Decoupling the spread of grasslands from the evolution of grazer-type herbivores in South America

Abstract

The evolution of high-crowned cheek teeth (hypsodonty) in herbivorous mammals during the late Cenozoic is classically regarded as an adaptive response to the near-global spread of grass-dominated habitats. Precocious hypsodonty in middle Eocene (B38 million years (Myr) ago) faunas from Patagonia, South America, is therefore thought to signal Earth's first grasslands, 20 million years earlier than elsewhere. Here, using a high-resolution, 43-18 million-year record of plant silica (phytoliths) from Patagonia, we show that although open-habitat grasses existed in southern South America since the middle Eocene (B40 Myr ago), they were minor floral components in overall forested habitats between 40 and 18 Myr ago. Thus, distinctly different, continent-specific environmental conditions (arid grasslands versus ash-laden forests) triggered convergent cheek-tooth evolution in Cenozoic herbivores. Hypsodonty evolution is an important example where the present is an insufficient key to the past, and contextual information from fossils is vital for understanding processes of adaptation.

Ontogenetic and life history trait changes associated with convergent ecological specializations in extinct ungulate mammals

“…Because they lived in environments on which soil minerals (dust and grit) were likely deposited to an important degree due to volcanic activity (16,17,29), high-crowned teeth may have allowed notoungulates to cope with the ingestion of such external abrasive particles that strongly wear teeth during chewing. In the presence of external abrasives, wear rates may have also been enhanced by an increase in the pressure and intensity of chewing, especially given a diet including many fibrous plants with a low nutritive value (e.g., grass and grass-like plants, some shrubs) (27).…”

“…Among factors leading to dietary changes, increasing aridity could have at least triggered the emergence and abundance of new types of vegetation, such as shrublands observed in Patagonia during the second part of the Paleogene (16,29) (Fig. 3C), although forests probably remained dominant until the Middle Miocene in South America (15,39), especially in the northern part (14).…”

“…This repeated innovation was likely selected to withstand high dental wear during mastication (17,27). It was also assumed to be related to the successive orogenic, volcanic, and erosive phases of the Andean Cordillera starting from the Late Paleocene (approximately 57 Ma) (28), which involved subsequent deposition of abrasive detritic particles on plants (16,29,30).…”

New highlights of phytolith structure and occluded carbon location: 3-D X-ray microscopy and NanoSIMS results

“…Inside the plant, Si is transported in the sap and deposited inside the cells, in the cell walls and in extracellular spaces of stems and leaves as micrometric hydrous amorphous silica particles called phytoliths. Upon plant decay, part of the phytolith production can be incorporated into soils or sediments and preserved for as long as millions of years (Alexandre et al, 2011;Miller et al, 2012;Strömberg et al, 2013). These fossil phytolith assemblages can be used for reconstructing past vegetation and climate conditions via their morphological and geochemical signatures (Piperno, 2006;Alexandre et al, 2012).…”

Computational biomes: The ecometrics of large mammal teeth

“…A large share of hypsodont species suggests an environment dominated by grasslands (Strömberg et al, 2013). Characterizing the African habitats around the equator with flat topography and high hypsodonty, which primarily signal grass eating, suggests that the African rain forest environments include a small share of grassland adapted fauna, and perhaps include open canopy with grasses.…”