We bring together lithostratigraphic, biostratigraphic, and magnetostratigraphic data from Nebraska and South Dakota to detail faunal change between 28-30 Ma in medial Oligocene time. This span records the transition from the White River chronofauna to the new assemblages that characterize the younger part of the Arikareean “age.” Although a regional disconformity of approximately a half-million year duration breaks the biostratigraphic sequence, the fossil record is reasonably continuous and mostly confined to the eolian facies. Between 28-30 Ma the White River chronofauna experienced significant enrichment in autochthonous clades especially hesperocyonine canids, oreodonts, camels, hypertragulids, and burrowing castoid and geomyoid rodents. Few allochthonous taxa are encountered so that the chronofauna was enriched without marked immigration or extinction. At approximately 28 Ma most of the White River genera leave the record, thus terminating the chronofauna. The fauna that emerges contains representatives of autochthonous lineages, some of which appeared during the enrichment phase of the White River chronofauna. In addition there are allochthonous genera that represent taxa new to midcontinental North America. The better resolved and calibrated fossil record allows re-examination of the definition and characterization of the beginning of the Arikareean mammal “age.” We propose that the initiation of the Arikareean Mammal “age” is signaled by the first appearance of taxa that enrich the White River chronofauna in latest Chron C11r and earliest Chron C11n (about 30 Ma).