Brazilian False Rice Rat
Encyclopedia
Pseudoryzomys simplex, also known as the Brazilian False Rice Rat or False Oryzomys, is a species
of rodent
in the family Cricetidae
from south-central South America. It is found in lowland palm savanna and thorn scrub
habitat
s. It is a medium-sized species, weighing about 50 grams (1.8 oz), 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 "Least Concern
", although almost nothing is known of 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 1887 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), and it was confirmed only in 1991 that both names in fact pertained to the same species.
history. It was first described in 1887 by Danish zoologist Herluf Winge
, 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. 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.
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
. 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, 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 1959—increased in the following years, and in 1975 the Bolivia
n 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, 1887), because simplex is the oldest specific name for the animal; Oryzomys wavrini Thomas, 1921, and Pseudoryzomys wavrini reigi Pine and Wetzel, 1975, are junior synonym
s. 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.
group. 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 vole
s, lemming
s, hamster
s, and deermice
, all mainly from Eurasia and North America.
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. 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. 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.
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. The head-body length is 94 to 140 mm (3.7 to 5.5 in), tail length 102 to 140 mm (4 to 5.5 in), hindfeet length 27 to 33 mm (1.1 to 1.3 in), ear length 13 to 19 mm (0.511811023622047 to 0.748031496062992 in) and body mass 45 to 55 g (1.6 to 1.9 oz).
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. 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.
The karyotype
includes 56 chromosome
s with a total of 54 major arms (2n = 56, FN = 54) in specimens from both Bolivia and Goiás, central Brazil; a poorly prepared Paraguayan specimen seems to have a similar karyotype. In the Bolivian specimen, all chromosomes appear telocentric, with the centromere
located at the end of the chromosome.
is long, extending past the molars and the maxilla
ry 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.
The nasal
bones end bluntly close to the hindmost extent of the premaxillary bones. The narrow interorbital region
, located between the eyes, converges towards the front and is flanked by low beads. 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.
The incisive foramina
, which perforate the palate between the incisor
s and the molars
, are long and narrow, extending between the first molars. The back margins of the zygomatic plate
s, the flattened front portions of the zygomatic arch
es (cheekbones), are located before the first molars. Like its close relatives Lundomys and Holochilus, Pseudoryzomys has spinous processes on its zygomatic plates. These genera also share relatively simple posterolateral palatal pits, perforations of the palate near the third molar. Unlike Holochilus and Lundomys, however, Pseudoryzomys has a flat palate, lacking a ridge on the middle that extends along the length of the palate. 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. The mastoid skull bone contains a conspicuous opening, as in most oryzomyines.
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.
As in all oryzomyines except Holochilus and its close relatives, the molars are brachyodont, low-crowned, and bunodont, with the cusps extending higher than the central parts of the molars. They are characterized by strong cusps and absence or reduction of accessory crests. The cusps of the upper molars are opposite, but in the lower molars the labial (outer) cusps are slightly further to the front than the lingual (inner) ones. On the upper first molar, one accessory ridge, the anteroloph, is lacking, but another, the mesoloph, is present. Unlike in most other oryzomyines, however, which have mesolophs reaching the labial margin of the molar, the mesolophs of Pseudoryzomys are short and protrude only slightly from the middle of the molar. The corresponding structure in the lower molars, the mesolophid, is completely absent. The hindmost valley between cusps on the lower first molar, the posteroflexid, is severely reduced, foreshadowing its loss in Lundomys and Holochilus. A number of molar traits support Pseudoryzomyss relationship with Holochilus and Lundomys, forming steps in the transition from the complex, low-crowned generalized oryzomyine molar pattern to the simpler, high-crowned pattern of Holochilus.
As in all oryzomyines, the upper molars all have one root on the inner (lingual) side and two on the outer (labial) side; in addition, the first upper molar in Pseudoryzomys and some other species has another labial root. The first lower molar has large roots at the front and back of the tooth and two smaller ones in between, at the labial and lingual side. The second and third lowers molars have two roots at the front, one labial and one lingual, and another at the back.
