A basal phiomorph (Rodentia, Hystricognathi) from the late Eocene of the Fayum Depression, Egypt

Abstract

The fossil record of phiomorph hystricognathous rodents from the Afro-Arabian Paleogene is important for understanding the origins and dispersal routes of the early crown hystricognaths. Here, we describe a “new” basal phiomorph genus and species, Acritophiomys bowni, based on complete upper and lower dentitions, mandibular fragments, and partial crania from the terminal late Eocene (~34 Ma) Locality 41 (L-41) in the Fayum Depression of northern Egypt. Acritophiomys bowni is the oldest and largest representative of the family “Phiomyidae”, being more or less the same size as contemporaneous gaudeamurids, and is one of the most abundant hystricognaths at L-41. The genus exhibits a mosaic of primitive and derived features, the former shared with primitive hystricognaths, such as Waslamys and Protophiomys from the earliest late Eocene, and the latter shared with Metaphiomys from early Oligocene (~31–29 Ma) sites in the upper sequence of the Jebel Qatrani Formation. Phylogenetic analysis of craniodental features, scored across a number of different hystricognathous groups, consistently places Acritophiomys bowni and members of the genus Phiomys as basal members of the phiomorph stem lineage, implying that the commonly used family “Phiomyidae” is a paraphyletic assemblage. Among other things, this material shows that basal members of the phiomorph clade consistently replaced dP4/4 with permanent P4/4, and suggests an African origin of stem and crown Phiomorpha.