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Two new genera and species of plagiomenids (Mammalia, ?Dermoptera, Plagiomenidae) are described from the North American Uintan (middle Eocene) and Chadronian (early Oligocene). A third genus and species, Ekgmowechashala philotau, from the early and middle Arikareean (late Oligocene) of the northern United States, is removed from the primate family Omomyidae and placed in the Plagiomenidae. All three newly recognized plagiomenids are placed in the Ekgmowechashalinae, sister subfamily to the subfamily Plagiomeninae (new rank). Ekgmowechashaline plagiomenids are somewhat primate-like, as is the plagiomenine genus Worlandia, but the Plagiomenidae are usually considered to be allied to the living colugos of southeast Asia, order Dermoptera. Analysis of that relationship is placed outside the scope of this paper. Tarka stylifera, the earliest known ekgmowechashaline, occurs in the type section of the Tepee Trail Formation, early Uintan (Shoshonian: late medial Eocene) of northwestern Wyoming. This locality falls in paleomagnetic Chron C20R, interpreted to be close to 47.5 Ma in age. A second, more primitive but later-occurring ekgmowechashaline genus and species, Tarkadectes montanensis, is from a nominally early Oligocene level (Chadronian) in the Kishenehn Formation of northern Montana. Ekgmowechashala is known from lower dentitions from the early Arikareean Sharps Formation of South Dakota and probably from an upper dentition reported from middle Arikareean rocks in the John Day Formation of Oregon. Ekgmowechashala is placed with the other two genera because of lower cheek-tooth morphology, but it lacks the enlarged incisor of Tarka. Ekgmowechashalines are hypothesized here to be primarily frugivores, folivores, and nectar- and exudate-feeders. Until now, known undoubted plagiomenids were restricted to the Paleocene and early Eocene (Wasatchian). The newly recognized post-Wasatchian occurrences are all in the northern part of the United States and are in keeping with previously known plagiomenid geographic distribution, which ranged from northern Wyoming to the Canadian arctic and possibly beyond.

Abstract The paleogeography and marine paleoclimate of the Arctic Ocean Basin have evolved considerably during the past 100 million years, since the beginning of the Late Cretaceous. The shallow-water marine faunas that are needed to understand much of this history have recently been found, so that paleontologic studies of the Arctic Ocean Basin and its margins are presently in their early stages. The paleogeographic history of the Arctic Ocean may be divided conveniently into four intervals, each characterized by different marine connections to the world ocean. The first interval extended through most of the Mesozoic, when the Arctic Ocean was a northern gulf of the Pacific Ocean and subsequently developed seaway connections to other oceans. The second interval was during the Late Cretaceous and early Paleogene, when the Arctic Ocean was more or less completely isolated geographically from the world ocean. The third interval lasted from the late Paleogene to the middle Pliocene, during which the Arctic Ocean was connected to the Atlantic but not to the Pacific; and the fourth interval, lasting to the present day, was ushered in at about 3.0–3.5 Ma by the opening of the Bering Strait. The marine paleoclimate of the Arctic Ocean during the Late Cretaceous and early Tertiary was remarkably warm and equable until the onset of gradual cooling that culminated in late Neogene and Pleistocene ice cover.