<p class="first" id="ID0EF">The Diatomyidae (Rodentia, Mammalia) are a group of medium to large rodents restricted to southern and eastern Asia. The skull, characterized by hystricomorphous musculature, a 4/4 cheek tooth formula, a sciurognathous jaw, and other features, places the Diatomyidae in a phylogenetic position near, but not within, Hystricognathi. Like living Ctenodactylidae, they are a product of the early radiation of Rodentia that includes, ultimately, the hystricognaths. The oldest fossil Diatomyidae occur in Oligocene terrestrial sediments of the Indian subcontinent. Younger sediments in Pakistan near the Oligocene/Miocene boundary produce a quite different diatomyid that presents unique, derived conditions, and demonstrates early diversification of the family. This diatomyid, described here as <b><span class="genus-species">Marymus dalanae</span>, new genus and species</b>, has cuspate anterior cheek tooth morphology interpreted as specialized, possibly for predation on invertebrates. All early diatomyids are relatively abundant components of their fossil assemblages. Later Diatomyidae from Thailand, China, and Japan suggest a clade characterized by increased body size, simplified dentition, greater molar crown height, and bilophodonty with planar wear indicating herbivory. Most of these later diatomyids are rare and infrequently recorded as fossils. This suggests a shift in habitat preference, with consequent under-representation in fluvial settings, or a new bias against preservation at sites where they do occur. That the habitat preferred by later diatomyids was different from that of early species and was peripheral to fluvial systems would be consistent with the recognition that <span class="genus-species">Laonastes</span> of rocky terrain in central Laos is a living diatomyid.