Carletonomys
Encyclopedia
Carletonomys cailoi is an extinct
rodent
from the Pleistocene
of Buenos Aires Province
, Argentina
. Although known only from a single maxilla
(upper jaw) 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
.
The single known molar is high-crowned (hypsodont
) and flat-crowned (planar) and is distinctive in lacking the ridge that connects the front to the middle part of the molar, the anterior mure, and in the configuration of another ridge, the mesoloph. Carletonomys was probably herbivorous
and lived in a wet habitat.
deposit in San Cayetano Partido
, southeastern Buenos Aires Province
. The stratigraphic
context suggests this locality is slightly over 1 million years old (Ensenadan
South American Land Mammal Age), making Carletonomys the oldest known oryzomyine. The single known specimen is now in the collections of the Museo de La Plata. It was initially referred to the genus Noronhomys, which is currently 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 of rodent in a publication in the Journal of Mammalogy
. The generic name, Carletonomys, combines the name of American mammalogist Michael Carleton with the Ancient Greek
μυς mys "mouse" and the specific name, cailoi, honors Argentinean biologist Carlos "Cailo" Galliari.
The fossil has a number of features that suggest a relation to a group of oryzomyine
rodents that includes the South American marsh rat Holochilus
, its living relatives Lundomys and Pseudoryzomys, and the extinct Noronhomys and Holochilus primigenus
. They share high-crowned (hypsodont
) molars and several simplifications of molar morphology, as well as other features that cannot be assessed in Carletonomys, which indicate specializations towards a semiaquatic lifestyle. 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.
This group of genera encompasses only a small part of the diversity of the tribe Oryzomyini
, a group of over a hundred species distributed mainly in South America, including nearby islands such as the Galápagos Islands
and some of the Antilles
. Oryzomyini is 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.
(upper jaw) with the upper first molar
(M1) in it. It is broken off behind the M1, but much of the front part is preserved, including the zygomatic plate
, the flattened front portion of the zygomatic arch
(cheekbone). The M1 is moderately worn, indicating that it is 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
and "Ekbletomys". 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 presence of a second labial root is a variable character among oryzomyines, occurring among others in Holochilus and Pseudoryzomys but not in Lundomys. The maxilla itself shows few significant characters. The back margin of the incisive foramen
, which perforates the palate
between the upper incisor
s and the molars, is not visible, suggesting that the foramen was short, as in Holochilus. The configuration of the zygomatic plate shows features that distinguish C. cailoi from some of its relatives.
The molar is plane and hypsodont: the crowns are relatively high and the main cusps are about as high as the other parts of the crown, as they are 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. As in closely related species, the front 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 front 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 back pair of cusps, is attached to the back of the paracone. A complete mesoloph is present, descending from the median mure slightly behind the paracone. The configuration of the paracone–median mure–mesoloph complex is unique to Carletonomys. The two posterior cusps, the hypocone
and the metacone
, are connected at the back margin of the molar. Unlike in most oryzomyines, no posteroflexus is present, so that the metacone is situated directly at the back margin.
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. 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, morphologically similar extant species.
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
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....
from the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
of 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
Argentina
Argentina , officially the Argentine Republic , is the second largest country in South America by land area, after Brazil. It is constituted as a federation of 23 provinces and an autonomous city, Buenos Aires...
. Although known only from a single 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...
(upper jaw) with the first 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"....
, its features are so distinctive that it is placed in its own 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...
, Carletonomys. Discovered in 1998 and formally described in 2008, it is part of a well-defined group of 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...
rodents that also includes 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...
, Noronhomys, Lundomys, and Pseudoryzomys. This group is characterized by progressive semiaquatic specializations and a reduction in the complexity of molar 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....
.
The single known molar is high-crowned (hypsodont
Hypsodont
Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...
) and flat-crowned (planar) and is distinctive in lacking the ridge that connects the front to the middle part of the molar, the anterior mure, and in the configuration of another ridge, the mesoloph. Carletonomys was probably herbivorous
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
and lived in a wet habitat.
Taxonomy
Carletonomys cailoi was discovered in 1998 in a siltSilt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
deposit in San Cayetano Partido
San Cayetano Partido
San Cayetano Partido is a partido of Buenos Aires Province in Argentina.The provincial subdivision has a population of about inhabitants in an area of 2,757.5 km², and its capital city is San Cayetano, which is located around 628 km from Buenos Aires....
, southeastern 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...
. The stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
context suggests this locality is slightly over 1 million years old (Ensenadan
Ensenadan
The Ensenadan age is a period of geologic time within the Early Pleistocene epoch of the Neogene used more specifically with South American Land Mammal Ages. It follows the Uquian and precedes the Lujanian age....
South American Land Mammal Age), making Carletonomys the oldest known oryzomyine. The single known specimen is now in the collections of the Museo de La Plata. It was initially referred to the genus Noronhomys, which is currently known only from the island of Fernando de Noronha
Fernando de Noronha
Fernando de Noronha is an archipelago of 21 islands and islets in the Atlantic Ocean, offshore from the Brazilian coast. The main island has an area of and had a population of 3,012 in the year 2010...
off northeastern 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...
