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Temporal range: Pleistocene – recent
Oryzomys palustris.jpg
Marsh rice rat (Oryzomys palustris)
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Cricetidae
Subfamily: Sigmodontinae
Vorontzov, 1959
Type genus
Baird, 1858
About 120 species in about 30 genera
  • Oryzomyini Vorontsov, 1959
  • Zygodontomyini Eisenberg, 1989 (nomen nudum)

Oryzomyini, or rice rat, is a tribe of rodents in the subfamily Sigmodontinae of family Cricetidae. It includes about 120 species in about thirty genera, [1] distributed from the eastern United States to the southernmost parts of South America, including many offshore islands. It is part of the clade Oryzomyalia, which includes most of the South American Sigmodontinae.

Rodent Diverse order of mammals

Rodents are mammals of the order Rodentia, which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents ; they are found in vast numbers on all continents except Antarctica. They are the most diversified mammalian order and live in a variety of terrestrial habitats, including human-made environments.

Sigmodontinae subfamily of mammals

The rodent subfamily Sigmodontinae 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 numbers at least 508. Their distribution includes much of the New World, but the genera are predominantly South American, such as brucies. They invaded South America from Central America as part of the Great American Interchange near the end of the Miocene, about 5 million years ago. Sigmodontines proceeded to diversify explosively in the formerly isolated continent. They inhabit many of the same ecological niches that the Murinae occupy in the Old World.

Cricetidae family of mammals

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. At almost 608 species, it is the second-largest family of mammals, and has members throughout the Americas, Europe and Asia.


The name Oryzomyini derives from that of its type genus, Oryzomys , which means "rice rat" or "rice mouse". Many species are also known as rice rats.

Type genus term in zoological nomenclature (also non-officially in botanical nomenclature)

In biological classification, especially zoology, the type genus is the genus which defines a biological family and the root of the family name.

<i>Oryzomys</i> A genus of semiaquatic rodents from southern North America and far northern South Americ

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 (O. palustris) of the United States and O. couesi of Mexico and Central America—are widespread; the six others have more restricted distributions. The species have had eventful taxonomic histories, and most species were at one time included in the marsh rice rat; additional species may be recognized in the future. The name Oryzomys was established in 1857 by Spencer Fullerton Baird for the marsh rice rat and was soon applied to over a hundred species of American rodents. Subsequently, the genus gradually became more narrowly defined until its current contents were established in 2006, when ten new genera were established for species previously placed in Oryzomys.


Contents of Oryzomyini

An oryzomyine group was first envisaged by Oldfield Thomas in the early 20th century. He defined it to include pentalophodont species, which have a mesoloph(id) on the upper and lower molars, with a long palate (extending past the third molars). Thomas included Oligoryzomys , Oecomys , and Oryzomys (which included many species now in other genera), as well as Rhagomys , which is currently classified in the related tribe Thomasomyini instead. In 1944, Hershkovitz proposed a more extended definition of the group, excluding Rhagomys, but including Nectomys (then including Sigmodontomys ), Neacomys , and Scolomys . Some subsequent authors did not separate the oryzomyines from the thomasomyines, which were distinguished from them by having a short palate, including Vorontsov, who in 1959 was the first to use Oryzomyini as a formal family-group name. He included most current oryzomyines as well as the thomasomyines and tylomyines, which are now known to be more distantly related. [2]

Oldfield Thomas British mammalogist

Michael Rogers Oldfield Thomas FRS FZS was a British zoologist.

Molar (tooth) large tooth at the back of the mouth

The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola, millstone and dens, tooth. Molars show a great deal of diversity in size and shape across mammal groups.

Palate roof of the mouth

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 bony hard palate and the posterior fleshy soft palate.

The genera Holochilus (including Lundomys at the time), Pseudoryzomys , and Zygodontomys were not included at the time because of their tetralophodont molars (lacking complete mesoloph(id)s); instead, Holochilus was considered to be a sigmodont, related to Sigmodon and Reithrodon , and Pseudoryzomys and Zygodontomys were considered to be members of Phyllotini, another large South American tribe. [2] Although lacking mesoloph(id)s, these genera share other characters with oryzomyines, and a series of papers by Robert Voss and coworkers in the early 1990s established their membership in Oryzomyini. [3]

Holochilus is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae, sometimes called marsh rats. It contains three living species, H. brasiliensis, H. chacarius, and H. sciureus, which are widely distributed in South America east of the Andes. A fourth species from the Pleistocene of Bolivia was formerly classified as H. primigenus, but is now placed in the genus Reigomys.

