Mesopropithecus is an extinctgenus of small to medium-sizedlemur, orstrepsirrhineprimate, fromMadagascar that includes three species,M. dolichobrachion,M. globiceps, andM. pithecoides. Together withPalaeopropithecus,Archaeoindris, andBabakotia, it is part of thesloth lemur family (Palaeopropithecidae). Once thought to be anindriid because itsskull is similar to that of livingsifakas, a recently discoveredpostcranial skeleton showsMesopropithecus had longer forelimbs thanhindlimbs—a distinctive trait shared by sloth lemurs but not by indriids. However, as it had the shortestforelimbs of all sloth lemurs, it is thought thatMesopropithecus was morequadrupedal and did not usesuspension as much as the other sloth lemurs.
All three species ate leaves, fruits, and seeds, but the proportions were different.M. pithecoides was primarily a leaf-eater (folivores), but also ate fruit and occasionally seeds.M. globiceps ate a mix of fruits and leaves, as well as a larger quantity of seeds thanM. pithecoides.M. dolichobrachion also consumed a mixed diet of fruits and leaves, but analysis of its teeth suggests that it was more of aseed predator than the other two species.
Although rare, the three species were widely distributed across the island yetallopatric to each other, withM. dolichobrachion in the north,M. pithecoides in the south and west, andM. globiceps in the center of the island.M. dolichobrachion was the most distinct of the three species due to its longer arms.Mesopropithecus was one of the smallest of the known extinctsubfossil lemurs, but was still slightly larger than the largest living lemurs. Known only fromsubfossil remains, it died out after the arrival of humans on the island, probably due to hunting pressure andhabitat destruction.
Mesopropithecus was named in 1905 byHerbert F. Standing using four skulls found at Ampasambazimba. He noted that the animal had characteristics of bothPalaeopropithecus and the livingsifakas (Propithecus).[5] In 1936, Charles Lamberton definedNeopropithecus globiceps (based on one skull from Tsirave) andN. platyfrons (based on two skulls from Anavoha). He thought thatNeopropithecus was a separate, intermediate genus betweenMesopropithecus andPropithecus. In 1971,paleoanthropologistIan Tattersall mergedN. platyfrons intoN. globiceps andNeopropithecus intoMesopropithecus.[2]
Until 1986,Mesopropithecus was only known fromcranial (skull) remains from central and southern Madagascar, and because these are similar to teeth and skulls of livingindriids, particularly those ofVerreaux's sifaka (Propithecus verreauxi),Mesopropithecus was often assigned to the family Indriidae.[1][6][7] For example, in 1974, Tattersall and Schwartz labeledMesopropithecus as asister group to sifakas.[3][2] With the discovery of an associated skeleton ofM. dolichobrachion near Ankarana in 1986, it became clear thatMesopropithecus shared distinct traits with sloth lemurs.[3][6][8][9] Unlike the indriids, but like the sloth lemurs, they had elongated forelimbs and other adaptations for arborealsuspension (hanging in trees), linking them most closely to family Paleaeopropithecidae.[1] A comparison of these morphological traits between the sloth lemurs and indriids suggest thatMesopropithecus was the first genus to diverge within the sloth lemur family.[3]
Three species are recognized withinMesopropithecus:[2]
M. pithecoides, described in 1905, was the first species to be formally named.[1] Itsspecific name,pithecoides, derives from the Greek wordpithekos, meaning "monkey" or "ape", and the Greek suffix-oides, meaning "like" or "form", and reflects Standing's impression that the animal resembled monkeys in form.[5][10][11] It was a small to medium-sized lemur,[12] weighing approximately 10 kg (22 lb) and having anintermembral index (ratio of limb proportions) of 99.[3] Its skull was similar to that ofM. globiceps, but had a broader snout and was more robust, particularly in itssagittal andnuchal crests (ridges on the skull for muscle attachments) and massivezygomatic arches (cheekbones).[3][1] Its skull length averaged 98 mm (3.9 in),[3] ranging from 94.0 to 103.1 mm (3.70 to 4.06 in).[7] It was predominantlyfolivorous (leaf-eating), but also consumed some fruit and (rarely) seeds.[13][14] It was moderately abundant on the high,central plateau of Madagascar.[12][13][15] It shared its range with the larger sloth lemurs,Palaeopropithecus maximus andArchaeoindris fontoynontii.[15] One sample of its subfossil remains has beenradiocarbon dated, yielding a date of 570–679CE.[3]
