Pseudoryzomys simplex, also known as the Brazilian false rice rat or false oryzomys,Musser and Carleton, 2005, p. 1164 is a species of rodent in the family Cricetidae from south-central South America. It is found in lowland palm savanna and thorn scrub habitats. It is a medium-sized species, weighing about , with gray–brown fur, long and narrow hindfeet, and a tail that is about as long as the head and body. The IUCN has assessed its conservation status as being of least concern, although almost nothing is known about its diet or reproduction.
The only species in the genus Pseudoryzomys, its closest living relatives are the large rats Holochilus and Lundomys, which are semiaquatic, spending much of their time in the water. The three genera share several characters, including specializations towards a semiaquatic lifestyle, such as the presence of membranes between the digits (interdigital webbing), and a reduction in the complexity of the molar crowns, both of which are at incipient stages in Pseudoryzomys. Together, they form a unique assemblage within the oryzomyine tribe, a very diverse group including over one hundred species, mainly in South America. This tribe is part of the subfamily Sigmodontinae and family Cricetidae, which include many more species, mainly from Eurasia and the Americas. Pseudoryzomys simplex was independently described in 1888 on the basis of subfossil cave specimens from Brazil (as Hesperomys simplex); and in 1921 on the basis of a live specimen from Paraguay (as Oryzomys wavrini). This was confirmed in 1991 that both names pertained to the same species.
Taxonomy
Discovery and recognition Pseudoryzomys simplex has had a complex taxonomic history. It was first described in 1888 by Danish zoologist Herluf Winge,Winge, 1888 who reviewed the materials Peter Wilhem Lund had collected in the caves of Lagoa Santa, Minas Gerais, Brazil. Winge described the species as Hesperomys simplex, and placed it in the same genus (Hesperomys) as the species now called Lundomys molitor and two species now placed in Calomys. Like most other species Winge proposed, H. simplex was mostly ignored in the systematic literature, but from 1952 it was used briefly, in the combination "Oecomys simplex", for an Oecomys species from central Brazil.Voss and Carleton, 1993, p. 31 In his 1960 review of Oecomys, Field Museum mammalogist Philip Hershkovitz denied any affinities between simplex and Oecomys, noting that various features of the H. simplex skull illustrated by Winge instead suggested affinities to the phyllotine or sigmodont groups.Hershkovitz, 1960, pp. 519–520
In 1921, renowned British mammalogist Oldfield Thomas described Oryzomys wavrini as a new species of Oryzomys from Paraguay. In the next decades, it was viewed as an aberrant species of Oryzomys (then used in a much broader sense than now), but it was moved to a separate genus, named Pseudoryzomys, by Hershkovitz in 1959, who noted that although it is similar to Oryzomys palustris in appearance, other features suggest it is more closely related to Phyllotis.Hershkovitz, 1959, pp. 8–9 Thus, he viewed the animal as a member of the phyllotine group of rodents, which includes Calomys and Phyllotis, not of the oryzomyine group, which includes Oryzomys,Hershkovitz, 1962, p. 208 and his opinion was mostly accepted in the next few decades. Scientific knowledge of the rare Pseudoryzomys wavrini—only three specimens were known when Hershkovitz described the genus Pseudoryzomys in 1959Hershkovitz, 1959, p. 9—increased in the following years, and in 1975 the Bolivian population was named as a separate subspecies, Pseudoryzomys wavrini reigi, because Bolivian animals are slightly larger and darker than those from Paraguay.
In 1980, Argentinean zoologist Elio Massoia suggested that Winge's Hesperomys simplex and the living Pseudoryzomys wavrini are in fact the same species. In a 1991 study, American zoologists Voss and Myers confirmed this suggestion after re-examining Winge's material, finding no appreciable differences among specimens of H. simplex and P. wavrini. Since then, the species has been known as Pseudoryzomys simplex (Winge, 1888), because simplex is the oldest specific name for the animal; Oryzomys wavrini Thomas, 1921, and Pseudoryzomys wavrini reigi Pine and Wetzel, 1975, are junior synonyms. Voss and Myers also re-evaluated the relationships of Pseudoryzomys; they considered it closer to oryzomyines than to phyllotines, but declined to formally place it in Oryzomyini in the absence of explicit phylogenetic justification for such a placement.Voss and Myers, 1991, p. 423
Oryzomyine relationships
When Voss and Carleton formally characterized Oryzomyini two years later, they did place Pseudoryzomys in the group, even though it lacks complete mesoloph(id)s. The mesoloph is an accessory crest on the upper molars and the mesolophid is the corresponding structure on the lower molars. Only a few other animals now considered oryzomyines lack complete mesoloph(id)s, but they are absent in various non-oryzomyines, some of which had previously been regarded as close relatives of the oryzomyines that lack them. Oryzomyines with and without complete mesoloph(id)s share various other characters, however, including presence of mammae on the chest, absence of a gall bladder, and some characters of the skull, suggesting that they form one natural, monophyletic group.Voss and Carleton, 1993, p. 31–32 Oryzomyini is now one of several tribes recognized within the subfamily Sigmodontinae, which encompasses hundreds of species found across South America and into southern North America. Sigmodontinae itself is the largest subfamily of the family Cricetidae, other members of which include voles, lemmings, hamsters, and deermice, all mainly from Eurasia and North America.Musser and Carleton, 2005, table 1
