Holochilus primigenus
Encyclopedia
Holochilus primigenus is an extinct
oryzomyine
rodent
known from Pleistocene
deposits in Tarija Department
, southeastern Bolivia
. It is known from a number of isolated jaw
s 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 in doubt.
, but the animal was not described until 1996, when Scott Steppan formally named and diagnosed it and contrasted it to related species. The material Riggs collected includes nine mandible
s, three maxilla
e, and five isolated molars. The specific name Steppan gave to the animal, primigenus, means "primitive" in Latin and refers to the primitive features of the animal when compared to its relatives, Holochilus and Lundomys. In order to determine the relationships of his new species, Steppan carried out a cladistic
analysis, in which he also included the oryzomyines
Holochilus
, Lundomys, Pseudoryzomys, and Cerradomys
, as well as the non-oryzomyine Sigmodon. His results supported a close relation between H. primigenus and extant Holochilus, with Lundomys and Pseudoryzomys more distantly related.
In 1999, another rodent from the same group was described: Noronhomys, a recently extinct species from the Brazilian island of Fernando de Noronha
. Michael Carleton and Storrs Olson, who described the animal, argued that Holochilus primigenus was likely outside the clade
of Noronhomys and extant Holochilus and that H. primigenus should likely be excluded from the genus. When Ulyses Pardiñas described another extinct rodent from this group, Carletonomys
from the Pleistocene of Argentina, he suggested that H. primigenus should be placed in its own genus because of its mosaic of Holochilus- and Lundomys-like features.
, short incisive foramina
(perforations of the front part of the palate
) that barely extend between the first molars, and a short bony palate that hardly extends beyonds the third molars, all of which are shared with extant species of Holochilus
to the exclusion of Lundomys. Also unlike in Lundomys, there is an additional small root present at the labial (outer) side of the first upper molar. In eight specimens that could be measured, the length of the lower toothrow is 6.79 to 7.58 mm, averaging 7.28 mm; the first lower molar is 2.62 to 3.08 mm long, averaging 2.89 mm, and 1.75 to 1.93 mm broad, averaging 1.85 mm in seven measured teeth; the only preserved complete upper toothrow is 6.64 mm long; and the first upper molar is 2.63 to 2.70 mm long and 2.03 mm broad in two specimens.
dated to about 0.7 to 1.0 million years ago (chrons Clr.ln to early Cln, Ensenadan
South American Land Mammal Age). Other sigmodontine
rodents found there include Andinomys, Calomys, Kunsia
, Nectomys
, Oxymycterus
, Phyllotis
, and another akodontine
, probably Akodon
, Necromys
, or a related genus. The deposits were deposited by a river and the paleoenvironment was likely a floodplain or channel. Holochilus primigenus is not known from any other localities and is considered to be extinct.
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...
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...
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....
known from 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 ....
deposits in Tarija Department
Tarija Department
Tarija is a department in Bolivia. It is located in south-eastern Bolivia bordering Argentina to the south and Paraguay to the east. According to the 2001 census, it has a population of 391,226 inhabitants. It has an area of 37.623 km²...
, southeastern Bolivia
Bolivia
Bolivia officially known as Plurinational State of Bolivia , is a landlocked country in central South America. It is the poorest country in South America...
. It is known from a number of isolated jaw
Jaw
The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of...
s and molars
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"....
, 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
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...
traits of 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...
which document a closer relationship to living 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...
, 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
Carletonomys
Carletonomys cailoi is an extinct rodent from the Pleistocene 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...
, which may be more closely related to extant Holochilus than H. primigenus is, have cast its placement in Holochilus in doubt.
Taxonomy
Material of Holochilus primigenus was collected in 1924 and 1927 by Elmer Riggs of the Field Museum of Natural HistoryField Museum of Natural History
The Field Museum of Natural History is located in Chicago, Illinois, USA. It sits on Lake Shore Drive next to Lake Michigan, part of a scenic complex known as the Museum Campus Chicago...
, but the animal was not described until 1996, when Scott Steppan formally named and diagnosed it and contrasted it to related species. The material Riggs collected includes nine mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
s, three 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...
e, and five isolated molars. The specific name Steppan gave to the animal, primigenus, means "primitive" in Latin and refers to the primitive features of the animal when compared to its relatives, Holochilus and Lundomys. In order to determine the relationships of his new species, Steppan carried out a cladistic
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
analysis, in which he also included the oryzomyines
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...
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...
, Lundomys, Pseudoryzomys, and Cerradomys
Cerradomys
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...
, as well as the non-oryzomyine Sigmodon. His results supported a close relation between H. primigenus and extant Holochilus, with Lundomys and Pseudoryzomys more distantly related.