(chest) and lumbar vertebrae
, 13 of which bear rib
s, as is characteristic of oryzomyines. The first ribs contact both the seventh
cervical (neck) vertebra and the first thoracic vertebra, an important character of the Sigmodontinae
. Unlike in most sigmodontines, including Holochilus and Lundomys, the fourth lumbar vertebra lacks the processes known as anapophyses. There are three or four sacral and about 29 caudal (tail) vertebrae. Between the second and third caudal vertebrae, separate bones called hemal arches are present. These display a spinous process at the back, as in both Holochilus and Lundomys. On the humerus
, the upper arm bone, the entepicondylar foramen
is absent, as in all members of the Sigmodontinae
; in some other cricetids, it perforates the far (distal) end of the humerus.
Pseudoryzomys simplex is known from northeastern Argentina, probably south to about 30°S, northward through western Paraguay to eastern Bolivia and from there eastward through Brazil in the states of Mato Grosso
, Goiás
, Tocantins
, Minas Gerais
, São Paulo
, Bahia
, and far in the northeast, Alagoas
and Pernambuco
. Paraguayan animals are somewhat smaller than those from Bolivia and Brazil and those from Bolivia have darker fur than Paraguayan specimens, but these differences are not considered significant enough to recognize subspecies
. Certain bat
s show a similar pattern of variation: they are smaller and paler in the Chaco
region, which includes much of Paraguay. Two specimens from Paraguay, collected 600 kilometres (372.8 mi) apart, differed by 1.4% in the sequence of the cytochrome b
gene, but nothing is known about genetic variation in other parts of the range. The species has long been rare in collections; in 1991, Voss and Myers could use less than 50 specimens for their study of the species, including Lund's fragmentary material from Lagoa Santa.
A fragmentary lower jaw of "Pseudoryzomys aff. P. simplex" (i.e., an unnamed species close to Pseudoryzomys simplex) is known from a cave deposit in Cueva Tixi, Buenos Aires Province
, Argentina, outside the current distribution of the species. It is dated from the first millennium CE. The jaw's morphology
agrees with that of P. simplex, but the toothrow is relatively long (5.78 mm; 4.61 to 5.60 mm in three specimens of P. simplex) and the first molar is relatively narrow (1.28 mm; 1.30 to 1.40 mm in five P. simplex).
P. simplex inhabits open, usually humid tropical and subtropical lowlands
. In Argentina, it is mainly a species of the eastern Chaco and in Brazil it is found in the Cerrado
and Caatinga
. Most specimens for which habitat data are known were caught on the ground in humid grassland, some in seasonally flooded areas; an Argentinean specimen was captured in dense swamp vegetation. It is terrestrial and semiaquatic, living on the ground but also spending time in the water.
Nothing is known about behavior or diet. P. simplex has frequently been found in pellets
of the Barn Owl
(Tyto alba) and also in those of the Great Horned Owl
(Bubo virginianus). It is a preferred prey of the Maned Wolf
(Chrysocyon brachyurus).
" by the IUCN. It is a widely distributed species without substantial threats to its continued existence, but degradation of its habitat may endanger some populations. It was assessed as "potentially vulnerable" in Argentina.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
in the family Cricetidae
Cricetidae
The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...
from south-central South America. It is found in lowland palm savanna and thorn scrub
Tropical and subtropical grasslands, savannas, and shrublands
Tropical and subtropical grasslands, savannas, and shrublands are a grassland terrestrial biome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes. Grasslands are dominated by grass and other herbaceous plants. Savannas are grasslands with scattered trees...
habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
s. It is a medium-sized species, weighing about 50 grams (1.8 oz), 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 "Least Concern
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
", although almost nothing is known of its diet or reproduction.
The only species
Monotypic
In biology, a monotypic taxon is a taxonomic group with only one biological type. The term's usage differs slightly between botany and zoology. The term monotypic has a separate use in conservation biology, monotypic habitat, regarding species habitat conversion eliminating biodiversity and...
in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Pseudoryzomys, its closest living relatives are the large rats Holochilus
Holochilus
Holochilus is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae, sometimes called marsh rats. It contains three living species, Holochilus brasiliensis, Holochilus chacarius, and Holochilus sciureus, which are widely distributed in South America east of the Andes, and a...
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
Interdigital webbing
Interdigital webbing is the presence of membranes of skin between the digits. Normally in mammals, webbing is present in the embryo but resorbed later in development, but in various mammal species it occasionally persists in adulthood...
