UNUSUAL OAK LEAF GALLS FROM THE MIDDLE MIOCENE OF NORTHWESTERN NEVADA

Distinctive galls have been found on a fossil oak leaf from the Miocen e Gillam Springs Flora of Washoe County, Nevada. The described galls a re located on the leaf surface of Quercus hannibali Dorf, an analogue of the modern species Q. chrysolepis Liebmann. Similar galls are found on extant Quercus, but the fossils seem distinctive enough to warrant description as Antronoides schorni new genus and species. The occurre nce of Antronoides schorni coincides with a rapid episode of change fr om a mesic to a more xeric habitat, with a concomitant shift from an o ak-dominated to a conifer-dominated paleoflora. Recent work suggests t hat speciation and radiation of galling insects is highest in xeric en vironments, possibly due to decreases in rates of parasitism and disea se. This pattern has been documented for modern galling insects and fi ts the qualitative fossil evidence we present. These galls also suppor t the hypothesis that cynipids in the Antron group originated in Nevad a or eastern California and migrated from their point of origin to the ir current range in the Sierra Nevada and Coast Ranges.