One of the striking evolutionary successes among vertebrates centers on the mammalian order Rodentia, which exhibits both a respectably long geologic history, extending back to the late Paleocene (c. 56 Ma), and significant modern diversity, with their 2277 extant species (Wilson and Reeder, 2005; presumably increased by today) occurring naturally on and around almost all terrestrial habitats except the Antarctic continent. Rodents, which form a little over 40% of all extant mammal species, are divided among about 474 genera that exhibit adaptations for terrestrial, fossorial and aquatic habitats, and as well as for aerial, at least gliding, locomotion; most are small and herbivorous to omnivorous. Rodents are characterized by the combination of enlargement of one pair of upper and of lower incisors, dI2/dI2 (Luckett, 1985), for gnawing and reduction or loss of other anterior teeth, with development of a marked diastema between the incisors and cheek teeth and associated modifications of masticatory muscles and their attachments on the skull and mandible (Korth, 1994). Scientific study of these mammals has included paleontologic (Marivaux et al., 2004), neontologic (Vaughan et al., 2013), and molecular approaches (Huchon et al., 2002, 2007; Fabre et al., 2012), the last frequently centered on laboratory animals (rats, mice, hamsters, guinea pigs). Currently, rodent monophyly as well as broad phylogenetic outlines have been established even if consensus related to patterns of rodent diversification has been elusive. Frequently occurring homoplasy within the order tends to interfere with interpretation of relationships (Marivaux et al., 2004).
The rodent fossil record has expanded at a significant pace. For example, Simpson's classification of mammals (1945) listed 349 extant and 275 extinct rodent genera, compared with the more recently compiled 474 genera of extant rodents and 743 extinct genera (McKenna and Bell, 1997). In the interval since the latter listing in 1997 the fossil record has continued to expand, an increase that can be related to screen washing techniques for small vertebrates that are now relatively standard for paleontological field collecting (Hibbard, 1949) as well as to augmented preparation of fossils from hard matrix, such as that from the Bridgerian NALMA Elderberry Canyon, Nevada, locality (e.g. Emry and Korth, 1989).