), and a reduction in the complexity of the molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
crowns, both of which are at incipient stages in Pseudoryzomys. Together, they form a unique assemblage within the oryzomyine
Oryzomyini
Oryzomyini is a tribe of rodents in the subfamily Sigmodontinae of family Cricetidae. It includes about 120 species in about thirty genera, distributed from the eastern United States to the southernmost parts of South America, including many offshore islands...
tribe, a very diverse group including over one hundred species, mainly in South America. This tribe is part of the subfamily Sigmodontinae
Sigmodontinae
The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...
and family Cricetidae
Cricetidae
The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...
, which include many more species, mainly from Eurasia and the Americas. Pseudoryzomys simplex was independently described in 1887 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), and it was confirmed only in 1991 that both names in fact pertained to the same species.
Discovery and recognition
Pseudoryzomys simplex has had a complex taxonomicTaxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
history. It was first described in 1887 by Danish zoologist Herluf Winge
Herluf Winge
Adolf Herluf Winge was a Danish zoologist.-Biography:As a young student, Winge was interested in small mammals, particularly moles, shrews and insectivora. He studied mammalian dentition and produced a comparison of cusp similarities...
, who reviewed the materials Peter Wilhem Lund had collected in the caves of Lagoa Santa
Lagoa Santa
For Lagoa Santa, a municipality in Goiás see Lagoa Santa, GoiásLagoa Santa is a municipality and region in the state of Minas Gerais, Brazil...
, Minas Gerais
Minas Gerais
Minas Gerais is one of the 26 states of Brazil, of which it is the second most populous, the third richest, and the fourth largest in area. Minas Gerais is the Brazilian state with the largest number of Presidents of Brazil, the current one, Dilma Rousseff, being one of them. The capital is the...
, Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. Winge described the species as Hesperomys simplex, and placed it in the same genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
(Hesperomys
Hesperomys
Hesperomys is an obsolete genus of American rodents. It was initially broadly defined to include most of the cricetid rodents of the Americas, except the Arvicolinae, but later became restricted to the members of the modern genus Calomys; the latter name has been in use for this genus instead of...
) as the species now called Lundomys molitor and two species now placed in Calomys
Vesper mouse
Calomys is a genus of rodents, also called vesper mice. The genus is widely distributed in South America.Some species are notable as the vectors of Argentinian hemorrhagic fever and Bolivian hemorrhagic fever.-References:...
. 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
Oecomys
Oecomys is a genus of rodent within the tribe Oryzomyini of family Cricetidae. It contains about 17 species, which live in trees and are distributed across forested parts of South America, extending into Panama and Trinidad.-Literature cited:...
species from central Brazil. In his 1960 review of Oecomys, Field Museum
Field Museum of Natural History
The Field Museum of Natural History is located in Chicago, Illinois, USA. It sits on Lake Shore Drive next to Lake Michigan, part of a scenic complex known as the Museum Campus Chicago...
mammalogist Philip Hershkovitz
Philip Hershkovitz
Philip Hershkovitz was an American mammalogist. Born in Pittsburgh, he attended the Universities of Pittsburgh and Michigan and lived in South America collecting mammals. In 1947, he was appointed a curator at the Field Museum of Natural History in Chicago and he continued to work there until his...
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.
In 1921, renowned British mammalogist Oldfield Thomas
Oldfield Thomas
Oldfield Thomas FRS was a British zoologist.Thomas worked at the Natural History Museum on mammals, describing about 2,000 new species and sub-species for the first time. He was appointed to the Museum Secretary's office in 1876, transferring to the Zoological Department in 1878...
described Oryzomys wavrini as a new species of Oryzomys
Oryzomys
Oryzomys is a genus of semiaquatic rodents in the tribe Oryzomyini living in southern North America and far northern South America. It includes eight species, two of which—the marsh rice rat of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
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
Marsh Rice Rat
The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. It is found mostly in the eastern and southern United States, from New Jersey and Kansas south to Florida and northeasternmost Tamaulipas, Mexico; its range previously extended further west and north, where it may...
in appearance, other features suggest it is more closely related to Phyllotis
Phyllotis
Phyllotis is a genus of rodent in the family Cricetidae.It contains the following species:* Phyllotis alisosiensis* Friendly Leaf-eared Mouse * Andean Leaf-eared Mouse...
