The curious case of Cenozoic South America: Assembling the grassland biome with almost no grasses.

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

Aix-en-Provence

Simposio; International Meeting for Phytolith Research; 2016

Phytolith Research Association

Grassland ecosystems constitute one of the most prominent biomes today, covering about40% of Earth?s land surface. The assembly of the grassland biome has been of vital concernamong paleontologists and evolutionary biologists for 140 years. Grassland evolution has primarilybeen studied using indirect lines of evidence, in particular the evolution of tall (hypsodont)cheek teeth of large mammalian herbivores, whereas direct paleobotanical data have historicallybeen scarce. However, in recent years, phytoliths analysis has emerged as a useful tool for trackingthe expansion of open habitats dominated by grassland vegetation, not least because of thepotential for phytoliths to preserve in a wide range of Cenozoic deposits that do not containother types of plant fossils. Phytolith records from North America and Eurasia have shownthat grass-dominated vegetation spread in the late Oligocene or early Miocene, many millionyears before hypsodont ungulate herbivores evolved, supporting a link between floral and faunalchange in these regions. In southern South America, hypsodont herbivores appeared anddiversified in the middle Eocene (38 Ma), a pattern that was long thought to mark the spreadof savanna vegetation 20 million years earlier than on any other continent. We have tested thishypothesis by analyzing early Eocene-middle Miocene (ca 49-12 Ma) phytolith assemblages fromPatagonia (Argentina and Chile). In addition to ?traditional? compositional analysis of phytolithassemblages, we used a new proxy for habitat openness (reconstructed Leaf Area Index; rLAI) toevaluate whether habitats were open or closed, regardless of the relative abundance of grasses.The rLAI method relies on the fact that anticlinal epidermal cells (reflected as phytoliths)of non-grass plants change shape and size depending on light environment (with larger, moreundulated cells in shaded environments) to infer light environment/habitat openness in fossilassemblages. Our phytolith assemblage compositional analysis showed that grasses were rare inPatagonia for most of the Cenozoic, and did not form grasslands until sometime after the middleMiocene. Instead woody plants, including palms, remained prominent in vegetation through themiddle Miocene. However, these relatively grass-free habitats became increasingly open duringthe middle Eocene (as inferred using rLAI ), culminating in non-analog palm shrublands roughlyconcurrently with the earliest hypsodont herbivores. Isotopic data from the same strata record