Pseudoryzomys

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Pseudoryzomys simplex
From top to bottom: side view of skull with mandible, missing much of the posterior part; text "1. Hesperomys simplex"; and views of the same skull from above and below.
Lectotype
skull as illustrated by Winge in 1888

Least Concern  (IUCN 3.1)[1]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Cricetidae
Subfamily: Sigmodontinae
Genus: Pseudoryzomys
Hershkovitz, 1962[fn 1]
Species:
P. simplex
Binomial name
Pseudoryzomys simplex
(Winge, 1888)
Map of South America, with a contiguous red area extending over northeastern Argentina, much of Paraguay, and eastern Bolivia, east into northeastern Brazil; and a small blue area in eastern Argentina
Distribution of Pseudoryzomys in South America (in red); fossil record from Cueva Tixi, eastern Argentina, in blue
Synonyms
  • Hesperomys simplex Winge, 1888
  • Oryzomys simplex: Trouessart, 1898
  • Oryzomys wavrini Thomas, 1921
  • Pseudoryzomys wavrini: Hershkovitz, 1959
  • Pseudoryzomys wavrini reigi Pine and Wetzel, 1975
  • Pseudoryzomys simplex: Massoia, 1980

Pseudoryzomys simplex, also known as the Brazilian false rice rat

least concern
, although almost nothing is known about its diet or reproduction.

The

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

sigmodont groups.[6]

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),[7] 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.[8] 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,[9] 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[10]—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.[7]

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.[11]

Oryzomyine relationships

Oryzomyini

Other Oryzomyini

Oryzomys

Pseudoryzomys

Oryzomyini

Other Oryzomyini

Lundomys

Pseudoryzomys

Holochilus

Relationships of Pseudoryzomys based on morphology (top) and IRBP (bottom).[12]

When Voss and Carleton formally characterized Oryzomyini two years later, they did place Pseudoryzomys in the group, even though it lacks complete

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.[14]

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.[15] 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.[16] 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.[17]

Description

Pseudoryzomys simplex is a nondescript, medium-sized rat with long, soft fur. The upperparts are gray–brown and the underparts are

tufts of hair on the toes and several of the pads are reduced, other common characteristics of semiaquatic oryzomyines.[19] The head-body length is 94 to 140 millimeters (3.7 to 5.5 in), tail length 102 to 140 mm (4.0 to 5.5 in), hindfeet length 27 to 33 mm (1.1 to 1.3 in), ear length 13 to 19 mm (0.5 to 0.7 in) and body mass 45 to 55 g (1.6 to 1.9 oz).[20]

Rat, yellow–brown above and white below, walking on mesh.
The marsh rice rat (Oryzomys palustris) of the eastern United States is similar in appearance to Pseudoryzomys.[8]

The female has four pairs of

gall bladder is absent, both important characters of Oryzomyini.[21] 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.[22]

Skull

The skull, which is short at the front, shows some typical oryzomyine characters. The

tegmen tympani, the roof the tympanic cavity. The front part is short.[23]

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.[26]

The

mastoid skull bone contains a conspicuous opening, as in most oryzomyines.[33]

The mandible (lower jaw) is short and deep.[23] The mental foramen, an opening at the front of the mandible, just before the first molar, opens to the side.[33] 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.[34]

Molars

Landscape with grass and scattered trees. The sky is blue, with a few scattered clouds.
Pseudoryzomys was first found as a living animal in the Chaco of Paraguay.[35]

As in all oryzomyines except Holochilus and its close relatives, the molars are

posteroflexid, is severely reduced, foreshadowing its loss in Lundomys and Holochilus.[37] A number of molar traits support Pseudoryzomys's 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.[38]

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.[23]

Postcranial skeleton

Pseudoryzomys has 19 or 20 

caudal (tail) vertebrae.[11] 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.[39] 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.[40]

Karyotype

The

Thaptomys.[43] Both sex chromosomes are acrocentric, and X is larger than Y. In addition to heterochromatin near the centromere, the Y chromosome contains two large blocks of heterochromatin on its long arm.[42] The karyotype is closely similar to that of Holochilus brasiliensis.[43]

Distribution, ecology, and variation

Owl, looking to the viewer's left, on a green background
Remains of Pseudoryzomys have been found in pellets of the barn owl (Tyto alba).[44]

Pseudoryzomys simplex is known from northeastern Argentina, probably south to about 30°S,[44] 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.[45] 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 bats show a similar pattern of variation: they are smaller and paler in the Chaco region, which includes much of Paraguay.[7] Two specimens from Paraguay, collected 600 kilometres (400 mi) apart, differed by 1.4% in the sequence of the cytochrome b gene,[46] 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.[47]

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).[48]