In 1999, another rodent from the same group was described: Noronhomys, a recently extinct species from the Brazilian 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...
. Michael Carleton and Storrs Olson, who described the animal, argued that Holochilus primigenus was likely outside the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
of Noronhomys and extant Holochilus and that H. primigenus should likely be excluded from the genus. When Ulyses Pardiñas described another extinct rodent from this group, Carletonomys
Carletonomys
Carletonomys cailoi is an extinct rodent from the Pleistocene 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...
from the Pleistocene of Argentina, he suggested that H. primigenus should be placed in its own genus because of its mosaic of Holochilus- and Lundomys-like features.
Description
Holochilus primigenus was a large rat, though smaller than both Lundomys and living Holochilus, characterized by an S-shaped ("sigmodont") crown on the third lower molar. Features of the molars are nearly indistinguishable from those of Lundomys and include cusps located opposite each other, enamel valleys that barely reach the midline of the molars, and short mesolophs and mesolophids (accessory crests). Other notable features include a robust mandible (lower jaw) with a steeply rising coronoid processCoronoid process of the mandible
The mandible's coronoid process is a thin, triangular eminence, which is flattened from side to side and varies in shape and size....
, short incisive foramina
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:...
(perforations of the front part of 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...
) that barely extend between the first molars, and a short bony palate that hardly extends beyonds the third molars, all of which are shared with extant species of 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...
to the exclusion of Lundomys. Also unlike in Lundomys, there is an additional small root present at the labial (outer) side of the first upper molar. In eight specimens that could be measured, the length of the lower toothrow is 6.79 to 7.58 mm, averaging 7.28 mm; the first lower molar is 2.62 to 3.08 mm long, averaging 2.89 mm, and 1.75 to 1.93 mm broad, averaging 1.85 mm in seven measured teeth; the only preserved complete upper toothrow is 6.64 mm long; and the first upper molar is 2.63 to 2.70 mm long and 2.03 mm broad in two specimens.
Distribution and ecology
Remains of Holochilus primigenus come from several localities with river sediments in Tarija Department, which have been paleomagneticallyPaleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
dated to about 0.7 to 1.0 million years ago (chrons Clr.ln to early Cln, 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). Other sigmodontine
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...
rodents found there include Andinomys, Calomys, Kunsia
Kunsia
Kunsia is the genus of giant rats. There are two South American species, both of which are classified as vulnerable by IUCN:*K. fronto *K. tomentosus...
, Nectomys
Nectomys
Nectomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is closely related to Amphinectomys and was formerly considered congeneric with Sigmodontomys...
, Oxymycterus
Oxymycterus
Oxymycterus is the genus of hocicudos. They are rat-like animals endemic to South America.Species listing:Oxymycterus is the genus of hocicudos. They are rat-like animals endemic to South America.Species listing:...
, Phyllotis
Phyllotis
Phyllotis is a genus of rodent in the family Cricetidae.It contains the following species:* Phyllotis alisosiensis* Friendly Leaf-eared Mouse * Andean Leaf-eared Mouse...
, and another akodontine
Akodontini
Akodontini is the second most speciose tribe of the subfamily Sigmodontinae. It includes at least 106 living species in 19 genera and is distributed mainly in the southern half of South America, with only two genera extending into Guyana and Venezuela . It also includes genera previously placed in...
, probably Akodon
Akodon
Akodon is a genus consisting of South American grass mice. They mostly occur south of the Amazon Basin and along the Andes north to Venezuela, but are absent from much of the basin itself, the far south of the continent, and the lowlands west of the Andes. Akodon is one of the most species-rich...
, Necromys
Necromys
Necromys is a genus of South American sigmodontine rodents allied to Akodon. This genus has also been known as Cabreramys or more recently Bolomys. N. urichi has recently been transferred from Akodon....
, or a related genus. The deposits were deposited by a river and the paleoenvironment was likely a floodplain or channel. Holochilus primigenus is not known from any other localities and is considered to be extinct.
Literature cited
- Carleton, M.D. and Olson, S.L. 1999. Amerigo Vespucci and the rat of Fernando de Noronha: a new genus and species of Rodentia (Muridae, Sigmodontinae) from a volcanic island off Brazil's continental shelf. American Museum Novitates 3256:1–59.
- 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.
- Steppan, S.J. 1996. A new species of Holochilus (Rodentia: Sigmodontinae) from the Middle Pleistocene of Bolivia and its phylogenetic significance (subscription required). Journal of Vertebrate Paleontology 16(3):522–530.
- Weksler, M., Percequillo, A.R. and Voss, R.S. 2006. Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae). American Museum Novitates 3537:1–29.