Megalomys audreyae, known as the Barbudan (?) Muskrat or the Barbuda giant rice-rat, is an 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....

 from Barbuda

Barbuda

Barbuda is an island in the Eastern Caribbean, and forms part of the state of Antigua and Barbuda. It has a population of about 1,500, most of whom live in the town of Codrington.-Location:...

 in the Lesser Antilles. Described on the basis of a single mandible

Mandible

The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

 (lower 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"....

 missing and an isolated upper incisor

Incisor

Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...

, both of uncertain but Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

 age, it is one of the smaller members of the genus 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...

. Little is known about the animal, and its provenance and distinction from "Ekbletomys hypenemus", an even larger extinct oryzomyine that also occurred on Barbuda, have been called into question. The toothrow in the lower jaw has a length of 8.7 mm at the alveoli

Dental alveolus

Dental alveolus are sockets in the jaws in which the roots of teeth are held in the alveolar process of maxilla with the periodontal ligament. The lay term for dental alveoli is tooth sockets...

. The third molar is relatively narrow and both the second and third molars have a wide valley between their outer cusps.

History

Remains of Megalomys audreyae were found by John Walter Gregory

John Walter Gregory

John Walter Gregory, FRS, was a British geologist and explorer, known principally for his work on glacial geology and on the geography and geology of Australia and East Africa.-Early life:...

 among cave breccia

Breccia

Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....

 on Barbuda around 1900. The exact locality is unknown. In his 1901 description of Oryzomys luciae, Charles Immanuel Forsyth Major

Charles Immanuel Forsyth Major

Charles Immanuel Forsyth Major was a Swiss zoologist and vertebrate palaeontologist.Major was born in Glasgow and studied at Basel and Zurich Universities in Switzerland and later Göttingen in Germany...

 mentioned the Barbuda animal as another member of the 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...

group, but he never published a description of the latter. Édouard Louis Trouessart

Édouard Louis Trouessart

Édouard Louis Trouessart was a French zoologist. He discovered the dust mite....

 gave the name Oryzomys (Megalomys) majori to it in his Catalogus Mammalium, but he did not describe it and therefore the name is a nomen nudum

Nomen nudum

The phrase nomen nudum is a Latin term, meaning "naked name", used in taxonomy...

. In 1926, Arthur Hopwood finally described it and named it Megalomys audreyae after Gregory's wife Audrey, following Major's intention.

The oryzomyines of the Caribbean were revised in 1962 by Clayton Ray, who examined the specimens Gregory had found and redescribed them. He suggested that M. audreyae may in fact have come from Barbados

Barbados

Barbados is an island country in the Lesser Antilles. It is in length and as much as in width, amounting to . It is situated in the western area of the North Atlantic and 100 kilometres east of the Windward Islands and the Caribbean Sea; therein, it is about east of the islands of Saint...

 instead of similarly named Barbuda, citing the occurrence of a different oryzomyine ("Ekbletomys hypenemus") in other cave deposits on Barbuda, circumstantial evidence for the occurrence of a native rodent on Barbados, uncertainty whether Gregory ever visited Barbuda, and biogeographical

Biogeography

Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...

 considerations.

In the subsequent literature, M. audreyae has seldom been mentioned and never been further described. In a 1999 review of recent extinctions in mammals, Ross MacPhee and C. Flemming reported that M. audreyae had been recovered from a locality on Barbuda known as Darby Sink, which had been radiocarbon dated

Radiocarbon dating

Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...

 to around 1200 CE. They also stated that M. audreyae and "Ekbletomys" may in fact be identical. However, in 2009 Samuel Turvey suggested that two different rice rats were in fact present in material from Barbuda, which would imply that M. audreyae is a valid species.

Description

The only remains of Megalomys audreyae that have been described in the literature are the original two specimens Gregory found, a left upper incisor

Incisor

Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...

 and a left mandible

Mandible

The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...

 (lower jaw). The upper incisor is not grooved and its diameter has a length of 2.6 mm and width of 1.5 mm, but exhibits no other significant characters.

