Carletonomys: Difference between revisions
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'''''Carletonomys cailoi''''' is a [[rodent]] known from the [[Pleistocene]] of [[Buenos Aires Province]], [[Argentina]]. Although known only from a single [[maxilla]] (upper jaw) with the first [[molar]] in it, 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]]'' and that is characterized by progressive [[semiaquatic]] specializations and a reduction in the complexity of molar [[morphology (biology)|morphology]].<ref>Pardiñas, 2008; Weksler, 2006, p. 113</ref> |
'''''Carletonomys cailoi''''' is a [[rodent]] known from the [[Pleistocene]] of [[Buenos Aires Province]], [[Argentina]]. Although known only from a single [[maxilla]] (upper jaw) with the first [[molar]] in it, 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]]'' and that is characterized by progressive [[semiaquatic]] specializations and a reduction in the complexity of molar [[morphology (biology)|morphology]].<ref>Pardiñas, 2008; Weksler, 2006, p. 113</ref> |
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Revision as of 20:53, 3 September 2009
| Carletonomys cailoi Temporal range: Pleistocene
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| Genus: | Carletonomys Pardiñas, 2008
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| Species: | C. cailoi
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| Carletonomys cailoi Pardiñas, 2008
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Carletonomys cailoi is a rodent known from the Pleistocene of Buenos Aires Province, Argentina. Although known only from a single maxilla (upper jaw) with the first molar in it, 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 and that is characterized by progressive semiaquatic specializations and a reduction in the complexity of molar morphology.[1]
Taxonomic history and relationships
The single known specimen of C. cailoi was discovered in 1998 in a silt deposit in San Cayetano Partido, southeastern Buenos Aires Province, which the stratigraphic context suggests is slightly over 1 million years old (Ensenadan South American Land Mammal Age), making Carletonomys the oldest known oryzomyine.[2] It is now in the collections of the Museo de La Plata as specimen MLP 98-I-15-1. The specimen was initially referred to the genus Noronhomys, which is known only from the island of Fernando de Noronha off northeastern Brazil, but in 2008 Argentinean mammalogist Ulyses Pardiñas established it as the holotype of a new genus and species in a publication in the Journal of Mammalogy. The generic name, Carletonomys, combines the name of distinguished American zoologist Michael Carleton with the Ancient Greek μυς "mouse" and the specific name, cailoi, honors Argentinean biologist Carlos "Cailo" Galliari.[3]
C. cailoi has a number of features that suggest a relation to a group of oryzomyine rodents that includes the South American marsh rat Holochilus, including high-crowned (hypsodont) molars and several details of molar morphology. It shows the most similarity to Noronhomys and Holochilus, so much so that Pardiñas considered placing it in either of these two genera, but its distinctive morphological features justify placement in a separate genus.[4]
Description
The holotype is a right maxilla with the upper first molar (M1) in it. It is broken off behind the M1, but much of the anterior part, including the zygomatic plate, is preserved. The M1 is moderately worn, indicating that it comes from an adult individual. With an M1 length of 3.59 mm and width of 2.53 mm, C. cailoi was one of the largest oryzomyines known, rivaled only by Lundomys and the extinct Antillean Megalomys.[5] The height of the M1 is 1.37 mm and it has four roots, including a large one in front, another large one on the inner (lingual) side, and two smaller ones on the outer (labial) side. The second labial root is variable among oryzomyines, occurring among others in Holochilus and Pseudoryzomys but not in Lundomys. The maxilla itself shows few significant characters. The posterior margin of the incisive foramen is not visible, suggesting that it was probably short, as in Holochilus. The configuration of the zygomatic plate shows some features that are variable among Noronhomys and its close relatives.[6]
The molar is plane and hypsodont; thus the crowns are relatively high and that the main cusps are about as high as the other parts of the crown, as in Holochilus. Most other oryzomyines have bunodont and brachydont molars, in which the crowns are lower and the cusps are higher than the rest of the crown. Like in its closest relatives, the anterior part of the molar is relatively simple, lacking an anteroloph, an additional ridge that is well-developed in most oryzomyines. A shallow anteromedian flexus is present, superficially dividing the anterior cusp (anterocone). Uniquely, the anterior mure, which connects the anterocone to the rest of the crown is absent; although this structure is sometimes missing in young individuals of other oryzomyines, it usually develops as a result of wear in adults. The two cusps on the middle part of the molar, the paracone and the protocone, are broadly connected. The median mure, which connects the middle to the posterior pair of cusps, is attached to the posterior side of the paracone. A complete mesoloph is present, descending from the median mure slightly behind the paracone. The morphology of the paracone—median mure—mesoloph complex is unique to Noronhomys. The two posterior cusps, the hypocone and the metacone, are connected at the posterior margin of the molar. Unlike in most oryzomyines, no posteroflexus is present, so that the metacone is situated directly at the posterior margin.
Ecology
C. cailoi was found in association with remains of several other animals, including fishes, chelid turtles, frogs, birds, armadillos, and several rodents, including Reithrodon auritus, the coypu (Myocastor), both of which still live in the area, the extinct echimyid Dicolpomys, and unidentified caviids and octodontids. C. cailoi probably lived in a wetland habitat under relatively warm and moist climatic conditions.[7] Although the limited material known permits few inferences as to the animal's natural history, it likely fed on hard plant material, as do related extant species.
References
- Pardiñas, U.F.J. 2008. A new genus of oryzomyine rodent (Cricetidae: Sigmodontinae) from the Pleistocene of Argentina. Journal of Mammalogy 89(5):1270-1278.
- 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 196:1-149.