P. simplex inhabits open, usually humid tropical and subtropical lowlands.[49] In Argentina, it is mainly a species of the eastern Chaco[50] and in Brazil it is found in the Cerrado and Caatinga.[51] Most specimens for which habitat data are known were caught on the ground in humid grassland, some in seasonally flooded areas;[52] an Argentinean specimen was captured in dense swamp vegetation.[50] It is terrestrial and semiaquatic, living on the ground but also spending time in the water.[53]

Nothing is known about behavior or diet. P. simplex has frequently been found in pellets of the barn owl (Tyto alba)[44] and also in those of the great horned owl (Bubo virginianus).[54] It is a preferred prey of the maned wolf (Chrysocyon brachyurus).[55]

Conservation status

The species is not known to be threatened and its conservation status is classified as

least concern 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.[1] It was assessed as "potentially vulnerable" in Argentina.[56]

Footnotes

  1. ^ Hershkovitz introduced the genus name Pseudoryzomys in a 1959 article, but failed to make the name available under the International Code of Zoological Nomenclature, because he did not explicitly name the diagnostic characteristics of the genus. In his 1962 monograph on phyllotine rodents, Hershkovitz properly identified diagnostic characters, making the name formally available for the first time. Accordingly, the correct year of publication for Pseudoryzomys is 1962, not 1959.[2]

References

  1. ^ a b c Percequillo et al., 2016
  2. ^ Voss and Myers, 1991, p. 418
  3. ^ Musser and Carleton, 2005, p. 1164
  4. ^ Winge, 1888
  5. ^ Voss and Carleton, 1993, p. 31
  6. ^ Hershkovitz, 1960, pp. 519–520
  7. ^ a b c Voss and Myers, 1991, p. 424
  8. ^ a b Hershkovitz, 1959, pp. 8–9
  9. ^ Hershkovitz, 1962, p. 208
  10. ^ Hershkovitz, 1959, p. 9
  11. ^ a b c Voss and Myers, 1991, p. 423
  12. ^ Weksler, 2006, figs. 34–36
  13. ^ Voss and Carleton, 1993, p. 31–32
  14. ^ Musser and Carleton, 2005, table 1
  15. ^ Pardiñas, 2008, p. 1275
  16. ^ Weksler, 2006, p. 72
  17. ^ Weksler, 2006, p. 131
  18. ^ Voss and Myers, 1991, p. 420
  19. ^ Weksler, 2006, pp. 23–25
  20. ^ Voss and Myers, 1991, tables 1, 2; Bonvicino et al., 2008, p. 54
  21. ^ Weksler, 2006, pp. 17, 19, 58–59
  22. ^ Weksler, 2006, pp. 55–56
  23. ^ a b c d e Voss and Myers, 1991, p. 422
  24. ^ Weksler, 2006, pp. 27–28, table 5
  25. ^ Weksler, 2006, pp. 28–30, table 5
  26. ^ Weksler, 2006, p. 31
  27. ^ Voss and Myers, 1991, p. 422; Weksler, 2006, p. 31
  28. ^ Weksler, 2006, p. 32, table 5
  29. ^ Weksler, 2006, pp. 31–32
  30. ^ Weksler, 2006, pp. 35–36, 131
  31. ^ Weksler, 2006, p. 35
  32. ^ Weksler, 2006, p. 36
  33. ^ a b Weksler, 2006, p. 41
  34. ^ Weksler, 2006, p. 42
  35. ^ Voss and Myers, 1991, p. 416
  36. ^ Weksler, 2006, pp. 43–44; Pardiñas, 2008, table 2
  37. ^ a b Weksler, 2006, p. 52
  38. ^ Weksler, 2006, p. 131; Carleton and Olson, 1999, pp. 49–50
  39. ^ a b Weksler, 2006, p. 53
  40. ^ Weksler, 2006, p. 55
  41. ^ Voss and Myers, 1991, p. 423; Bonvicino et al., 2005, p. 399; Moreira et al., 2013, p. 202
  42. ^ a b Moreira et al., 2013, p. 202
  43. ^ a b Moreira et al., 2013, p. 204
  44. ^ a b c Pardiñas et al., 2004, p. 105
  45. ^ Musser and Carleton, 2005, p. 1164; Bonvicino et al., 2008, p. 54
  46. ^ D'Elia et al., 2008, p. 49
  47. ^ Voss and Myers, 1991, pp. 426–427
  48. ^ Pardiñas, 1995, pp. 199–201
  49. ^ Voss and Myers, 1991, p. 425
  50. ^ a b Pardiñas et al., 2004, p. 108
  51. ^ Bonvicino et al., 2008, p. 54
  52. ^ Voss and Myers, 1991, p. 426; Bonvicino et al., 2005, p. 399; D'Elia et al., 2008, p. 49
  53. ^ Hershkovitz, 1962, p. 213; Bonvicino et al., 2008, p. 54; Wetzel and Lovett, 1974, p. 211
  54. ^ Chebez et al., 2005, p. 484
  55. ^ Belentani et al., 2005, p. 96
  56. ^ Díaz and Ojeda, 2000

Literature cited

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