LATE MIOCENE (CLARENDONIAN) FOSSIL PLANTS AND ANIMALS FROM UNITY, BAKER COUNTY, OREGON

A small assemblage of vertebrates including Clarendonian (late Miocene) hipparionine horses and proboscideans has been known for some time from Ironside, Malheur County, Oregon. In 1941 Lon Hancock found a fossil proboscidean skull in the Ironside Formation near Unity, Baker County. It was identified as "Miomastodon merriami" by G.G. Simpson during a visit in 1953, and remains so-labelled at the Oregon Museum of Science and Industry, Portland. "Miomastodon merriami" has since been transferred to Zygolophodon proavus and has very different teeth from Hancock's skull, which is conspecific with Gomphotherium cingulatum Downs, a taxon included within G. osborni by Madden and Storer. New Clarendonian fossils of proboscidean, hipparionine horse, peccary, and rhino have now been found in the Ironside Formation near Unity, where tephrochronological correlations indicate an age of 11-12 Ma. Proboscideans, peccaries and rhinos dominate Inceptisol paleosols of stream margins, and form a mammal community comparable with Shotwell's Clarendonian "pond bank community". Hipparionine horses dominate Alfisol paleosols comparable with Shotwell's Clarendonian "savanna community". In Juniper Gulch, near Hereford, north of Unity, diatomaceous shale 35 m above the mammal-bearing beds has recently yielded partly articulated specimens of bullhead catfish [Ictalurus (Ameiurus) vespertinus] as well as an assemblage of fossil leaves dominated by live oak (Quercus hannibali), but with other oaks (Q. lobata, Q. simulata), maple (Acer chaneyi), ash (Fraxinus dayana), hickory (Carya bendirei), mountain mahogany (Cercocarpus ovatifolius) and water chestnut (Trapa americana). This assemblage and paleosols indicate vegetation and climate comparable to open grassy live oak forests on the western slopes of the Sierra Nevada in northern California today. This was a summer-dry, open vegetation, with mean annual precipitation of 540-900 mm, distinct from humid middle Miocene (Hemingfordian-Barstovian) forests of eastern Oregon, and also different from arid latest Miocene (Hemphillian) tall grassland and sagebrush paleosols and faunas.