<i>Lundomys</i> A semiaquatic rat species from southeastern South America.

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

<i>Pseudoryzomys</i> A genus of rodent from South America with one species

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

In a 1993 paper, Voss and Carleton proposed the first cladistic diagnosis of Oryzomyini. They included twelve genera and proposed five synapomorphies for the tribe: presence of a pair of mammae on the chest; a long palate marked by posterolateral palatal pits, perforations near the third molar; absence of an alisphenoid strut, which in some sigmodontines separates two foramina (openings) in the skull; absence of a suspensory process of the squamosal bone attached to the roof of the tympanic cavity, the tegmen tympani; and absence of a gall bladder. [4] Some of these have been reversed in some oryzomyines; for example, an alisphenoid strut is present in several oryzomyines, including Eremoryzomys . [5]

Cladistics A method of biological systematics in evolutionary biology

Cladistics is an approach to biological classification in which organisms are categorized in groups ("clades") based on the most recent common ancestor. Hypothesized relationships are typically based on shared derived characteristics (synapomorphies) that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors. A key feature of a clade is that a common ancestor and all its descendants are part of the clade. Importantly, all descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms animals, bilateria/worms, fishes/vertebrata, or monkeys/anthropoidea were used, these terms would include humans. Many of these terms are normally used paraphyletically, outside of cladistics, e.g. as a 'grade'. Radiation results in the generation of new subclades by bifurcation.

In anatomy, posterolateral palatal pits are gaps at the sides of the back of the bony palate, near the last molars. Posterolateral palatal pits are present, in various degrees of development, in several members of the rodent family Cricetidae. Many members of the family lack them or have only simple pits, but Arvicolinae and Oryzomyini have more highly developed posterolateral palatal pits. Posterolateral palatal pits are also present in some other rodents, including Glis, Jaculus, Hystrix, Abrocoma, Ctenomys, Chinchilla, and Lagidium.

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 accessorium. The presence or absence of this strut is variable between species, but also within them; some Oryzomyini even have a strut on one side of the skull but lack it on the other side.

The contents of Oryzomyini have been largely stable since, [6] but the allocation of some animals has been contentious. Megaoryzomys , an extinct giant rat from the Galápagos Islands, has been allocated to both Oryzomyini and Thomasomyini, but its correct classification currently remains unclear. [7] The genus Scolomys has been excluded from Oryzomyini on the basis of studies of the mitochondrial cytochrome b gene, [8] but the nuclear IRBP gene provides evidence for its membership in Oryzomyini. [9] Handleyomys fuscatus was placed in the thomasomyine genus Aepeomys before its close relationship to H. intectus was recognized in 2002. [10] Microakodontomys transitorius was described as a transitional form between oryzomyines and akodontines, but later allied with Oryzomyini [11] and even summarily dismissed as an anomalous Oligoryzomys . [12]

<i>Megaoryzomys</i> genus of mammals

Megaoryzomys curioi, also known as the Galápagos giant rat, is an extinct species of sigmodontine rodent, known only from Santa Cruz Island in the Galápagos Islands. It likely met its demise when European settlers introduced invasive species to the island. It is the only species in the genus Megaoryzomys. Its relationships are currently unclear; it has been placed in both Oryzomyini and Thomasomyini in the past.

Galápagos Islands Achipelago and protected area of Ecuador in the Pacific Ocean

The Galápagos Islands, part of the Republic of Ecuador, are an archipelago of volcanic islands distributed on either side of the equator in the Pacific Ocean surrounding the centre of the Western Hemisphere, 906 km (563 mi) west of continental Ecuador. The islands are known for their large number of endemic species and were studied by Charles Darwin during the second voyage of HMS Beagle. His observations and collections contributed to the inception of Darwin's theory of evolution by means of natural selection.

Scolomys is a genus of rodent in the tribe Oryzomyini of the family Cricetidae. Some evidence suggests that it is related to Zygodontomys. It is characterized, among other traits, by spiny fur. It contains two species, S. melanops and S. ucayalensis.

