Holochilus primigenus

Reigomys primigenus is an extinct oryzomyine rodent known from deposits in Tarija Department, southeastern Bolivia. It is known from a number of isolated jaws and molars which show that its molars were almost identical to those of the living Lundomys. On the other hand, the animal possesses a number of derived traits of the palate which document a closer relationship to living Holochilus, the genus of South American marsh rats, and for this reason it was placed in the genus Holochilus when it was first described in 1996. The subsequent discoveries of Noronhomys and Carletonomys, which may be more closely related to extant Holochilus than H. primigenus is, have cast its placement in Holochilus into doubt, and it was ultimately made the type species of a separate genus, Reigomys.

Material of Reigomys primigenus was collected in 1924 and 1927 by Elmer Riggs of the Field Museum of Natural History, but the animal was not described until 1996, when Scott Steppan formally named it as a new species of the genus Holochilus and diagnosed it, contrasting it to related species. The material Riggs collected includes nine mandibles, three maxillae, and five isolated molars. The specific name Steppan gave to the animal, primigenus, means "primitive" in Latin and refers to the primitive features of the animal when compared to its relatives Holochilus and Lundomys. In order to determine the relationships of his new species, Steppan carried out a cladistic analysis, in which he also included the oryzomyines Holochilus, Lundomys, Pseudoryzomys, and Cerradomys, as well as the non-oryzomyine Sigmodon. His results supported a close relation between R. primigenus and extant Holochilus, with Lundomys and Pseudoryzomys more distantly related.