Habitat shift during the Middle Miocene Climatic Optimum of Southern Patagonia recorded in phytolith assemblages

JOSE CUTIÑO; ROBIN TRAYLER; CAROLINE STRÖMBERG; CAMILLA CRIFO; RICHARD KAY; SERGIO VIZCAı́NO; M. SUSANA BARGO; MATTHEW KOHN; ALEJANDRO ZUCOL

Paris

Congreso; 5th International Paleontological Congress; 2018

International Paleontological

High global temperature and pCO2 characterized the Middle Miocene Climatic Optimum(MMCO; ca. 17?14.5 Ma) and are thought to have promoted highly diverse ecosystems inwarm and wet climates at high latitudes. However, only sparse observations inform inter-pretations of regional climate and biotas outside the northern hemisphere. The Santa CruzFormation (SCF; 47-52◦S) of coastal Patagonia (ca. 17.8?16.6 Ma) represents the southern-most sedimentary sequence in the world recording the onset of the MMCO. SCF fossiliferoushorizons yield one of the most species-rich and well-preserved vertebrate assemblages onEarth. Plant macrofossils and well preserved phytolith assemblages also occur in the SCFwithin the same strata as the faunas. These linked fossil records allow us, for the first time, tocompare in detail animal and plant paleoecology and vegetation structure through the onsetof the MMCO in southern South America. Here we present results from analysis of phytolithassemblage composition (% plant functional types) documenting vegetation change duringthe early part of the MMCO, and interpret it in light of our current climate proxy data.Stable isotope ratios from fossil enamel and bones from the same strata further allow us toreconstruct local Mean Annual Temperature (MAT) and Mean Annual Precipitation (MAP)to test for potential climatic drivers of biotic change. Our preliminary results show highvalues for forest:open-habitat indicator ratios (indicating dominance of woody vegetation) inmost phytolith assemblages, excepting the oldest assemblage. While there is no clear trend inoverall forest:open-habitat ratio through time, the dominant forest indicators do vary acrossassemblages ? palm phytoliths decrease through time while other forest indicators increase.This pattern could be explained by vegetation shifts through time, spatial heterogeneity, orboth. Both phytolith and isotopic data indicate that grass communities consisted mainly ofC3 pooid grasses. Interestingly, grass phytoliths display a considerable decrease in size fromolder to younger strata, which may reflect the onset of a dryer and warmer climate. Whileisotopic data further support this climatic trend, our data suggest that drying did not resultin an expansion of open-habitat grasses.