Lund's Amphibious Rat

''Lundomys molitor'', also known as Lund's amphibious ratMusser and Carleton, 2005, p. 1124 or the greater marsh rat, is a semiaquatic rat species from southeastern South America.

Its distribution is now restricted to Uruguay and nearby Rio Grande do Sul, Brazil, but it previously ranged northward into Minas Gerais, Brazil, and southward into eastern Argentina. The Argentine form may have been distinct from the living form from Brazil and Uruguay. ''L. molitor'' is a large rodent, with the head and body length averaging , characterized by a long tail, large hindfeet, and long and dense fur. It builds nests above the water, supported by reeds, and it is not currently threatened.

Its external morphology is similar to that of ''Holochilus brasiliensis'', and over the course of its complex taxonomic history it has been confused with that species, but other features support its placement in a distinct genus, ''Lundomys''. Within the family Cricetidae and subfamily Sigmodontinae, it is a member of a group of specialized oryzomyine rodents that also includes ''Holochilus'', ''Noronhomys'', '' Carletonomys'', and ''Pseudoryzomys''.

Taxonomy

''Lundomys molitor'' was first described in 1888 by Danish zoologist Herluf Winge, who reviewed the materials Peter Wilhelm Lund had collected in the caves of Lagoa Santa, Minas Gerais, Brazil. Winge used four specimens for his description, including two skull fragments and an isolated maxilla (upper jaw) from the cave chamber Lapa da Escrivania Nr. 5 and a mandible (lower jaw) from Lapa da Serra das Abelhas, but the latter later turned out to be from a different species, probably '' Gyldenstolpia fronto''. Lund named the animal ''Hesperomys molitor'' and placed it in the same genus (''Hesperomys'') as what is now '' Pseudoryzomys simplex'' and two species of ''Calomys''. Subsequently, it was rarely mentioned in the literature on South American rodents; those authors who did mention it placed it in either ''Oryzomys'' or ''Calomys''.Voss and Carleton, 1993, p. 3

In 1926, American zoologist Colin Campbell Sanborn collected some rodents in Uruguay, which he identified as ''Holochilus vulpinus'' (currently ''Holochilus brasiliensis'') in his 1929 report on the collection. When his successor at the Field Museum of Natural History, Philip Hershkovitz, reviewed ''Holochilus'' in 1955, he recognized that the series from Uruguay contained two species, one close to the forms of ''Holochilus'' found across much of South America, and another unique to Uruguay and southern Brazil; he named the latter as a new species, ''Holochilus magnus''. Hershkovitz identified ''Holochilus'' as one of the members of a "sigmodont" group of American rodents, also including '' Sigmodon'', ''Reithrodon'', and '' Neotomys'', on the basis of its flat-crowned molars, which are lophodont (the crown consists of transverse ridges). In 1981, ''H. magnus'' was also recognized in the Late Pleistocene of Buenos Aires Province, Argentina,Voss and Carleton, 1993, p. 10 and in 1982 it was recorded from Rio Grande do Sul in southern Brazil.

In a 1980 article, Argentine zoologist Elio Massoia recognized the resemblance between Winge's ''Hesperomys molitor'' and Hershkovitz's ''Holochilus magnus'', and recommended that the former be reclassified as a species of ''Holochilus'', ''Holochilus molitor''. When American zoologists Voss and Carleton restudied Winge's material in a 1993 paper, they were unable to find any consistent differences between the two and accordingly considered them to pertain to the same species. In addition, they reviewed the differences between this species and other ''Holochilus'' and concluded that these were significant enough to place the former in a distinct genus, which they named ''Lundomys'' after Lund, who had collected the original material. Since then, the species has been known as ''Lundomys molitor''.

In the same paper in which they described ''Lundomys'', Voss and Carleton also, for the first time, diagnosed the tribe Oryzomyini in a phylogenetically valid way.Voss and Carleton, 1993, p. 31 Previously, Oryzomyini had been a somewhat loosely defined group defined among others by a long palate and the presence of a crest known as the mesoloph on the upper molars and mesolophid on the lower molars; this crest is absent or reduced in ''Holochilus'' and ''Lundomys''. Voss and Carleton recognized five synapomorphies for the group, all of which are shared by ''Lundomys''; the placement in Oryzomyini of ''Lundomys'' and of three other genera—''Holochilus'', ''Pseudoryzomys'', and ''Zygodontomys''—which also lack complete mesoloph(id)s has been universally supported since.

Voss and Carleton had found some support for a close relationship between ''Holochilus'', ''Lundomys'', and ''Pseudoryzomys'' within Oryzomyini. In subsequent years, the related species ''Holochilus primigenus'' and '' Noronhomys vespuccii'' were discovered, providing additional evidence for this grouping. The allocation of the former, which is similar to ''Lundomys'' in features of the dentition, to ''Holochilus'' is controversial, and placement as a second species of ''Lundomys'' has been suggested as an alternative. A comprehensive phylogenetic analysis of oryzomyines by Marcelo Weksler, published in 2006, supported a close relationship among ''Lundomys'', ''Holochilus'', and ''Pseudoryzomys''; the other species of the group were not included. Data from the sequence of the IRBP gene supported a closer relationship between ''Holochilus'' and ''Pseudoryzomys'', with ''Lundomys'' more distantly related, but morphological data placed ''Holochilus'' and ''Lundomys'' closer together, as did the combined analysis of both morphological and IRPB data. Subsequently, '' Carletonomys cailoi'' was described as an additional relative of ''Holochilus'' and ''Lundomys''.

