Brachytarsomys mahajambaensis
Encyclopedia
Brachytarsomys mahajambaensis is an extinct rodent
from northwestern Madagascar. It is known from nine isolated molars
found in several sites during field work that started in 2001. First described in 2010, it is placed in the genus Brachytarsomys
together with two larger living species, which may differ in some details of molar morphology. The presence of B. mahajambaensis, a rare element in the local rodent fauna, suggests that the region was previously more humid.
. The specific name, mahajambaensis, refers to Mahajamba Bay, which is close to the places where the species has been found. It is placed in the genus Brachytarsomys
, together with two larger living species, B. albicauda and B. villosa
. Brachytarsomys is classified in the exclusively Madagascan subfamily Nesomyinae
of the family Nesomyidae
, which includes various African rodents.
, including two first upper molars (M1), one of which is broken, two second upper molars (M2), one M3, two second lower molars (m2), and two m3. It is generally similar to the other two species, but smaller. There are a few differences in molar structure, which may not hold in larger samples.
Both the M1 and M2 have flat crowns. The M1 is convex on the lingual (inner) side and concave on the outer side, rendering it kidney-shaped. The anterocone, the cusp at the front of the tooth, appears not to be divided in two, as it is in living Brachytarsomys. It is narrowly connected to the protocone
, another cusp behind it on the labial side, which is connected in the same way to the paracone
. This lingually located cusp displays a broader connection to the hypocone
on the labial side, which is isolated from the metacone
lingually behind it. The metacone is broadly connected to another labial cusp behind it, referred to as the "post-hypocone", which it is expected to fuse to with increasing wear. Behind this pair of cusps, a small posteroloph (a crest at the back of the tooth) is present. In general, the back part of the tooth is more highly developed than in B. albicauda. The anterolingual cingulum on the M2, a crest on the front lingual corner, is absent or very small; it is well-developed in B. albicauda. The cusps form three transverse crests, with the labial cusp behind the lingual one. In one specimen, the second and third lophs are weakly connected. The posteroloph is more highly developed than on the M1. The M3 also has three such crests, which display narrow connections along the length of the tooth. In the first crest, the two cusps are about next to each other, but in the second one, the hypocone (on the labial side) is a bit behind the paracone. The post-hypocone and the metacone, in the third crest, are joined at the back. There are three roots, two at the front and one at the back, on the M1 and M2; the roots are missing from the only known M3.
The m1 is unknown. In m2, there are two well-developed transverse crests and one smaller one behind them. The first one consists of two cusps, the anteroconid (labial) and the metaconid (lingual) and the second one joins the protoconid at the labial side with the entoconid at the lingual side. The second crest is weakly joined to the third one, which joins the hypoconid at the labial side to the posteroconid at the back. In one specimen, a cingulum
(ridge) is present at the back of the tooth and a small cusp, the ectostylid, is also present. The m3 becomes narrower towards the back, rendering its form triangular, and contains three crests. The first one again consists of the anteroconid and the metaconid and the second of the protoconid and the entoconid. The third is smaller and consists of the hypoconid with the posteroconid in one specimen, but in the other the posteroconid is reduced to a narrow crest, the posterolophid. Unlike in B. albicauda, the hypoconid remains separate from the posterolophid and is not fused to it. Both m2 and m3 have two roots.
(126,000 to 10,000 years ago) and Holocene
(less than 10,000 years ago) in age. Brachytarsomys is a rare element of the rodent fauna, which is dominated by multiple species of Eliurus
and Macrotarsomys
. Modern Brachytarsomys are large rats that live in trees and eat fruits at middle to high altitudes. The modern, dry environment in northwestern Madagascar is decidedly inhospitable to these animals, and they no longer occur there; the former presence of B. mahajambaensis could indicate that the region was more humid in the past.
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 northwestern Madagascar. It is known from nine isolated 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"....
found in several sites during field work that started in 2001. First described in 2010, it is placed in the genus Brachytarsomys
Brachytarsomys
Brachytarsomys is a genus of rodent in the family Nesomyidae.It contains the following species:* White-tailed Antsangy * Brachytarsomys mahajambaensis * Hairy-tailed Antsangy...
together with two larger living species, which may differ in some details of molar morphology. The presence of B. mahajambaensis, a rare element in the local rodent fauna, suggests that the region was previously more humid.
Taxonomy
Remains of Brachytarsomys mahajambaensis were found during fieldwork in northwestern Madagascar that started in 2001. The species was described in a 2010 paper by Pierre Mein and colleagues, together with another extinct rodent, Nesomys narindaensisNesomys narindaensis
Nesomys narindaensis is an extinct rodent that lived in northwestern Madagascar. It is known from subfossil skull bones and isolated molars found in several sites during field work that started in 2001. First described in 2010, it is placed in the genus Nesomys together with three smaller living...
. The specific name, mahajambaensis, refers to Mahajamba Bay, which is close to the places where the species has been found. It is placed in the genus Brachytarsomys
Brachytarsomys
Brachytarsomys is a genus of rodent in the family Nesomyidae.It contains the following species:* White-tailed Antsangy * Brachytarsomys mahajambaensis * Hairy-tailed Antsangy...
, together with two larger living species, B. albicauda and B. villosa
Brachytarsomys villosa
Brachytarsomys villosa is a species of rodent in the family Nesomyidae.It is found only in Madagascar....