. Thus, he viewed the animal as a member of the phyllotine group of rodents, which includes Calomys and Phyllotis, not of the oryzomyine
Oryzomyini
Oryzomyini is a tribe of rodents in the subfamily Sigmodontinae of family Cricetidae. It includes about 120 species in about thirty genera, distributed from the eastern United States to the southernmost parts of South America, including many offshore islands...
group, which includes Oryzomys, 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 1959—increased in the following years, and in 1975 the Bolivia
Bolivia
Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...
n population was named as a separate subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
, 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, 1887), because simplex is the oldest specific name for the animal; Oryzomys wavrini Thomas, 1921, and Pseudoryzomys wavrini reigi Pine and Wetzel, 1975, are junior synonym
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
s. 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
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
justification for such a placement.
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, monophyleticMonophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
group. Oryzomyini is now one of several tribes recognized within the subfamily Sigmodontinae
Sigmodontinae
The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...
, which encompasses hundreds of species found across South America and into southern North America. Sigmodontinae itself is the largest subfamily of the family Cricetidae
Cricetidae
The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice...
, other members of which include vole
Vole
A vole is a small rodent resembling a mouse but with a stouter body, a shorter hairy tail, a slightly rounder head, smaller ears and eyes, and differently formed molars . There are approximately 155 species of voles. They are sometimes known as meadow mice or field mice in North America...
s, lemming
Lemming
Lemmings are small rodents, usually found in or near the Arctic, in tundra biomes. They are subniveal animals, and together with voles and muskrats, they make up the subfamily Arvicolinae , which forms part of the largest mammal radiation by far, the superfamily Muroidea, which also includes rats,...
s, hamster
Hamster
Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera....
s, and deermice
Peromyscus
The genus Peromyscus contains the animal species commonly referred to as deer mice. This is a genus of New World mouse only distantly related to the common house mouse and laboratory mouse, Mus musculus...
, all mainly from Eurasia and North America.
Several phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
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
Holochilus
Holochilus is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae, sometimes called marsh rats. It contains three living species, Holochilus brasiliensis, Holochilus chacarius, and Holochilus sciureus, which are widely distributed in South America east of the Andes, and a...
. The extinct genera Noronhomys and Carletonomys
Carletonomys
Carletonomys cailoi is an extinct rodent from the Pleistocene of Buenos Aires Province, Argentina. Although known only from a single maxilla with the first molar, its features are so distinctive that it is placed in its own genus, Carletonomys...
, described in 1999 and 2008 respectively, were also recognized as members of the group. In 2006, a broad morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
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. 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
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
, as do Pseudoryzomys and its relatives. Morphological data indicate that the genus Oryzomys
Oryzomys
Oryzomys is a genus of semiaquatic rodents in the tribe Oryzomyini living in southern North America and far northern South America. It includes eight species, two of which—the marsh rice rat of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
is the closest relative of the group that includes Pseudoryzomys, but DNA sequence data from the nuclear IRBP
RBP3
Retinol-binding protein 3, interstitial , also known as IRBP is a protein that in humans is encoded by the RBP3 gene. RBP3 orthologs have been identified in most eutherians except tenrecs and armadillos.- Function :...
gene did not support this relationship; convergent
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
adaptations towards a semiaquatic lifestyle may explain the morphological support for a relation between Oryzomys and the other three genera.
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. WebbingInterdigital webbing
Interdigital webbing is the presence of membranes of skin between the digits. Normally in mammals, webbing is present in the embryo but resorbed later in development, but in various mammal species it occasionally persists in adulthood...
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. The head-body length is 94 to 140 mm (3.7 to 5.5 in), tail length 102 to 140 mm (4 to 5.5 in), hindfeet length 27 to 33 mm (1.1 to 1.3 in), ear length 13 to 19 mm (0.511811023622047 to 0.748031496062992 in) and body mass 45 to 55 g (1.6 to 1.9 oz).
Baculum
The baculum is a bone found in the penis of most mammals. It is absent in humans, but present in other primates, such as the gorilla and chimpanzee.The bone aids in sexual intercourse.-Purpose:...
(penis bone) displaying large protuberances at the sides. In the cartilaginous
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
part of the baculum, the central digit is smaller than those at the sides.
The karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
includes 56 chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s with a total of 54 major arms (2n = 56, FN = 54) in specimens from both Bolivia and Goiás, central Brazil; a poorly prepared Paraguayan specimen seems to have a similar karyotype. In the Bolivian specimen, all chromosomes appear telocentric, with the centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
located at the end of the chromosome.
Skull
The skull, which is short at the front, shows some typical oryzomyine characters. The palatePalate
The palate is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but, in most other tetrapods, the oral and nasal cavities are not truly separate. The palate is divided into two parts, the anterior...
is long, extending past the molars and the maxilla
Maxilla
The maxilla is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible , which is also a fusion of two halves at the mental symphysis. Sometimes The maxilla (plural: maxillae) is a fusion of two bones along the palatal fissure that form the upper...
ry bones. The alisphenoid strut
Alisphenoid strut
In some rodents, the alisphenoid strut is an extension of the alisphenoid bone that separates two foramina in the skull, the masticatory–buccinator foramen and the foramen ovale accessorius...
, which in some sigmodontines separates two foramina (openings) in the skull, is absent. The squamosal
Squamosal
The squamosal is a bone of the head of higher vertebrates. It is the principal component of the cheek region in the skull, lying below the temporal series and otic notch and bounded anteriorly by postorbital. Posteriorly, the squamosal articulates with the posterior elements of the palatal complex,...
bone lacks a suspensory process
Process (anatomy)
In anatomy, a process is a projection or outgrowth of tissue from a larger body. The vertebra has several kinds of processes,such as: transverse process, prezygapophysis, postzygapophysis.-Examples:Examples of processes include:...
contacting the tegmen tympani, the roof the tympanic cavity
Tympanic cavity
The tympanic cavity is a small cavity surrounding the bones of the middle ear.It is formed from the tubotympanic recess, an expansion of the first pharyngeal pouch....
. The front part is short.
The nasal
Nasal bone
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face, and form, by their junction, "the bridge" of the nose.Each has two surfaces and four borders....
bones end bluntly close to the hindmost extent of the premaxillary bones. The narrow interorbital region
Interorbital region
The interorbital region of the skull is located between the eyes, anterior to the braincase. The form of the interorbital region may exhibit significant variation between taxonomic groups....
, located between the eyes, converges towards the front and is flanked by low beads. The interparietal bone, located in the roof of the skull on the braincase, is nearly as wide as the frontals
Frontal bone
The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....
, but does not reach the squamosals.
The incisive foramina
Incisive foramen
The fossa incisiva is an opening in the bone of the oral hard palate where blood vessels and nerves may pass. There are four of these openings in the incisive fossa.-Formation:...
, which perforate the palate between the incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s and the molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
, are long and narrow, extending between the first molars. The back margins of the zygomatic plate
Zygomatic plate
In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch . At the back, it connects to the front root of the zygomatic arch, and at the top it is connected to the rest of the skull via the antorbital bridge. It is part of the maxillary...
s, the flattened front portions of the zygomatic arch
Zygomatic arch
The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...
es (cheekbones), are located before the first molars. Like its close relatives Lundomys and Holochilus, Pseudoryzomys has spinous processes on its zygomatic plates. These genera also share relatively simple posterolateral palatal pits, perforations of the palate near the third molar. Unlike Holochilus and Lundomys, however, Pseudoryzomys has a flat palate, lacking a ridge on the middle that extends along the length of the palate. 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. The mastoid skull bone contains a conspicuous opening, as in most oryzomyines.
The mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
(lower jaw) is short and deep. The mental foramen
Mental foramen
The mental foramen is one of two holes located on the anterior surface of the mandible. It permits passage of the mental nerve and vessels. The mental foramen descends slightly in edentulous individuals.- Variations :...
, an opening at the front of the mandible, just before the first molar, opens to the side. The capsular process
Capsular process
In rodents, the capsular process or projection is a bony capsule that contains the root of the lower incisor. It is visible on the labial side of the mandible as a raising in the bone...
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.