Description

''Lundomys molitor'' is among the largest living oryzomyines, rivaled only by some large forms of ''Holochilus'' and ''Nectomys'', but it is substantially smaller than some of the recently extinct Antillean species, such as "'' Ekbletomys hypenemus''" and '' Megalomys desmarestii''. Unlike in ''Holochilus brasiliensis'', which occurs in the same area, the tail is longer than the head and body. It is sparsely haired and dark, and there is no difference in color between the upper and lower side. The coat, which is long, dense, and soft, is yellow–brown at the sides, but becomes darker on the upperparts and lighter on the underparts.Voss and Carleton, 1993, p. 7 The large hindfeet are characterized by conspicuous interdigital webbing, but they lack tufts of hair on the digits and several of the pads are reduced. As in some other semiaquatic oryzomyines, fringes of hair are present along the plantar margins and between some of the digits. The forefeet also lack tufts on the digits and show very long claws, a character unique among oryzomyines. The female has four pairs of teats, and the gall bladder is absent, both important characters of oryzomyines. The head and body length is , averaging , the tail length is 195 to 255 mm (7.68 to 10.04 mm), averaging , and the length of the hindfoot is , averaging .Measurements for head and body length and tail length are from 10 specimens, and those for hindfoot length are from 12 specimens, all from Uruguay.

The front part of the skull is notably broad. As in ''Holochilus'', the zygomatic plate, the flattened front portion of the cheek bone, is expansive and produced into a spinous process at the anterior margin. The jugal bone is small, but less reduced than in ''Holochilus''.Voss and Carleton, 1993, p. 15 The interorbital region of the skull is narrow and flanked by high beads. The incisive foramina, which perforate the palate between the incisors and the upper molars, are long, extending between the molars. The palate itself is also long, extending beyond the posterior margin of the maxillary bones, and it is perforated near the third molars by conspicuous posterolateral palatal pits. As in all oryzomyines, the squamosal bone lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity, but ''Lundomys'' is unusual in that the squamosal and the tegmen tympani usually overlap when viewed from the side. In the mandible, the angular and coronoid processes are less well-developed than in ''Holochilus''. The capsular process of the lower incisor, a slight raising of the mandibular bone at the back end of the incisor, near the coronoid process, is small. The two masseteric ridges, to which some of the chewing muscles are attached, are entirely separate, joining only at their anterior edges, which are located below the first molar.

The molars are slightly more high-crowned (hypsodont) than in most oryzomyines, and many of the accessory crests are reduced, but they are sharply distinct from the highly derived, hypsodont molars of ''Holochilus''.Voss and Carleton, 1993, p. 19 The main cusps are located opposite each other and have rounded edges. The enamel folds do not extend past the midlines of the molars. The mesoloph, an accessory crest on the upper molars that is usually well-developed in oryzomyines, is present but short on the first and second upper molar; it is much more reduced in ''Holochilus'' and ''Pseudoryzomys''. The corresponding structure on the lower molars, the mesolophid, is present on the first and second molars in ''Lundomys'', but absent in both ''Holochilus'' and ''Pseudoryzomys''. Another accessory crest, the anteroloph, is present, though small, on the first upper molar in ''Lundomys'', but entirely absent in both other genera. As in ''Holochilus'' and ''Pseudoryzomys'', the anterior cusp on the first lower molar, the anteroconid, contains a deep pit. Each of the three upper molars has three roots; unlike in both ''Holochilus'' and ''Pseudoryzomys'', the first upper molar lacks an accessory fourth root. The first lower molar has four roots, including two small accessory roots located between larger anterior and posterior roots. The second molar has either two or three roots, with the anterior root split into two smaller roots in some specimens.

The karyotype contains 52 chromosomes with a total of 58 major arms (2n = 52, FN = 58). The non-sex chromosomes (autosomes) are mostly acrocentric, having a long and a short arm, or telocentric, having only one arm, but there are also three large metacentric pairs, which have two major arms, and a small metacentric pair. The Y chromosome is metacentric and the X chromosome is variable, ranging from nearly metacentric to acrocentric in five specimens studied.

Distribution and ecology

''Lundomys molitor'' has been found as a living animal only in Uruguay and nearby Rio Grande do Sul; records of live specimens from eastern Argentina and Lagoa Santa, Minas Gerais, have not been confirmed. It is rarely encountered, and has been collected in only one location in Rio Grande do Sul, but this may be due to insufficient efforts to locate it, rather than genuine rarity. Its distribution is generally limited to areas with mean winter temperatures over , mean annual temperatures over , annual rainfall over , and a long rainy season averaging over 200 days. It is usually found in swamps or near streams.

Pleistocene fossils have been found throughout its current range and beyond it. In Uruguay and Rio Grande do Sul, the Lujanian (Late Pleistocene to Early Holocene) Sopas Formation

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