This study describes and summarizes the Temnocyoninae (Mammalia, Carnivora), a subfamily of amphicyonid carnivores of considerable diversity and singular ecomorphology within Cenozoic faunas of North America. In temnocyonines, we see the first carnivorans to occupy an ecological niche as large cursorial predators. The subfamily is confined to the Arikareean NALMA, ranging in age from the latest early Oligocene to the early Miocene. Distributed from the Pacific Northwest to the Great Plains and Florida, there are four genera (Temnocyon, Mammacyon, Delotrochanter n. gen., Rudiocyon n. gen.) and 12 species, of which eight are newly described (Temnocyon subferox, T. fingeruti, T. macrogenys; Rudiocyon amplidens; Mammacyon ferocior; Delotrochanter petersoni, D. oryktes, D. major). Among the specimens examined are eight skulls, three with intact basicranial morphology that establish the presence of a plesiomorphic arctoid auditory region in the subfamily. Temnocyonine dentitions and postcranial skeletons reveal a blend of morphological characteristics not previously nor subsequently seen among the Carnivora. From a stem species, Temnocyon altigenis, there evolve both large hypercarnivorous (Temnocyon) and durophagous forms (Mammacyon, Delotrochanter); these genera share a derived dentition that defines the Temnocyoninae. Delotrochanter oryktes n. sp., an early Miocene species, was found in a den, suggesting a possible burrowing capability and sheltering of offspring.

The John Day basin of Oregon and the central Great Plains (western Nebraska, southeast Wyoming) are the source of most temnocyonine fossils; a few have been found in southern California and Florida, indicative of a continent-wide distribution. Temnocyonines have often been confused with canids, however their basicranial anatomy places them securely within the Amphicyonidae. First discovered in the 1870s, only ∼30 individuals comprise the entire record of the subfamily. Many were found in proximity to radioisotopically calibrated tuffs and ignimbrites and/or were closely associated with mammals of established biochronologic age. Thus, most species can be placed in a temporal context. With rare exception, the fossils represent isolated occurrences, hence estimates of variation within a population are lacking.

Cursorial postcranial features characterize several lineages (Mammacyon, Delotrochanter) and probably were present in other temnocyonines known only from dental remains. Late Oligocene Mammacyon ferocior and early Miocene Delotrochanter oryktes evolved uniquely configured crushing cheek teeth and cursorial limbs, combining distinctive dental and skeletal traits in a manner not seen in any living carnivore. These species are interpreted as large durophagous predators with craniodental characteristics that parallel living hyaenids (Crocuta crocuta) and postcranial adaptations approaching those of cursorial canids such as the wolf (Canis lupus). Expansion of semiarid grasslands and savanna during the late Oligocene and early Miocene in the central Great Plains seems to have favored the evolution of these wide-ranging