Molars
As in all oryzomyines, the upper molars all have one root on the inner (lingual) side and two on the outer (labial) side; in addition, the first upper molar in Pseudoryzomys and some other species has another labial root. The first lower molar has large roots at the front and back of the tooth and two smaller ones in between, at the labial and lingual side. The second and third lowers molars have two roots at the front, one labial and one lingual, and another at the back.
Postcranial skeleton
Pseudoryzomys has 19 or 20 thoracicThoracic vertebrae
In human anatomy, twelve thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. They are intermediate in size between those of the cervical and lumbar regions; they increase in size as one proceeds down the spine, the upper...
(chest) and lumbar vertebrae
Lumbar vertebrae
The lumbar vertebrae are the largest segments of the movable part of the vertebral column, and are characterized by the absence of the foramen transversarium within the transverse process, and by the absence of facets on the sides of the body...
, 13 of which bear rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s, as is characteristic of oryzomyines. The first ribs contact both the seventh
Vertebra prominens
Vertebra prominens is the proper name for the seventh cervical vertebra. The most distinctive characteristic of this vertebra is the existence of a long and prominent spinous process which is palpable from the skin surface, hence the name...
cervical (neck) vertebra and the first thoracic vertebra, an important character of the Sigmodontinae
Sigmodontinae
The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...
. Unlike in most sigmodontines, including Holochilus and Lundomys, the fourth lumbar vertebra lacks the processes known as anapophyses. There are three or four sacral and about 29 caudal (tail) vertebrae. Between the second and third caudal vertebrae, separate bones called hemal arches are present. These display a spinous process at the back, as in both Holochilus and Lundomys. On the humerus
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
, the upper arm bone, the entepicondylar foramen
Entepicondylar foramen
The entepicondylar foramen is an opening in the distal end of the humerus present in some mammals. It is often present in primitive placentals, such as the enigmatic Madagascan Plesiorycteropus...
is absent, as in all members of the Sigmodontinae
Sigmodontinae
The subfamily Sigmodontinae is one of the most diverse groups of mammals. It includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count...
; in some other cricetids, it perforates the far (distal) end of the humerus.
Distribution, ecology, and variation
Mato Grosso
Mato Grosso is one of the states of Brazil, the third largest in area, located in the western part of the country.Neighboring states are Rondônia, Amazonas, Pará, Tocantins, Goiás and Mato Grosso do Sul. It also borders Bolivia to the southwest...
, Goiás
Goiás
Goiás is a state of Brazil, located in the central part of the country. The name Goiás comes from the name of an indigenous community...
, Tocantins
Tocantins (state)
Tocantins is one of the states of Brazil. . The state was formed in 1988 out of the northern part of Goiás, and construction began on the capital, Palmas, in 1989, in contrast to most of the other cities in the state which date back to the Portuguese colonial period...
, Minas Gerais
Minas Gerais
Minas Gerais is one of the 26 states of Brazil, of which it is the second most populous, the third richest, and the fourth largest in area. Minas Gerais is the Brazilian state with the largest number of Presidents of Brazil, the current one, Dilma Rousseff, being one of them. The capital is the...
, São Paulo
São Paulo (state)
São Paulo is a state in Brazil. It is the major industrial and economic powerhouse of the Brazilian economy. Named after Saint Paul, São Paulo has the largest population, industrial complex, and economic production in the country. It is the richest state in Brazil...
, Bahia
Bahia
Bahia is one of the 26 states of Brazil, and is located in the northeastern part of the country on the Atlantic coast. It is the fourth most populous Brazilian state after São Paulo, Minas Gerais and Rio de Janeiro, and the fifth-largest in size...
, and far in the northeast, Alagoas
Alagoas
Alagoas is one of the 27 federative units of Brazil and is situated in the eastern part of the Northeast Region. It borders: Pernambuco ; Sergipe ; Bahia ; and the Atlantic Ocean . It occupies an area of 27,767 km², being slightly larger than Haiti...
and Pernambuco
Pernambuco
Pernambuco is a state of Brazil, located in the Northeast region of the country. To the north are the states of Paraíba and Ceará, to the west is Piauí, to the south are Alagoas and Bahia, and to the east is the Atlantic Ocean. There are about of beaches, some of the most beautiful in the...
