Photosynthetic pathway of grass fossils from the upper Miocene Dove Spring Formation, Mojave Desert, California

The spread of grasslands in the Miocene and of C-4 grasses in the late Miocene-Pliocene represents a major development in terrestrial plant evolution that affected the climate system and faunal evolution. The macrofossil record of grasses is sparse, likely due to the limited preservation potential of grasses. Diagnosis of the C-3 or C-4 photosynthetic pathway depends on preservation of both cellular structures and organic carbon for isotope analysis. Here we analyze the anatomical and isotopic composition of newly-collected grass fossils from the Dove Spring Formation, Red Rock Canyon State Park, California, USA, located in the El Paso Basin on the western side of the Basin and Range Province, a site previously identified as one of the earliest known C-4 grass fossil bearing localities. We analyzed the anatomical and geochemical characteristics of these new grass fossils dated to 12.01-12.15 Ma. The fossils analyzed in this study include grass shoots and in cross-section display anatomy indicative of the C-3 photosynthetic pathway. We isolated organic carbon from the stem fossils and determined the carbon isotopic composition to be -24.8 +/- 0.5 parts per thousand. Together, the anatomical and geochemical analyses confirm that these plants used the C-3 photosynthetic pathway. Our findings are consistent with dietary evidence based on tooth enamel from grazing mammals of available C-3 resources in the same sections. These newly reported Miocene-age C-3 grass fossils contribute to a sparse macrofossil record of grass evolution. Overall, paleoecological reconstructions at this site indicate more humid conditions during the Miocene compared to the modern Mojave Desert with C-3 grasses and diverse grazing mammals.