In the early 2000s, the advent of molecular phylogenetics has led to progress in the understanding of the relationships of oryzomyines. They are currently classified in the family Cricetidae, which includes the voles, hamsters, deermice, and many other species, chiefly in the Americas and Eurasia. Within this family, they are part of the subfamily Sigmodontinae, which is mainly distributed in South America but also extends into southern North America. Sigmodontinae includes several tribes, most of which cluster into a clade now known as Oryzomyalia, which includes Oryzomyini, Akodontini, Phyllotini, Thomasomyini, and other, smaller groups, but not the cotton rats (Sigmodon) and the Ichthyomyini. [13]

Internal classification

The relationships among the oryzomyine genera have long been obscure, although several studies provided insights into the relations of some genera. [14] The single most significant problem in oryzomyine taxonomy has been the definition of the type genus, Oryzomys , [15] which in one classification included all animals then recognized as oryzomyines. [16] Many groups were later excluded from the genus, but even so it included forty species that did not form a monophyletic group. [17]

In the 2000s, Marcelo Weksler published several studies in which he used evidence from IRBP, a nuclear gene, and morphology to assess the relationships among the members of Oryzomyini. He provided support for several intergeneric relationships and clarified the scale of the Oryzomys problem, as species of Oryzomys appeared in about ten separate clades. [18] In a 2006 publication, he and coworkers described ten new genera for species previously placed in Oryzomys and transferred some others to Handleyomys, leaving only about six species in Oryzomys. [1]


Clade A

Clade B

Clade C

Clade D

Possible relationships of major oryzomyine clades. [19]

Weksler's analyses suggested that oryzomyines fall into four major clades, which were largely congruent across his analyses of morphology and IRBP, but support for all of those was limited and the placement of some genera remained unclear. He dubbed these clades "clade A" through "clade D". Some analyses supported a relationship between clades C and D, which in turn were related to clade B, with clade A at a basal position, but other analyses could not resolve the relationships among the major clades. [18] The four clades are as follows:

The affinities of some species remain unclear. Many oryzomyines are known from the Lesser Antilles, including " Ekbletomys hypenemus " and species of Megalomys and Oligoryzomys, but most remain undescribed. [33]


Most oryzomyines are nondescript rodents that look like common house mice and rats, but the tribe also includes some spectacularly specialized forms. [34] The smallest members, mainly in clade C, may have a head-body length of as little as 65 millimetres (2.6 in) and mass of 10 grams (0.4 oz), but the largest living oryzomyine, Nectomys, reaches head and body lengths over 250 millimetres (9.8 in) and mass of about 300 grams (10 oz); Lundomys and Holochilus are only a little smaller. [35] Some of the extinct species from the Lesser Antilles, such as "Ekbletomys hypenemus" and Megalomys desmarestii , were even larger. [36]

Distribution and ecology

Oryzomyines range from New Jersey in the north, where the Marsh Rice Rat (Oryzomys palustris) is found, to Tierra del Fuego in the south, where Oligoryzomys magellanicus occurs. Extinct species are known from Jamaica ( Oryzomys antillarum ), the Galápagos Islands (Nesoryzomys and Aegialomys galapagoensis ), Fernando de Noronha (Noronhomys), and the Lesser Antilles north to Anguilla (Megalomys, Oligoryzomys victus , and several unidentified genera). [7] They are abundant in many environments, from rainforest to grassland. [34] Most live in the forest, but Zygodontomys, Lundomys, Pseudoryzomys, Aegialomys, and Nesoryzomys live exclusively in open vegetation and some other genera include both forest and non-forest forms. [37] Most oryzomyines are relatively unspecialized animals that live on the ground, [34] but Oecomys is specialized to live in trees and various members of clade D, including Holochilus, Oryzomys, and Nectomys, are semiaquatic, spending at least some of their time in the water. [38]

Related Research Articles

New World rats and mice informal group of mammals

The New World rats and mice are a group of related rodents found in North and South America. They are extremely diverse in appearance and ecology, ranging in from the tiny Baiomys to the large Kunsia. They represent one of the few examples of muroid rodents in North America, and the only example of muroid rodents to have made it into South America.

Oreoryzomys balneator, also known as the Peruvian rice rat or Ecuadoran oryzomys, is a species of rodent in the tribe Oryzomyini of family Cricetidae. It is found in Ecuador and northern Peru in cloud forest at elevations from 1500 to 1800 m. It is the only species in the genus Oreoryzomys, which was included in Oryzomys until 2006. The genus name Oreoryzomys is a combination of ορος the Greek word for "mountain" with the old genus name Oryzomys and refers to the mountainous habitat of O. balneator. Recent research suggests that O. balneator is not closely related to Oryzomys, but instead is probably related to Microryzomys within a clade also including Neacomys and Oligoryzomys.

<i>Aegialomys galapagoensis</i> species of mammal

Aegialomys galapagoensis, also known as the Galápagos rice rat or Galápagos oryzomys, is a species of rodent in the family Cricetidae from the Galápagos Islands.

