CENOZOIC ANDEAN GEOCHRONOLOGY, PALEOENVIRONMENTS AND TECTONIC HISTORY: EVIDENCE FROM SOUTH AMERICAN FOSSIL MAMMALS

Mammals provide key data for understanding evolution, documenting faunal change through time, and assessing environmental transformations.  For >100 years, knowledge of South American fossil mammals has been derived almost exclusively from the remarkable, but incomplete and sparsely-calibrated record from Patagonia and other lowland, high-latitude sites.  Broader availability of data from the tropics and montane regions permits development of a precise terrestrial geochronology, integrating magnetostratigraphy, radioisotopic calibration, and biochronology (South American Land Mammal “Ages”, SALMAs).

Exceptionally diverse or important Tertiary mammal faunas from the Andes are now known from Chile, Bolivia, Colombia, and Ecuador, helping to “fill in the picture” of evolution on a more continental scale.  This paper reviews geochronologic advances, and environmental and tectonic inferences derived from Chilean Andean faunas.  Key among these are >12 new assemblages from the central Chilean Abanico Formation (spanning ~ 4º of latitude and >25 m.y., from the Eocene to mid-Miocene).  Fossils are so ubiquitous that this unit now represents a premiere archive of SA mammal evolution, shedding light on periods of environmental restructuring, diversification pulses for “archaic” endemic lineages, and initial diversification of autochthonous clades (e.g., caviomorph rodents, platyrrhine primates).  The first-discovered of these assemblages represents a recently named SALMA (Tinguirirican), and documents the earliest appearance of open-habitat/grassland environments (~15-20 m.y. earlier in SA than elsewhere), possibly related to major climate changes near the Eo/Oligo boundary.

Perhaps the best-sampled temporal interval is the early-mid Miocene (~20-11 Ma), with important Andean assemblages (complementing the classical record from lowland Argentina) now known from S. Chile (47°S) to Colombia (3°N).  Intervening sequences occur throughout central Chile, the Chilean and Bolivian Altiplano, and Ecuador.  This interval now appears to represent a time of pervasive orogenesis and rapid uplift throughout much of the Andes, undoubtedly with major biotic ramifications (reflecting environmental change, increased topographic complexity, and geographic isolation creating endemism).