, but in 2008 Argentinean mammalogist Ulyses Pardiñas established it as the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
of a new genus and species of rodent in a publication in the Journal of Mammalogy
Journal of Mammalogy
The Journal of Mammalogy is the flagship publication of the American Society of Mammalogists. Both the society and the journal were founded in 1919. The peer-reviewed journal publishes papers about mammals throughout the world and their conservation...
. The generic name, Carletonomys, combines the name of American mammalogist Michael Carleton with the Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
μυς mys "mouse" and the specific name, cailoi, honors Argentinean biologist Carlos "Cailo" Galliari.
The fossil has a number of features that suggest a relation to a group of 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...
rodents that includes the South American marsh rat 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...
, its living relatives Lundomys and Pseudoryzomys, and the extinct Noronhomys and Holochilus primigenus
Holochilus primigenus
Holochilus 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...
. They share high-crowned (hypsodont
Hypsodont
Hypsodont dentition is characterized by high-crowned teeth and enamel which extends past the gum line. This provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cows, horses and deer; all animals that feed on gritty, fibrous material. The opposite...
) molars and several simplifications of molar morphology, as well as other features that cannot be assessed in Carletonomys, which indicate specializations towards a semiaquatic lifestyle. 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.
This group of genera encompasses only a small part of the diversity of the tribe Oryzomyini
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...
, a group of over a hundred species distributed mainly in South America, including nearby islands such as the Galápagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
and some of the Antilles
Antilles
The Antilles islands form the greater part of the West Indies in the Caribbean Sea. The Antilles are divided into two major groups: the "Greater Antilles" to the north and west, including the larger islands of Cuba, Jamaica, Hispaniola , and Puerto Rico; and the smaller "Lesser Antilles" on the...
. Oryzomyini is 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.
Description
The holotype is a right maxillaMaxilla
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...
(upper jaw) with the upper first 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"....
(M1) in it. It is broken off behind the M1, but much of the front part is preserved, including 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...
, the flattened front portion 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...
(cheekbone). The M1 is moderately worn, indicating that it is 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
Megalomys
Megalomys is a genus of rodent in the family Cricetidae, part of the tribe Oryzomyini. The genus contains four large rodents from various Caribbean islands, all of which are now extinct. The last species to survive was M...
and "Ekbletomys". 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 presence of a second labial root is a variable character among oryzomyines, occurring among others in Holochilus and Pseudoryzomys but not in Lundomys. The maxilla itself shows few significant characters. The back margin of the incisive foramen
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 perforates the palate
Palate
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...
between the upper 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, is not visible, suggesting that the foramen was short, as in Holochilus. The configuration of the zygomatic plate shows features that distinguish C. cailoi from some of its relatives.
The molar is plane and hypsodont: the crowns are relatively high and the main cusps are about as high as the other parts of the crown, as they are 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. As in closely related species, the front 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 front 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
Paracone
An atmospheric reentry or spaceflight mission abort concept using an inflatable cone.A notable feature of the paracone concept is that it facilitates an abort throughout the entire flight profile....
and the protocone
Protocone
thumb|500px|right|Right upper molar showing the four main upper molars cusps.The protocone is a cusp of the molars of the upper dentition in Placental and Marsupial vertebrates .It is found at the mesiolingual area of the tooth...
, are broadly connected. The median mure, which connects the middle to the back pair of cusps, is attached to the back of the paracone. A complete mesoloph is present, descending from the median mure slightly behind the paracone. The configuration of the paracone–median mure–mesoloph complex is unique to Carletonomys. The two posterior cusps, the hypocone
Hypocone
The hypocone is the name for a main cusp found on the molars of the upper dentition of hominids. It is found on the distal lingual side of the tooth. It fits into the grooves of the lower dentition and is an adaptation for the overall grinding and tearing of foods using the occlusal of the tooth...
and the metacone
Metacone
thumb|500px|left|Right upper molar showing the four main upper molars cusps.The metacone is a cusp on the molars of the upper dentition in hominids. It is found at the buccal distal area of the tooth...
, are connected at the back margin of the molar. Unlike in most oryzomyines, no posteroflexus is present, so that the metacone is situated directly at the back margin.
Ecology
Carletonomys was found in association with remains of several other animals, including fishes, chelidChelidae
The Chelidae are one of the three living families of the turtle suborder Pleurodira and are commonly called the Austro-South American Side Neck turtles. The Family is distributed in Australia, New Guinea, parts of Indonesia and throughout most of South America. It is a large family of turtles with...
turtles, frogs, birds, armadillos, and several rodents, including Reithrodon auritus, the coypu
Coypu
The coypu , , also known as the river rat, and nutria, is a large, herbivorous, semiaquatic rodent and the only member of the family Myocastoridae. Originally native to subtropical and temperate South America, it has since been introduced to North America, Europe, Asia, and Africa, primarily by...
(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
Wetland
A wetland is an area of land whose soil is saturated with water either permanently or seasonally. Wetlands are categorised by their characteristic vegetation, which is adapted to these unique soil conditions....
habitat under relatively warm and moist climatic conditions. 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, morphologically similar extant species.
Literature cited
- 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, ISBN 978-0-8018-8221-0
- 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.
- Ray, C.E. 1962. The Oryzomyine Rodents of the Antillean Subregion. Doctor of Philosophy thesis, Harvard University, 211 pp.
- 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.
- 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.