<i>Mindomys</i> A species of rodent in the family Cricetidae from Ecuador

Mindomys hammondi, also known as Hammond's rice rat or Hammond's oryzomys, is a species of rodent in the tribe Oryzomyini of family Cricetidae. Formerly considered to be related with Nectomys, Sigmodontomys, Megalomys, or Oryzomys, it is now placed in its own genus, Mindomys, but its relationships remain obscure; some evidence supports a placement near Oecomys or as a basal member of Oryzomyini.

<i>Eremoryzomys</i> A rodent species in the family Cricetidae from central Peru

Eremoryzomys polius, also known as the gray rice rat or the Marañon oryzomys, is a rodent species in the tribe Oryzomyini of the family Cricetidae. Discovered in 1912 and first described in 1913 by Wilfred Osgood, it was originally placed in Oryzomys and named Oryzomys polius. In 2006, a cladistic analysis found that it was not closely related to Oryzomys in the strict sense or to any other oryzomyine then known, so that it is now placed in its own genus, Eremoryzomys. The Brazilian genus Drymoreomys, named in 2011, is probably the closest relative of Eremoryzomys. Eremoryzomys has a limited distribution in the dry upper valley of the Marañón River in central Peru, but may yet contain more than one species.

Aegialomys xanthaeolus, also known as the yellowish oryzomys or yellowish rice rat, is a species of rodent in the family Cricetidae. It belongs to the genus Aegialomys in tribe Oryzomyini, which was not recognized as distinct from Oryzomys until 2006. It is found in coastal Ecuador and Peru. Though it is currently the only formally recognized mainland species of Aegialomys, at least one other exists. The specific name is sometimes incorrectly spelled "xantheolus", without the second "a".

Sigmodontomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is related to Nectomys and Melanomys and used to be included in Nectomys. It includes two species, Sigmodontomys alfari and the much rarer Sigmodontomys aphrastus, but whether these are indeed each other's closest relatives is uncertain.

Cerradomys is a genus of oryzomyine rodents from eastern Bolivia, Paraguay, and central Brazil found in cerrado, Caatinga and Gran Chaco habitats. The species in this genus have historically been placed in Oryzomys, but according to cladistic research, they are not more closely related to the type species of Oryzomys than species in some other genera are. Its sister group may be a clade of oryzomyines living in open or aquatic habitats, comprising, among others, Aegialomys, Nectomys and Sigmodontomys. Sooretamys angouya, also formerly in Oryzomys, is another relative; it has been placed in the same group as the species of Cerradomys in the past. The generic name Cerradomys is a compound of the word "Cerrado" and the Ancient Greek μῦς mys "mouse" and therefore means "Cerrado mouse".

<i>Carletonomys</i> species of mammal

Carletonomys cailoi is an extinct rodent from the Pleistocene (Ensenadan) 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. Discovered in 1998 and formally described in 2008, it is part of a well-defined group of oryzomyine rodents that also includes Holochilus, Noronhomys, Lundomys, and Pseudoryzomys. This group is characterized by progressive semiaquatic specializations and a reduction in the complexity of molar morphology.

Reigomys primigenus is an extinct oryzomyine rodent known from Pleistocene deposits in Tarija Department, southeastern Bolivia. It is known from a number of isolated jaws and molars which show that its molars were almost identical to those of the living Lundomys. On the other hand, the animal possesses a number of derived traits of the palate which document a closer relationship to living Holochilus, the genus of South American marsh rats, and for this reason it was placed in the genus Holochilus when it was first described in 1996. The subsequent discoveries of Noronhomys and Carletonomys, which may be more closely related to extant Holochilus than H. primigenus is, have cast its placement in Holochilus into doubt, and it was ultimately made the type species of a separate genus, Reigomys.

In rodents, sphenopalatine vacuities are perforations of the roof of the mesopterygoid fossa, the open space behind the palate, in between the parapterygoid fossae. They may perforate the presphenoid or basisphenoid bone. Their development and form are variable between and within species, and features of the sphenopalatine vacuities have been used as characters in cladistic analyses.

In mammals, ungual tufts are tufts of hairs at the base of claws of the fore- and hindfeet. Their presence has been used as a character in cladistic studies of Cricetidae.

