WHERE HAVE ALL THE GRASSES GONE?: NEW MIDDLE MIOCENE PHYTOLITH RECORDS REVEAL THAT GRASSLANDS PLAYED A MINOR ROLE IN HYPSODONTY EVOLUTION IN SOUTHERN SOUTH AMERICA

STRÖMBERG C.; DUNN R.; MADDEN R.H.; KOHN, M.; CARLINI A.A.

Los Angeles

Congreso; 73 Meeting of the Society of Vertebrate Paleontology; 2013

SVP

The incremental evolution of high-crowned (hypsodont) and ever-growing (elodont) cheek teeth in numerous herbivore lineages in southern South America, starting in the middle Eocene, was long thought to signal the spread of savanna vegetation 20 million years earlier than on any other continent. In contrast, recent pollen and plant silica (phytolith) data have indicated that Patagonia was covered in forests for much of the Cenozoic, and that grasslands emerged sometime after the early Miocene. These results contradict the notion that hypsodonty initially evolved in response to a shift from forest to open, grass-dominated vegetation. However, they do not preclude that a subsequent, middle or late Miocene spread of grasslands influenced crown-height evolution in Patagonian meridiungulates, rodents, and marsupials. We sought to test this hypothesis by reconstructing vegetation using phytolith assemblages sampled from several, late early-middle Miocene (16.87?11.78 Ma) faunal sites along the Argentine and Chilean Andes. Phytoliths are appropriate for tracking the spread of grasslands as they can provide information about both the relative abundances of grass and woody vegetation, as well as the types of grasses present. Plant silica recovery from many sampled sections was poor, but enough sediment samples yielded phytoliths to allow for reconstruction of vegetation history in this region. Specifically, we found that woody plants, including palms, continued to dominate vegetation through the middle Miocene. Although grass phytoliths are abundant in some samples (~30?45%), there is no convincing evidence for plant communities dominated by open-habitat grasses such as pooids and PACMADs. Instead, bamboos or similar forest-dwelling grasses appeared to have comprised a substantial part of the grassy understory. This pattern suggests that, unlike for North American and Eurasian ungulates, grass-dominated habitats likely played a minor role in Cenozoic hypsodonty evolution in South American herbivores. Instead, the availability of inorganic dietary abrasives (e.g., volcanic ash) may have driven evolutionary changes in cheek tooth crown heights in southern South America.