of the paleoclimate conditions over Patagonia (Armijo et al., 2015). During the Miocene paleocli- mate settings along southern Patagonia were fluctuating, but showed an overall downward trend in temperature and upward trend in aridity (Blisniuk et al., 2005, 2006; Kay et al. 2012a). Animal and plant communities changed according to these oscillations. Toward the end of the Paleogene and the beginning of the Neogene (Late Oligocene–Early Miocene) warm climates allowed the development of rainforest elements in the flora of extra-Andean Patagonia, with dominance of gallery forests to the east and a subtle emergence of xerophytic assemblages in coastal salt environments and pockets of inland areas. Even though drier conditions would have prevailed in lowland areas during the late Early Miocene (Late Aquitanian), the latest Early Miocene (Burdigalian) was characterized by a predominance of aquatic herbs and hydrophyte vegetation (Palazzesi et al., 2003; Barreda and Palazzesi, 2007, 2010), suggesting plentiful water availability. Moreover, Palazzesi et al. (2003) suggested that the infer- red paleofloristics is consistent with the paleovertebrate record,
as deduced from the work of Patterson and Pascual (1972) and
Pascual et al. (2002), who highlighted that the greatest diversi- fication of hypsodont-grazer mammals in Patagonia occurred during the Oligocene and Early Miocene; this supports a paleoenvironmental change toward an increasing predominance of grassy vegetation. Several works on fossil vertebrates have assessed the paleoenvironmental shift that occurred in southern Patagonia throughout the
Miocene.Among others, these studies addressed a diverse group of mammals including arboreal por- cupines (Echimyidae, Erethizontidae), microbiotheres similar to the extant “monito del monte” (Microbiotheriidae), and new world primates (Platyrrhini), all of which have been regarded as indicators of warm and humid climates and of forest environ- ments (Pascual and Ortiz-Jaureguizar, 1990; Tauber, 1997b, 1999; Vizcaíno et al. 2006; Kay et al., 2012b). Patterson and Pascual (1972) and Pascual et al. (2002). In this regard, Tauber (1997b) also described a relative increase in the percentage of organisms with euhypsodont dentition—which are considered specialized grazers—against organisms with brachydont and brachydont-mesodont dentition toward the upper levels of Santa Cruz Formation. In addition, a general decrease in taxonomic and body size diversity of small-sized mammals and increasing taxonomic diversity and size of other mammals like glyptodonts and toxodonts was found. Overall, this points to a climate turnover throughout the Miocene from warm, moist, stable conditions to drier conditions with marked seasonality, and a shift from environments with predominant tree or shrubby vegetation to environments more open with predominantly grassy vegetation. Similar inferences resulting from analysis based on the diversity of the Santacrucian armadillos by Viz- caíno et al. (2006) reinforced the climate-turnover hypothesis. In addition, analyses of sedimentological, paleopedologi-
cal, and geomorphological evidence (Tauber, 1994, 1996), support the aforementioned Miocene paleoenvironment progression. Immature and mature paleosols, the latter with a high degree of animal and plant bioturbation, have been found in the lower beds of the Santa Cruz Formation, suggesting humid and stable environmental conditions; while toward the upper levels occurrence of evaporite crystals, geomorphological structures (interpreted as desiccation cracks) and inferred seasonal variations in rivers paleo-flux, indicate increasing seasonality and a drying trend. However, paleoclimatic approximations based on analyses
of community structure (cenograms), which allowed to infer variables such as rainfall and vegetation structure (Croft 2001), showed a less marked shift in paleoenvironmental conditions suggesting a slightly greater rainfall in the upper levels of Santa Cruz Formation that described in previous studies. Following Kay et al. (2012a), the Early Miocene
(17.4–17.5 Myr) Santacrucian-age fauna thrived in a mosaic of open temperate humid and semi-arid forests, with paleoclimatic reconstructions indicating annual rainfalls greater than 1000mm and a mean annual temperature above 14°C in a period characterized by wet winters and dry summers, and a marked seasonality in the length of daylight. In addition to the presence of lakes, seasonal flooding led to the formation of marshlands dominated by herbaceous (both grass and forb) vegetation. To this extent, various elements of the vertebrate associations of the Santa Cruz Formation, other than mammals, suggest a mosaic of
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