. Brachytarsomys is classified in the exclusively Madagascan subfamily Nesomyinae
Nesomyinae
The Malagasy rats and mice are the sole members of the subfamily Nesomyinae. These animals are the only native rodents of Madagascar, come in many shapes and sizes, and occupy a wide variety of ecological niches. There are nesomyines that resemble gerbils, rats, mice, voles, and even rabbits...
of the family Nesomyidae
Nesomyidae
Nesomyidae is a family of African rodents in the large and complex superfamily Muroidea. It includes several subfamilies, all of which are native to either continental Africa or to Madagascar...
, which includes various African rodents.
Description
Brachytarsomys mahajambaensis is known from nine isolated molarsMolar (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"....
, including two first upper molars (M1), one of which is broken, two second upper molars (M2), one M3, two second lower molars (m2), and two m3. It is generally similar to the other two species, but smaller. There are a few differences in molar structure, which may not hold in larger samples.
Both the M1 and M2 have flat crowns. The M1 is convex on the lingual (inner) side and concave on the outer side, rendering it kidney-shaped. The anterocone, the cusp at the front of the tooth, appears not to be divided in two, as it is in living Brachytarsomys. It is narrowly connected to 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...
, another cusp behind it on the labial side, which is connected in the same way to 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....
. This lingually located cusp displays a broader connection to 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...
on the labial side, which is isolated from 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...
lingually behind it. The metacone is broadly connected to another labial cusp behind it, referred to as the "post-hypocone", which it is expected to fuse to with increasing wear. Behind this pair of cusps, a small posteroloph (a crest at the back of the tooth) is present. In general, the back part of the tooth is more highly developed than in B. albicauda. The anterolingual cingulum on the M2, a crest on the front lingual corner, is absent or very small; it is well-developed in B. albicauda. The cusps form three transverse crests, with the labial cusp behind the lingual one. In one specimen, the second and third lophs are weakly connected. The posteroloph is more highly developed than on the M1. The M3 also has three such crests, which display narrow connections along the length of the tooth. In the first crest, the two cusps are about next to each other, but in the second one, the hypocone (on the labial side) is a bit behind the paracone. The post-hypocone and the metacone, in the third crest, are joined at the back. There are three roots, two at the front and one at the back, on the M1 and M2; the roots are missing from the only known M3.
| Species | M1 | M2 | M3 | m1 | m2 | m3 |
|---|---|---|---|---|---|---|
| B. albicauda | 3.5 x 2.38 | 2.75 x 2.27 | 2.08 x 1.94 | 3.04 x 2.03 | 2.74 x 2.24 | 2.46 x 2.03 |
| B. mahajambaensis | 2.73 x 1.79, – x 1.87 |
2.27 x 1.90, 2.43 x 2.02 |
1.78 x 1.75 | – | 2.41 x 1.93, 2.41 x 1.98 |
1.89 x 1.57, 2.23 x 1.76 |
The m1 is unknown. In m2, there are two well-developed transverse crests and one smaller one behind them. The first one consists of two cusps, the anteroconid (labial) and the metaconid (lingual) and the second one joins the protoconid at the labial side with the entoconid at the lingual side. The second crest is weakly joined to the third one, which joins the hypoconid at the labial side to the posteroconid at the back. In one specimen, a cingulum
Cingulum (tooth)
In dentistry, cingulum refers to an anatomical feature of the anterior teeth . It refers to the portion of the teeth, occurring on the lingual or palatal aspects, that forms a convex protuberance at the cervical third of the anatomic crown. It represents the lingual or palatal developmental lobe...
(ridge) is present at the back of the tooth and a small cusp, the ectostylid, is also present. The m3 becomes narrower towards the back, rendering its form triangular, and contains three crests. The first one again consists of the anteroconid and the metaconid and the second of the protoconid and the entoconid. The third is smaller and consists of the hypoconid with the posteroconid in one specimen, but in the other the posteroconid is reduced to a narrow crest, the posterolophid. Unlike in B. albicauda, the hypoconid remains separate from the posterolophid and is not fused to it. Both m2 and m3 have two roots.
Distribution and ecology
Teeth of Brachytarsomys mahajambaensis have been found at the sites of Antsingiavo, Belobaka, and Ambatomainty in northwestern Madagascar, which are late PleistocenePleistocene
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 ....
(126,000 to 10,000 years ago) and Holocene
Holocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
(less than 10,000 years ago) in age. Brachytarsomys is a rare element of the rodent fauna, which is dominated by multiple species of Eliurus
Eliurus
Eliurus is a genus of rodent in the family Nesomyidae.It contains the following species:* Tsingy Tufted-tailed Rat * Ankarana Special Reserve Tufted-tailed Rat...
and Macrotarsomys
Macrotarsomys
Macrotarsomys is a genus of rodent in the family Nesomyidae.It contains the following species:Genus Macrotarsomys - Big-footed mice*Bastard Big-Footed Mouse, Macrotarsomys bastardi Milne-Edwards and G. Grandidier, 1898...
. Modern Brachytarsomys are large rats that live in trees and eat fruits at middle to high altitudes. The modern, dry environment in northwestern Madagascar is decidedly inhospitable to these animals, and they no longer occur there; the former presence of B. mahajambaensis could indicate that the region was more humid in the past.
Literature cited
- Mein, P., Sénégas, F., Gommery, D., Ramanivoso, B., Randrianantenaina, H. and Kerloc'h, P. 2010. Nouvelles espèces subfossiles de rongeurs du Nord-Ouest de Madagascar (subscription required). Comptes Rendus Palevol 9(3):101–112 (in French, with abridged English version).
- 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, 2 vols., 2142 pp. ISBN 978-0-8018-8221-0