[2] 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.

[5] When Ulyses Pardiñas described another extinct rodent from this group, Carletonomys from the Pleistocene of Argentina, he suggested that H. primigenus should be placed in its own genus because of its mosaic of Holochilus- and Lundomys-like features.

[6] The phylogenetic analysis conducted by Machado et al. (2014) confirmed that living members of the genus Holochilus are more closely related to Noronhomys and Carletonomys than to H. primigenus.

[7] Reigomys primigenus was a large rat, though smaller than both Lundomys and living Holochilus, characterized by an S-shaped ("sigmodont") crown on the third lower molar.

Other sigmodontine rodents found there include Andinomys, Calomys, Kunsia, Nectomys, Oxymycterus, Phyllotis, and another akodontine, probably Akodon, Necromys, or a related genus.