Eusciurida, Flynn & Jacobs & Kimura & Lindsay, 2019

Flynn, Lawrence J., Jacobs, Louis L., Kimura, Yuri & Lindsay, Everett H., 2019, Rodent Suborders, Fossil Imprint 75 (3 - 4), pp. 292-298 : 294-295

Suborder Eusciurida , new

The Suborder Eusciurida is a crown group, the least inclusive clade of all living “squirrel-like” Sciuridae , Aplodontiidae , and Gliridae (see Appendix). The name is based on squirrels and their relatives, plus the prefix “eu”, Greek for “true, well”. We follow the analysis of Marivaux et al. (2004) to include extinct Family Theridomyidae . Members have sciurognathous jaw structure, primitively lack hypocones on upper molars, and primitively have low crowned cheek teeth. Incisor enamel is derived, as highly organized and advanced as multiserial enamel, but in a different way. The decussating prism pattern, termed “uniserial enamel”, is organized into thin bands, one prism thick, and is seen in all three extant families. (Extinct theridomyids, basal to the extant eusciuridans ( Marivaux et al. 2004), have primitive pauciserial enamel as sampled by Wahlert (1968)).

Eusciurida families display different arrays of jaw muscle anatomy, showing independent evolutionary paths. Aplodontiids have, arguably, the least derived jaw musculature, the masseter being unexpanded beyond the zygomatic arch. Sciurids are the namesake of the sciuromorphous condition in which the masseter seats broadly on the side of the snout in a distinct fossa that constricts the infraorbital foramen ventrally; it transmits only nerve and blood vessels. Glirids show independent modification of the musculature in which a primitively unexpanded masseter migrates onto the snout, in part through an enlarged infraorbital foramen ( Vianey-Liaud 1985, Hautier et al. 2008). The extinct theridomyids are hystricomorphous ( Marivaux et al. 2004).

Traditionally considered members of the old subordinal grouping Sciuromorpha, beavers ( Castoridae ) do have a sciuromorphous zygoma. Carleton (1984) realized that beavers are not closely related to squirrels, and subsequent studies consistently interpret the shared zygomasseteric morphology as a case of convergence. Because the content of Sciuromorpha has been so unstable, and Theridomyidae appear to be associated with it, we prefer a new name and utilize the prefix “eu” for Eusciurida .