PALEOECOLOGICAL ASPECTS OF WESTERN UNITED STATES NONMARINE OSTRACODS DURING THE EOCENE–OLIGOCENE TRANSITION: THE EARLY OLIGOCENE FAUNAS OF THE RENOVA FORMATION, SOUTHWESTERN MONTANA

ABSTRACT The Renova Formation is a Paleogene sedimentary sequence from southwestern Montana that was deposited in a more extensive basin dominated by mid- to high altitude streams and small-lake paleoenvironments. The sediment infill was dissected during the Miocene–Pliocene into the remnants currently observed in several places, including the Upper Ruby River valley, Medicine Lodge valley and Horse Prairie. Ostracods from these early Oligocene strata belong to five genera of the suborder Cypridocopina: Amplocypris?, Candona (including the species Candona rangliensis? and Candona sp. aff. Candona ohioensis), Cyprois, Eucypris, and Paracypridopsis?. These ostracods, capable of producing drought-resistant eggs, likely inhabited fairly stable small lakes, characterized by arboreal surroundings reminiscent of forests in highland regions that were buffered against the drastic climatic variations of the late Eocene–early Oligocene transition (EOT) during the Eocene–Oligocene climatic deterioration. Further south, climatic change during the EOT, coupled with volcanic events, impacted the establishment of long-lasting, highly diverse ostracod faunas in lowland areas. The Upper Ruby River, Medicine Lodge and Horse Prairie early Oligocene deposits in southwestern Montana may have acted as refugia for Eocene ostracod genera that became extinct elsewhere or represented Lazarus taxa that would later diversify during the Miocene.