Several phylogenetic studies published during the 1990s and 2000s supported a close relationship between Pseudoryzomys and two other oryzomyines with reduced or absent mesoloph(id)s, Lundomys and Holochilus. The extinct genera Noronhomys and Carletonomys, described in 1999 and 2008 respectively, were also recognized as members of the group.Pardiñas, 2008, p. 1275 In 2006, a broad morphological and molecular phylogenetic study of Oryzomyini provided further support for the relationship between Holochilus, Lundomys, and Pseudoryzomys. Within this group, morphological data supported a closer relationship between Holochilus and Lundomys to the exclusion of Pseudoryzomys, but DNA sequence data favored a clustering between Holochilus and Pseudoryzomys to the exclusion of Lundomys; among all oryzomyines, this was the only case where relationships which received strong support from morphological and DNA sequence data conflicted.Weksler, 2006, p. 72 Together, the three genera form part of a large group of oryzomyines ("clade D"), which contains tens of other species. Several of those display some adaptations to life in the water, being partially aquatic, as do Pseudoryzomys and its relatives. Morphological data indicate that the genus Oryzomys is the closest relative of the group that includes Pseudoryzomys, but DNA sequence data from the nuclear IRBP gene did not support this relationship; convergent adaptations towards a semiaquatic lifestyle may explain the morphological support for a relation between Oryzomys and the other three genera.Weksler, 2006, p. 131
Description Pseudoryzomys simplex is a nondescript, medium-sized rat with long, soft fur. The upperparts are gray–brown and the underparts are buff; the color changes gradually over the body. The small ears are covered with short hairs. The tail is as long as or slightly longer than the head and body, and is dark above and light below. Despite the presence of short hairs, the scales on the tail are clearly visible. The hairs on the feet are pale. The hindfeet are long and narrow and have five toes, the first and fifth of which are short.Voss and Myers, 1991, p. 420 Webbing is present between the second, third, and fourth toes, but the membranes are not as large as in Lundomys or Holochilus. The tufts of hair on the toes and several of the pads are reduced, other common characteristics of semiaquatic oryzomyines.Weksler, 2006, pp. 23–25 The head-body length is , tail length , hindfeet length , ear length and body mass .Voss and Myers, 1991, tables 1, 2; Bonvicino et al., 2008, p. 54
The female has four pairs of teats, including one on the chest and three on the belly, and the gall bladder is absent, both important characters of Oryzomyini.Weksler, 2006, pp. 17, 19, 58–59 As is characteristic of Sigmodontinae, Pseudoryzomys has a complex penis, with the baculum (penis bone) displaying large protuberances at the sides. In the cartilaginous part of the baculum, the central digit is smaller than those at the sides.Weksler, 2006, pp. 55–56
Skull The skull, which is short at the front, shows some typical oryzomyine characters. The palate is long, extending past the molars and the maxillary bones. The alisphenoid strut, which in some sigmodontines separates two foramina (openings) in the skull, is absent. The squamosal bone lacks a suspensory process contacting the tegmen tympani, the roof the tympanic cavity. The front part is short.Voss and Myers, 1991, p. 422
The nasal bones end bluntly close to the hindmost extent of the premaxillary bones.Weksler, 2006, pp. 27–28, table 5 The narrow interorbital region, located between the eyes, converges towards the front and is flanked by low beads.Weksler, 2006, pp. 28–30, table 5 The interparietal bone, located in the roof of the skull on the braincase, is nearly as wide as the frontals, but does not reach the squamosals.Weksler, 2006, p. 31
The incisive foramina, which perforate the palate between the incisors and the molars, are long and narrow, extending between the first molars.Voss and Myers, 1991, p. 422; Weksler, 2006, p. 31 The back margins of the zygomatic plates, the flattened front portions of the zygomatic arches (cheekbones), are located before the first molars.Weksler, 2006, p. 32, table 5 Like its close relatives Lundomys and Holochilus, Pseudoryzomys has spinous processes on its zygomatic plates.Weksler, 2006, pp. 31–32 These genera also share relatively simple posterolateral palatal pits, perforations of the palate near the third molar.Weksler, 2006, pp. 35–36, 131 Unlike Holochilus and Lundomys, however, Pseudoryzomys has a flat palate, lacking a ridge on the middle that extends along the length of the palate.Weksler, 2006, p. 35 The parapterygoid fossae, which are located behind the third molars, are excavated beyond the level of the palate, but not as deeply as in Holochilus and Lundomys.Weksler, 2006, p. 36 The mastoid skull bone contains a conspicuous opening, as in most oryzomyines.Weksler, 2006, p. 41
The mandible (lower jaw) is short and deep. The mental foramen, an opening at the front of the mandible, just before the first molar, opens to the side. The capsular process of the lower incisor, a raising of the mandibular bone at the back end of the incisor, is well developed. The two masseteric ridges, to which some of the chewing muscles are attached, are entirely separate, joining only at their front edges, which are located below the first molar.Weksler, 2006, p. 42
Molars