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 characters, analyses on phylogeny, discussions on divergence time of the Glires, and analyses on functional morphology of mastication and locomotion.

In comparing all known specimens assigned to the genus, we recognize only a single species, Rhombomylus turpanensis, for the genus and consider “R. laianensis” a junior synonym of R. turpanensis. This monotypic genus is rediagnosed. As a typical gliroid mammal, Rhombomylus possesses the following features: a single pair of enlarged, ever-growing incisors in both upper and lower jaws, the enamel band covering primarily the anterior and lateral surface of the incisor, a large diastema between the incisor and premolars, and a dental formula of dI2, C0, P3-P4, M1-M3/di2, c0, p3-p4, m1-m3. The single pair of upper incisors distinguishes Rhombomylus from duplicendentate Glires. Rhombomylus is more similar to Matutinia than to other simplicidentate Glires in having the following features: a broad hypocone shelf on the upper cheek tooth, a specialized zygomatic arch formed by the maxilla and jugal, an expanded mastoid of the petrosal that is exposed on the skull roof between the squamosal and parietal, an extensively interdigital suture between the premaxilla and frontal, a contribution of the ectotympanic to the medial wall of the glenoid fossa, and a distinct external meatus contacting the squamosal and blocking the postglenoid fossa posteriorly. Rhombomylus differs from Matutinia in being generally larger, in having higher tooth crowns, a stronger protocone and paracone, a complete vertical groove on the lingual surface of cheek teeth, and stronger crests on cheek teeth, and in lacking the carotid foramen in the auditory region. In general, Rhombomylus is more derived in morphology than is Matutinia.

Although the general skull morphology of Rhombomylus is rodentlike, the dental morphology and detailed anatomy of the skull display unique specialization of the genus. Upper tooth crowns are higher lingually than labially. Lophs are well developed, whereas cusps are not distinct. The incisor enamel is pauciserial and has two layers, with the outer layer being poorly differentiated. The enamel is thicker basally than apically in the same incisor and thicker in incisors of adult individuals than in juveniles. The basic enamel microstructure does not vary much at different sections of the same incisor, nor does it differ significantly in incisors f