From the Late Paleocene.
dental formula 1/1, 0/0, 2/1, 3/3; incisors have 2 layers of enamel on labial face & no enamel on lingual side; wear creates sharp, beveled shape to incisors; paraconids absent on cheek teeth; complex occlusal surfaces; large diastema; mandibular symphysis flexible (many forms can spread incisors slightly) zygomatic arch begins anterior to cheek teeth; zygomatic arch complete, large & dominated by jugal; masseters large, with complex subdivisions, some quite far anterior and at least one almost always originating on rostrum; temporalis relatively small; orbit just dorsal to cheek teeth; glenoid fossa of squamosal elongate, allowing for propalinal motion of jaw; large paroccipital process; post-cranial skeleton unspecialized; clavicle present; herbivorous or omnivorous; gnawing and chewing separate with gnawing requiring distinct forward shift of lower jaw; gnawing makes use of anteriorly shifted jaw muscles (both messeteric & pterygoideus); propalinal movement also for chewing in many later forms; tend to be highly specialized.
Note: Since there are many fine collections of rodent links on the Net, I have chosen only a few sites. Extremely speciose order. Half (2000) of all species of R mammals are rodents, & more than half of these are murids.
Image: Rattus norvegicus from the Mammalian Crania Picture Archive of the Department of Anatomy, Dokkyo University School of Medicine.
Links: Rodentia; Animal Diversity Web: Order Rodentia (very important site); Rodentia; Yahoo! Rodents; Whole Mouse Catalog (many dead links but still a great site); Order Rodentia; Electronic Zoo / NetVet Veterinary Resources Rodent Sites; BIOSIS | Resource Guide | Mammalia - Rodentia.(numerous links); Rodentia.htm">Rodentia skull images etc of Phillipine rodents); Rodentia collection of SZMN, Novosibirsk, Russia always a good resource); APUS.RU | Отряд ·Грызуны - Rodentia. ATW030729