Diverse evidence for grasslands since the Eocene in Patagonia

Abstract

Timing and ecological steps of the rise and expansion of grasslands differ not only geographically but also according to the type of available information. Patagonia (Argentina) was largely considered the region where grasslands and grazers early evolved in the Eocene, mostly based on functional morphology of mammals. However, recent paleobotanical studies question this traditional view and maintain that this occurred after early Miocene. To tackle this discrepancy, we conduct a multidisciplinary and integrative approach using diverse lines of evidence obtained from middle Eocene - middle Miocene tuffaceous deposits of central and northern Patagonia. Gathered data correspond to sedimentary facies, paleosols, insect trace fossils, opal phytoliths and fossil mammals (tooth morphology, body size). Long-lasting and frequent volcanic ashfalls caused permanent disturbances in physical scenarios, biotic systems and grassy habitats; just like herbivory of mammals through coevolutionary grass-grazer mechanisms. The results indicate that soils, vegetation, insects and mammal herbivores begun synchronously to record diverse traits related to grasslands showing mosaic vegetation since middle Eocene (~44 Ma). These traits include mollic paleosols, abundant and persistent dung beetle trace fossils (Coprinisphaera ichnofacies), phytolith assemblages attributable to savannas and medium to large-size hypsodont mammals. We also track the evolutionary history after the reduction of closed forests, recognizing alternating subhumid savannas with riparian forests, where the relative frequency between grasses, palms, trees and shrubs fluctuated in time and space; and desert vegetation (shrublands) dominated by palms and shrubs during dryer periods. The advent and expansion of grasslands can be linked to the cooling and drying period subsequent to the Early Eocene Climate Optimum.