Aridity, Cooling, Open Vegetation, and the Evolution of Plants and Animals During the Cenozoic

Abstract

The development of grassland ecosystems across most continents was a multistage process involving the appearance of open-habitat grasses in the Paleogene, the mid-late Cenozoic spread of C3 grass-dominated habitats, and, finally, the Late Neogene expansion of C4 grasses at tropical and subtropical latitudes. In addition, the timing of these evolutionary and ecological events varied across continents and between regions. The middle Miocene witnessed a climate optimum at a global scale, but, soon thereafter, beginning about 14 million years ago, a global cooling trend commenced that was accompanied by environmental change. This drop in global temperature resulted in the spread of more arid and seasonally dry conditions that, in turn, resulted in the expansion of increasingly open woodlands. Changes in resources also led to modifications in the distribution and abundance of mammalian herbivores, favoring forms with hypsodont teeth. In Europe, where humid climates generally persisted throughout the Neogene, the Iberian Peninsula was the first region to experience increasingly seasonal aridity in the early Miocene. Open environments under an arid climate remained confined to Southern Europe and only expanded into Northern Italy in the latest Miocene. The spread of aridity to this region is associated with the final stage of the Messinian Salinity Crisis. In Eastern Asia, generally humid conditions existed before the late middle Miocene but were replaced by a midlatitude arid belt in the late middle–late Miocene. The East Asian summer monsoon, an atmospheric circulation pattern of the present, only influenced the region later, with the beginning of eolian red clay deposition, sometime between seven and eight million years ago. A similar pattern is recorded elsewhere. Biomes changed from tropical forest to steppes across broad areas of southern South America during the early–middle Miocene. Hence, a general global pattern of vegetational turnover is witnessed in the Neogene. More open woodlands and/or grasslands appeared over all continental landmasses in response to changes from rather humid conditions to more seasonally dry, semiarid, and arid climates at various times during the Miocene. In this chapter, we will explore these vegetational changes to understand if they were accompanied by changes in faunal composition, reshaping the world’s biosphere.