M. globiceps was discovered in 1936 and originally classified in its own genus,Neopropithecus.[1] The nameglobiceps comes from its prominent forehead[16] and derives from theLatin wordglobus, meaning "ball", and theNeo-Latin suffix-ceps, meaning "head".[17][18] LikeM. pithecoides, it was a small to medium-sized lemur,[12] weighing approximately 11 kg (24 lb) and having an intermembral index of 97.[3] It had the most narrow snout and gracile skeleton of theMesopropithecus species, similar to but smaller thanM. pithecoides, making it more like the living sifakas.[3][1] Its teeth were similar to but larger than those of living sifakas, except for its lower premolars, which were shorter, and the M3 (third upper molar), which was moderately constricted by the cheek and tongue. Its skull length averaged 94 mm (3.7 in),[3] ranging from 93.4 to 94.8 mm (3.68 to 3.73 in).[7] It was a mixed feeder, eating fruit, leaves, and a moderate amount of seeds,[13] having a diet similar to that of the livingindri (Indri indri).[14] Although its forelimbs were more like those of living indriids, its hindlimbs andaxial skeleton (skull, spine, and ribs) were more specialized for suspension, as inPalaeopropithecus andBabakotia.[3] It was found in the south and west of Madagascar.[15] Three samples of its subfossil remains have been radiocarbon dated, yielding dates of 354–60 BCE, 58–247 CE, and 245–429 CE.[3]
M. dolichobrachion was discovered in 1986 and formally described in 1995. It was found in the caves ofAnkarana, northern Madagascar, around the same time that the first remains ofBabakotia were unearthed.[7] The species namedolichobrachion is Greek, coming fromdolicho- ("long") andbrachion ("arm"), and means "long-armed".[7][19][20] It was a medium-sized lemur,[12] slightly larger than the other two members of its genus,[7] weighing approximately 14 kg (31 lb).[3] It differed significantly from the other two in its limb proportions and itspostcranial morphology.[7][15] Most notably, it was the only species in the genus to haveforelimbs that were longer than thehindlimbs, due to a substantially longer and more robusthumerus (yielding an intermembral index of 113), as well as more curvedphalanges (finger and toe bones).[3][7][21][22] For these reasons, it is thought to have been moresloth-like in its use of suspension.[3][12][21] This was further supported by a study of a singlelumbar vertebra. This vertebra was similar to that ofBabakotia in having a moderately reduced, dorsally orientedspinous process and a transverse processes (plates of bone that protrude from thevertebrae) that points to the side (laterally). The vertebra was intermediate in length when compared with other sloth lemurs, and itslaminae (two plates of bone that connect to the spinous process) were not as broad as seen inPalaeopropithecus.[23] InM. dolichobrachion, skull length averaged 102 mm (4.0 in),[3] ranging from 97.8 to 105.5 mm (3.85 to 4.15 in).[7] The only notable difference from the two other species in its teeth was that the third upper molar had a relatively wider trigon and smaller talon (groups ofcusps on the molar teeth).[3] It was a mixed feeder, eating leaves, fruits, and seeds.[13][14] This species was more of aseed predator than the other two species, but was not as specialized as closely relatedBabakotia radofilai.[14]M. dolichobrachion was rare[12] and shared its range with two other sloth lemurs,Babakotia radofilai andPalaeopropithecus maximus.[2][15] It was the most distinct member of its genus and was geographically restricted to the extreme north of the island.[3]
The genusMesopropithecus includes some of the smallest of the recently extinctsubfossil lemurs, but all species were still noticeably larger than all living (extant) lemurs. They ranged in weight from 10 to 14 kg (22 to 31 lb).[3][1][13] They were also the least specialized of the sloth lemurs, more closely resembling living indriids in both skull and postcranial characteristics.[15] Skull length ranged from 93.4 to 105.5 mm (3.68 to 4.15 in).[7] Thedentition and cranial proportions, however, more closely resembled those of the sifakas.[1] Thedental formula ofMesopropithecus was the same as in the other sloth lemur and indriids: either2.1.2.31.1.2.3[1][6] or2.1.2.32.0.2.3 × 2 = 30.[2]Mesopropithecus had a four-toothedtoothcomb, like all indriids and most other sloth lemurs.[3][9] It is unclear whether one of thepermanent teeth in the toothcomb is anincisor orcanine, resulting in the two conflicting dental formulae.[26] Like other sloth lemurs and indriids,Mesopropithecus had rapidtooth development.[3]