. Paraguayan animals are somewhat smaller than those from Bolivia and Brazil and those from Bolivia have darker fur than Paraguayan specimens, but these differences are not considered significant enough to recognize subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
. Certain bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s show a similar pattern of variation: they are smaller and paler in the Chaco
Gran Chaco
The Gran Chaco is a sparsely populated, hot and semi-arid lowland region of the Río de la Plata basin, divided among eastern Bolivia, Paraguay, northern Argentina and a portion of the Brazilian states of Mato Grosso and Mato Grosso do Sul, where it is connected with the Pantanal region...
region, which includes much of Paraguay. Two specimens from Paraguay, collected 600 kilometres (372.8 mi) apart, differed by 1.4% in the sequence of the cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...
gene, but nothing is known about genetic variation in other parts of the range. The species has long been rare in collections; in 1991, Voss and Myers could use less than 50 specimens for their study of the species, including Lund's fragmentary material from Lagoa Santa.
A fragmentary lower jaw of "Pseudoryzomys aff. P. simplex" (i.e., an unnamed species close to Pseudoryzomys simplex) is known from a cave deposit in Cueva Tixi, Buenos Aires Province
Buenos Aires Province
The Province of Buenos Aires is the largest and most populous province of Argentina. It takes the name from the city of Buenos Aires, which used to be the provincial capital until it was federalized in 1880...
, Argentina, outside the current distribution of the species. It is dated from the first millennium CE. The jaw's morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
agrees with that of P. simplex, but the toothrow is relatively long (5.78 mm; 4.61 to 5.60 mm in three specimens of P. simplex) and the first molar is relatively narrow (1.28 mm; 1.30 to 1.40 mm in five P. simplex).
P. simplex inhabits open, usually humid tropical and subtropical lowlands
Tropical and subtropical grasslands, savannas, and shrublands
Tropical and subtropical grasslands, savannas, and shrublands are a grassland terrestrial biome located in semi-arid to semi-humid climate regions of subtropical and tropical latitudes. Grasslands are dominated by grass and other herbaceous plants. Savannas are grasslands with scattered trees...
. In Argentina, it is mainly a species of the eastern Chaco and in Brazil it is found in the Cerrado
Cerrado
The Cerrado, is a vast tropical savanna ecoregion of Brazil, particularly in the states of Gioas and Minas Gerais...
and Caatinga
Caatinga
Caatinga is a type of vegetation, and an ecoregion characterized by this vegetation in the northeastern part of Brazil. The name "Caatinga" is a Tupi word meaning "white forest" or "white vegetation"...
. Most specimens for which habitat data are known were caught on the ground in humid grassland, some in seasonally flooded areas; an Argentinean specimen was captured in dense swamp vegetation. It is terrestrial and semiaquatic, living on the ground but also spending time in the water.
Nothing is known about behavior or diet. P. simplex has frequently been found in pellets
Pellet (ornithology)
A pellet, in ornithology, is the mass of undigested parts of a bird's food that some bird species occasionally regurgitate. The contents of a bird's pellet depend on its diet, but can include the exoskeletons of insects, indigestible plant matter, bones, fur, feathers, bills, claws, and teeth...
of the Barn Owl
Barn Owl
The Barn Owl is the most widely distributed species of owl, and one of the most widespread of all birds. It is also referred to as Common Barn Owl, to distinguish it from other species in the barn-owl family Tytonidae. These form one of two main lineages of living owls, the other being the typical...
(Tyto alba) and also in those of the Great Horned Owl
Great Horned Owl
The Great Horned Owl, , also known as the Tiger Owl, is a large owl native to the Americas. It is an adaptable bird with a vast range and is the most widely distributed true owl in the Americas.-Description:...
(Bubo virginianus). It is a preferred prey of the Maned Wolf
Maned Wolf
The maned wolf is the largest canid of South America, resembling a large fox with reddish fur.This mammal is found in open and semi-open habitats, especially grasslands with scattered bushes and trees, in south, central-west and south-eastern Brazil The maned wolf (Chrysocyon brachyurus) is the...
(Chrysocyon brachyurus).
Conservation status
The species is not known to be threatened and its conservation status is classified as "Least ConcernLeast Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
" by the IUCN. It is a widely distributed species without substantial threats to its continued existence, but degradation of its habitat may endanger some populations. It was assessed as "potentially vulnerable" in Argentina.
