Abstract Rhombomylus is a gliroid mammal endemic to several early Eocene localities of central and eastern Asia. It is best represented by a collection of numerous jaws and teeth, dozens of juvenile and adult skulls, and associated postcranial specimens from the early Eocene Yuhuangding Formation at the Dajian village, Junxian County, Hubei Province, China. These specimens are the best preserved material of early nonrodent, nonlagomorph Glires known to date, and they provide a unique opportunity for us to understand anatomy, intraspecies variation, postnatal ontogeny, phylogeny, and evolution of basal gliroid mammals. This study opens with an introduction to the current knowledge of the phylogeny of gliroid mammals, a brief review of the research history of Rhombomylus, and a description of the locality where the specimens were collected. The core of the study consists of a taxonomic revision of the genus, a detailed description of the osteological morphology, extensive analyses on morphological character...