Evolutionary transformations of hystricognath Rodentia and the climatic change in the Eocene to Late Oligocene time interval

Abstract

The Eocene-Oligocene transition interval is characterized by global cooling and the development of large Antarctic ice sheets, in response to global cooling, the Paleogene vegetation shifted from dominantly tropical/subtropical Eocene forests to more “mixed mesophytic forests” and floras adapted to (seasonal) dry conditions during the Oligocene. Fully established grass-dominated environments, i.e. grasslands, developed not until the Miocene. The oldest fossils of Hystricognathi (Rodentia) are of Late Eocene age and the group was already diverse in the Late Oligocene. In South America seven distinct higher taxa are recorded for the Late Oligocene, followed by a subsequent radiation during the Miocene. According to a reconstruction based on morphology of Recent taxa and paleontological data, the last common ancestor of Recent (crown group) Hystricognathi possessed tooth structures, mouth cavity organisation and a nasal apparatus suggesting a mainly herbivorous diet with a wide range of nutritional resources. This pattern most probably evolved before the Late Eocene. The fossil record shows that further evolutionary transformations within Hystricognathi, i.e. the development of high crowned (hypsodont) cheek teeth, occurred from the Oligocene onward. They made possible a more pronounced use of plants with high fibre content, such as grasses. The reconstructed stem species pattern (ancestral character set) of Hystricognathi is characterised by a less energy consuming mode of reproduction which is hypothesised to be a prerequisite to herbivory with pronounced utilisation of grasses. When more open ecosystems originated in the Oligocene, several lineages of Hystricognathi show adaptations to the utilisation of plant material including grasses. However, these features were present long before the establishment of widespread grasslands during the Miocene when particularly the South American Hystricognathi experienced a second major phase of radiation. Most likely, the early evolution of Hystricognathi from the Oligocene onward occurred primarily in parallel to the development of more temperate forest communities and more open ecosystems, e.g. woodlands, wooded savannah communities, and shrubland.