Natatory fringes are rows of stiff hairs that occur along the margins of the hindfeet in some rodents. They occur along the plantar margins and in some cases also between the toes. Among sigmodontines, a mostly South American groups, natatory fringes are present in Ichthyomyini and some Oryzomyini. Among ichthyomyines, the fringes are poorly developed in Neusticomys but well-developed in other genera, and in Rheomys mexicanus the hairs of the fringes may exceed 3 millimetres (0.12 in) in length. Amphinectomys, Holochilus, Lundomys, and Nectomys are the only oryzomyines with natatory fringes, but have them only weakly developed; one study also records them in Oryzomys. In oryzomyines, the fringes are an adaptation for a semiaquatic lifestyle that appeared convergently in the Holochilus-Lundomys and Nectomys-Amphinectomys lineages. The term was introduced in 1993 by Voss and Carleton in describing Lundomys.

<i>Agathaeromys</i> genus of mammals

Agathaeromys is an extinct genus of oryzomyine rodents from the Pleistocene of Bonaire, Netherlands Antilles. Two species are known, which differ in size and some details of tooth morphology. The larger A. donovani, the type species, is known from hundreds of teeth, found in four localities that are probably 900,000 to 540,000 years old. A. praeuniversitatis, the smaller species, is known from 35 teeth found in a single fossil site, which is probably 540,000 to 230,000 years old.

<i>Pennatomys</i> An extinct rodent from the islands of Sint Eustatius, Saint Kitts, and Nevis in the Lesser Antilles

Pennatomys nivalis is an extinct oryzomyine rodent from the islands of Sint Eustatius, Saint Kitts, and Nevis in the Lesser Antilles. The only species in the genus Pennatomys, it is known from skeletal remains found in Amerindian archeological sites on all three islands, with dates ranging from 790–520 BCE to 900–1200 CE. No live specimens are known, but there are several historical records of rodents from Saint Kitts and Nevis that could conceivably refer to Pennatomys. The animal apparently belongs to a group within the tribe Oryzomyini that includes many other island-dwelling species.

<i>Drymoreomys</i> A rodent genus with one species in the family Cricetidae from the Atlantic Forest of Brazil.

Drymoreomys is a rodent genus in the tribe Oryzomyini that lives in the Atlantic Forest of Brazil. The single species, D. albimaculatus, is known only from the states of São Paulo and Santa Catarina and was not named until 2011. It lives in the humid forest on the eastern slopes of the Serra do Mar and perhaps reproduces year-round. Although its range is relatively large and includes some protected areas, it is patchy and threatened, and the discoverers recommend that the animal be considered "Near Threatened" on the IUCN Red List. Within Oryzomyini, Drymoreomys appears to be most closely related to Eremoryzomys from the Andes of Peru, a biogeographically unusual relationship, in that the two populations are widely separated and each is adapted to an arid or a moist environment.


  1. 1 2 Weksler et al., 2006, table 1
  2. 1 2 Weksler, 2006, p. 4
  3. Weksler, 2006, p. 5
  4. Voss and Carleton, 1993, p. 31
  5. Weksler, 2006, p. 38
  6. Musser and Carleton, 2005; Weksler, 2006; Weksler et al., 2006
  7. 1 2 Musser and Carleton, 2005
  8. Musser and Carleton, 2005, p. 1117
  9. Weksler, 2006, p. 124
  10. Voss et al., 2002, p. 2
  11. Musser and Carleton, 2005, p. 1126
  12. Weksler et al., 2006, table 1, footnote d
  13. Steppan et al., 2004
  14. Weksler, 2006, p. 7
  15. Weksler, 2006, p. 10
  16. Ray, 1962
  17. Weksler, 2006, p. 10, table 2
  18. 1 2 3 4 Weksler, 2006, figs. 34–39
  19. Weksler, 2006, figs. 37, 39; figs. 34–36, 38 provided less resolved topologies.
  20. Weksler, 2006, p. 75
  21. 1 2 3 4 Weksler et al., 2006
  22. Weksler, 2006, figs. 34, 35, 38
  23. 1 2 Weksler, 2006, p. 133
  24. Weksler et al., 2006, p. 17
  25. Percequillo et al., 2011, p. 372
  26. Weksler et al., 2006, p. 18
  27. Zijlstra et al., 2010, p. 869
  28. Turvey et al., 2010, p. 766
  29. 1 2 Weksler, 2006, figs. 37–39
  30. Turvey et al., 2010, pp. 759–760
  31. Pardiñas, 2008
  32. Weksler, 2006, p. 131
  33. Turvey, 2009
  34. 1 2 3 Weksler, 2006, p. 3
  35. Weksler, 2006, p. 3, table 8
  36. Ray, 1962, tables 7, 11
  37. Weksler, 2006, pp. 81–82
  38. Weksler, 2006, pp. 78–79

Literature cited