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Oryzomyini is a tribe of in the subfamily Sigmodontinae of the family Cricetidae. It includes about 120 species in about thirty genera,Weksler et al., 2006, table 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.
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.
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. 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.Weksler, 2006, p. 5
In a 1993 paper, Voss and Carleton proposed the first cladistics diagnosis of Oryzomyini. They included twelve genera and proposed five synapomorphy 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.Voss and Carleton, 1993, p. 31 Some of these have been reversed in some oryzomyines; for example, an alisphenoid strut is present in several oryzomyines, including Eremoryzomys.Weksler, 2006, p. 38
The contents of Oryzomyini have been largely stable since,Musser and Carleton, 2005; Weksler, 2006; Weksler et al., 2006 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.Musser and Carleton, 2005 The genus Scolomys has been excluded from Oryzomyini on the basis of studies of the mitochondrial cytochrome b gene,Musser and Carleton, 2005, p. 1117 but the nuclear DNA IRBP gene provides evidence for its membership in Oryzomyini.Weksler, 2006, p. 124 Handleyomys fuscatus was placed in the thomasomyine genus Aepeomys before its close relationship to H. intectus was recognized in 2002.Voss et al., 2002, p. 2 Microakodontomys was described as a transitional form between oryzomyines and Akodontini, but later allied with OryzomyiniMusser and Carleton, 2005, p. 1126 and even summarily dismissed as an anomalous Oligoryzomys.Weksler et al., 2006, table 1, footnote d
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 , , deermouse, 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 ( Sigmodon) and the Ichthyomyini.Steppan et al., 2004
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 .Weksler, 2006, figs. 34–39 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.
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. The four clades are as follows:
The affinities of some species remain unclear. Many oryzomyines are known from the Lesser Antilles, including " Ekbletomys" and species of Megalomys and Oligoryzomys, but most remain undescribed.Turvey, 2009
Internal classification
The relationships among the oryzomyine genera have long been obscure, although several studies provided insights into the relations of some genera.Weksler, 2006, p. 7 The single most significant problem in oryzomyine taxonomy has been the definition of the type genus, Oryzomys,Weksler, 2006, p. 10 which in one classification included all animals then recognized as oryzomyines.Ray, 1962 Many groups were later excluded from the genus, but even so it included forty species that did not form a monophyly group.Weksler, 2006, p. 10, table 2
Description
Most oryzomyines are nondescript rodents that look like common House Mouse and Rattus, but the tribe also includes some spectacularly specialized forms. The smallest members, mainly in clade C, may have a head-body length of as little as and mass of , but the largest living oryzomyine, Nectomys, reaches head and body lengths over and mass of about ; Lundomys and Holochilus are only a little smaller.Weksler, 2006, p. 3, table 8 Some of the extinct species from the Lesser Antilles, such as " Ekbletomys hypenemus" and Megalomys desmarestii, were even larger.Ray, 1962, tables 7, 11
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). They are abundant in many environments, from rainforest to grassland.Weksler, 2006, p. 3 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.Weksler, 2006, pp. 81–82 Most oryzomyines are relatively unspecialized animals that live on the ground, but Oecomys is specialized to arboreal and various members of clade D, including Holochilus, Oryzomys, and Nectomys, are semiaquatic, spending at least some of their time in the water.Weksler, 2006, pp. 78–79
Literature cited
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