Holochilus Primigenus

''Reigomys primigenus'' is an extinct oryzomyine rodent known from Pleistocene 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''.

Taxonomy

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.Steppan, 1996, p. 523 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''.Steppan, 1996, p. 524 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.

In 1999, another rodent from the same group was described: '' Noronhomys'', a recently extinct species from the Brazilian island of Fernando de Noronha. Michael Carleton and Storrs Olson, who described the animal, argued that ''Holochilus primigenus'' was probably outside the clade of ''Noronhomys'' and extant ''Holochilus'' and that ''H. primigenus'' should probably be excluded from the genus. 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.

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''. The authors moved ''H. primigenus'' to its own genus, which they named ''Reigomys''.

Description

''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. Features of the molars are nearly indistinguishable from those of ''Lundomys'' and include cusps located opposite each other, enamel valleys that barely reach the midline of the molars, and short mesolophs and mesolophids (accessory crests). Other notable features include a robust mandible (lower jaw) with a steeply rising coronoid process, short incisive foramina (perforations of the front part of the palate) that barely extend between the first molars, and a short bony palate that hardly extends beyonds the third molars, all of which are shared with extant species of ''Holochilus'' to the exclusion of ''Lundomys''. Also unlike in ''Lundomys'', there is an additional small root present at the labial (outer) side of the first upper molar. In eight specimens that could be measured, the length of the lower toothrow is 6.79 to 7.58 mm, averaging 7.28 mm; the first lower molar is 2.62 to 3.08 mm long, averaging 2.89 mm, and 1.75 to 1.93 mm broad, averaging 1.85 mm in seven measured teeth; the only preserved complete upper toothrow is 6.64 mm long; and the first upper molar is 2.63 to 2.70 mm long and 2.03 mm broad in two specimens.

Distribution and ecology

Remains of ''Reigomys primigenus'' come from several localities in river sediments of the Tarija Formation in Tarija Department, which have been paleomagnetically dated to about 0.7 to 1.0 million years ago ( chrons Clr.ln to early Cln, Ensenadan South American land mammal age). Other sigmodontine rodents found there include '' Andinomys'', ''Calomys'', '' Kunsia'', '' Nectomys'', '' Oxymycterus'', ''Phyllotis'', and another akodontine, probably ''Akodon'', '' Necromys'', or a related genus. The deposits were deposited by a river and the paleoenvironment was probably a floodplain or channel. ''Reigomys primigenus'' is not known from any other localities and is considered to be extinct.Steppan, 1996, p. 527

References

Bibliography

* Carleton, M.D. and Olson, S.L. 1999
Amerigo Vespucci and the rat of Fernando de Noronha: a new genus and species of Rodentia (Muridae, Sigmodontinae) from a volcanic island off Brazil's continental shelf
''American Museum Novitates'' 3256:1–59.
* Pardiñas, U.F.J. 2008
A new genus of oryzomyine rodent (Cricetidae: Sigmodontinae) from the Pleistocene of Argentina
(subscription required). ''Journal of Mammalogy'' 89(5):1270–1278.
* Steppan, S.J. 1996
A new species of ''Holochilus'' (Rodentia: Sigmodontinae) from the Middle Pleistocene of Bolivia and its phylogenetic significance
(subscription required). ''Journal of Vertebrate Paleontology'' 16(3):522–530.
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Oryzomyini
Monotypic prehistoric rodent genera
Pleistocene rodents
Pleistocene mammals of South America
Ensenadan
Pleistocene Bolivia
Fossils of Bolivia
Fossil taxa described in 1996