The mandible, which is severely damaged and lacks the condyloid

Condyloid process

The condyloid process is part of the mandible and is thicker than the coronoid, and consists of two portions: the condyle, and the constricted portion which supports it, the neck.-Condyle :...

, coronoid

Coronoid 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....

, and angular processes at the back of the bone, contains the second and third 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"....

 and part of the lower incisor, but the first molar is missing. The capsular process

Capsular process

In rodents, the capsular process or projection is a bony capsule that contains the root of the lower incisor. It is visible on the labial side of the mandible as a raising in the bone...

 of the lower incisor, a slight raising of the mandibular bone at the back end of the incisor, is small. The preserved alveoli

Dental alveolus

Dental alveolus are sockets in the jaws in which the roots of teeth are held in the alveolar process of maxilla with the periodontal ligament. The lay term for dental alveoli is tooth sockets...

, the impressions left by the roots, show that the first molar was supported by large roots at the front and back and a smaller root in between these. The second molar is about square and shows the four main cusps commonly present in 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....

s: the protoconid, metaconid, hypoconid, and entoconid. A strongly developed mesolophid (a crest) is also present, as in most oryzomyines. The main valley between the cusps, the hypoflexid, is broad and V-shaped. The third molar is as long as the second, but it is narrower and the entoconid is poorly developed. Again, the hypoflexid is broad and V-shaped. The length of the toothrow at the alveoli is 8.7 mm. The length of the second molar is 2.5 mm and the width is 2.2 mm. The third molar has a length of 2.5 mm and width of 1.8 mm.

When Clayton Ray described "Ekbletomys hypenemus" on the basis of abundant skeletal remains from both Barbuda and Antigua, he carefully distinguished it from M. audreyae, the only other native rodent recorded from those islands. M. audreyae is much smaller than "Ekbletomys"; for example, 72 specimens of the latter had the alveolar length of the lower molars ranging from 10.3 to 12.6 mm (mean 11.6 mm, standard deviation 0.49 mm; compare 8.7 mm for M. audreyae). In addition, the V-shaped hypoflexids and narrow third molar of M. audreyae contrast with the narrow, parallel-sided hypoflexids and broad third molar of "Ekbletomys". These characters, and others observable in species of Megalomys represented by more complete material, convinced Ray that M. audreyae and "Ekbletomys" are not only distinct species, but indeed share no close relationship. Instead, he proposed that the combination of large size, occurrence in the Lesser Antilles, and similarity in molar morphology indicated a relationship between M. audreyae and other species of Megalomys, and he suggested that the similarly sized M. curazensis

Megalomys curazensis

Megalomys curazensis is a species of rodent from the Pleistocene of the island of Curaçao, off northwestern Venezuela. It is a member of the genus Megalomys, which also includes species from other islands of the Lesser Antilles. It is known from abundant but fragmentary material found throughout...

from Curaçao

Curaçao

Curaçao is an island in the southern Caribbean Sea, off the Venezuelan coast. The Country of Curaçao , which includes the main island plus the small, uninhabited island of Klein Curaçao , is a constituent country of the Kingdom of the Netherlands...

, off Venezuela, may be most closely related to M. audreyae.

Literature cited

• Hopwood, A.T. 1926. A fossil rice-rat from the Pleistocene of Barbuda. Annals and Magazine of Natural History (9)17:328–330.
• MacPhee, R.D.E. and Flemming, C. 1999. Requiem Æternam: The last five hundred years of mammalian species extinctions. Pp. 333–371 in MacPhee, R.D.E. (ed.). Extinctions in Near Time: Causes, Contexts, and Consequences. New York: Plenum Press, 384 pp. ISBN 978-0-306-46092-0
• Ray, C.E. 1962. The Oryzomyine Rodents of the Antillean Subregion. Doctor of Philosophy thesis, Harvard University, 211 pp.
• Turvey, S.T. 2009. Holocene Extinctions. Oxford University Press US, 359 pp. ISBN 9780199535095