Literature cited
- Belentani, S.C. da S., Motta-Junior, J.C. and Talamoni, S.A. 2005. Notes on the food habits and prey selection of the maned wolf (Chrysocyon brachyurus) (Mammalia, Canidae) in southeastern Brazil. Biocièncias 13(1):95–98.
- Bonvicino, C.R., Lemos, B. and Weksler, M. 2005. Small mammals of Chapada dos Veadeiros National Park (Cerrado of Central Brazil): Ecologic, karyologic, and taxonomic considerations. Brazilian Journal of Biology 65(3):395–406.
- Bonvicino, C.R., Oliveira, J.A. and D'Andrea, P.S. 2008. Guia dos Roedores do Brasil, com chaves para gêneros baseadas em caracteres externos. Rio de Janeiro: Centro Pan-Americano de Febre Aftosa – OPAS/OMS, 120 pp. (in Portuguese). ISSN 0101-6970
- 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.
- Chebez, J.C., Pereira, J., Massoia, E., Di Giacomo, A.G. and Fortabat, S.H. 2005. Mamíferos de la Reserva El Bagual. Temas de Naturaleza y Conservación 4:467–499 (in Spanish).
- D'Elia, G., Mora, I., Myers, P. and Owen, R.D. 2008. New and noteworthy records of Rodentia (Erethizontidae, Sciuridae, and Cricetidae) from Paraguay (subscription required for full paper). Zootaxa 1784:39–57.
- Díaz, G. and Ojeda, R. 2000. Libro Rojo de los mamíferos amenazados de la Argentina. Mendoza: Sociedad Argentina para el Estudio de los Mamíferos, 106 pp. (in Spanish). ISBN 978-987-98497-0-5
- Hershkovitz, P.M. 1959. Two new genera of South American rodents (Cricetinae). Proceedings of the Biological Society of Washington 72:5–10.
- Hershkovitz, P.M. 1960. Mammals of northern Colombia, preliminary report no. 8: Arboreal rice rats, a systematic revision of the subgenus Oecomys, genus Oryzomys. Proceedings of the United States National Museum 110:513–568.
- Hershkovitz, P. 1962. Evolution of Neotropical cricetine rodents (Muridae) with special reference to the phyllotine group. Fieldiana Zoology 46:1–524.
- Musser, G.G. and Carleton, M.D. 2005. Superfamily Muroidea. Pp. 894–1531 in Wilson, D.E. and Reeder, D.M. (eds.). Mammal Species of the World: a taxonomic and geographic reference. 3rd ed. Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. ISBN 978-0-8018-8221-0
- Pardiñas, U.F.J. 1995. Novedosos cricetidos (Mammalia, Rodentia) en el Holoceno de la Región Pampeana, Argentina. Ameghiniana 32(2):197–203 (in Spanish).
- 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.
- Pardiñas, U.F.J., Cirignoli, S. and Galliari, C.A. 2004. Distribution of Pseudoryzomys simplex (Rodentia: Cricetidae) in Argentina. Mastozoología Neotropical 11(1):105–108.
- Percequillo, A., Weksler, M., Pardiñas, U. and D'Elía, G. 2008. . In IUCN. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org> . Downloaded on November 9, 2009.
- Voss, R.S. and Carleton, M.D. 1993. A new genus for Hesperomys molitor Winge and Holochilus magnus Hershkovitz (Mammalia, Muridae) with an analysis of its phylogenetic relationships. American Museum Novitates 3085:1–39.
- Voss, R.S. and Myers, P. 1991. Pseudoryzomys simplex (Rodentia: Muridae) and the significance of Lund's collections from the caves of Lagoa Santa, Brazil. Bulletin of the American Museum of Natural History 206:414–432.
- Weksler, M. 2006. Phylogenetic relationships of oryzomyine rodents (Muroidea: Sigmodontinae): separate and combined analyses of morphological and molecular data. Bulletin of the American Museum of Natural History 296:1–149.
- Wetzel, R.M. and Lovett, J.W. 1974. A collection of animals from the Chaco of Paraguay. University of Connecticut Occasional Papers 2(13):203–216.
- Winge, H. 1887. Jordfundne og nulevende Gnavere (Rodentia) fra Lagoa Santa, Minas Geraes, Brasilien. E Museo Lundii 1(3):1–200.