Unusual oak leaf galls from the middle Miocene of northwestern Nevada

ABSTRAcr-Distinctive galls have been found on a fossil oak leaf from the Miocene Gillam Springs Flora of Washoe County, Nevada. The described galls are located on the leaf surface of Quercus hannibali Dorf, an analogue of the modem 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 occurrence of Antronoides schorni coincides with a rapid episode of change from a mesic to a more xeric habitat, with a concomitant shift from an oak-dominated to a conifer-dominated paleoflora. Recent work suggests that speciation and radiation of galling insects is highest in xeric environments, possibly due to decreases in rates of parasitism and disease. This pattern has been documented for modem galling insects and fits the qualitative fossil evidence we present. These galls also support the hypothesis that cynipids in the Antron group originated in Nevada or eastern California and migrated from their point of origin to their current range in the Sierra Nevada and Coast Ranges.