Despite the similarities, there are several features that distinguishMesopropithecus skulls from those of living indriids. The skull, including the zygomatic arch, is more robustly built. Thetemporal lines join together anteriorly into a sagittal crest and there is a distinct nuchal ridge that joins the rear of the zygomatic arch. The skull has a more roundedbraincase, slightly smaller and more convergentorbits, more pronouncedpostorbital constriction (narrowing of the skull behind the eye sockets), more robustpostorbital bar (bone that encircles the eye socket), a steeper facial angle, more robust and cranially convex zygomatic bone, and a broader, squared snout. The upperincisors andcanines are larger.[3][1][2][7] The more robustmandible (lower jaw) andmandibular symphysis (point where the two halves of the lower jaw meet) suggest a more folivorous diet, which requires extra grinding. The orbits are as large (in absolute size) as those in smaller living indriids,[15] which suggests lowvisual acuity.[27]Mesopropithecus and its closest sloth lemur relative,Babakotia, did share a few ancestral traits with indriids, unlike the largest sloth lemurs,Palaeopropithecus andArchaeoindris. These include the aforementioned four-toothed toothcomb, an inflatedauditory bulla (bony structure that encloses part of the middle and inner ear), and an intrabullarectotympanic ring (bony ring that holds the eardrum).[3]
While the skull ofMesopropithecus most closely resembles that of modern sifakas, the postcranial skeleton is quite different. Rather than having elongated hindlimbs forleaping,Mesopropithecus had elongatedforelimbs, suggesting they predominantly usedquadrupedal locomotion, slow climbing, with some forelimb andhindlimb suspension.[1][8][9][13] In fact, they were the most quadrupedal of the sloth lemurs,[13][15][21] having an intermembral index between 97 and 113, compared to the lower value for indriids and higher values for the other sloth lemurs.[3][15] (In arboreal primates, an intermembral index of 100 predicts quadrupedalism, higher values predict suspensory behavior, and lower values predict leaping behavior.)[28] Wrist bones found in 1999 further demonstrate thatMesopropithecus was a vertical climber[29] and the mostloris-like of the sloth lemurs.[9] Analysis of a lumbar vertebra ofM. dolichobrachion further supported this conclusion.[23]
Our understanding of the morphology ofMesopropithecus has not always been so complete. Until recently, important pieces of the skeleton had not been discovered, including theradius,ulna,vertebrae, hand and foot bones, and thepelvis. In 1936, Alice Carleton mistakenly associated postcranial remains of thediademed sifaka (Propithecus diadema) from Ampasambazimba withMesopropithecus pithecoides and came to the false conclusion that its morphology was like that of a monkey. This mistaken attribution was corrected in 1948 by Charles Lamberton.[9]
Mesopropithecus species appear to have been generally rare within their wide range. Collectively, the three species have been found in the north, south, west, and center of Madagascar,[15] although they appear to have been geographically separated (allopatric) from each other.[2]Subfossil discoveries indicate that they lived in the same region (sympatric) with other sloth lemurs in the north and center of Madagascar.[15] The subfossil remains ofM. globiceps have been found at sevensubfossil sites on Madagascar:Anavoha,Ankazoabo Cave,Belo sur Mer,Manombo-Toliara,Taolambiby,Tsiandroina,Tsirave.[3] The subfossil remains of bothM. pithecoides andM. dolichobrachion have only been found at one site each,Ampasambazimba andAnkarana respectively.[3]
M. pithecoides from the central plateau was a specialized leaf-eater (folivore), but the other two species had a more mixed diet, eating fruits and seeds in addition to leaves.[13][14][15] The level of seed predation varies between the three species, with tooth wear indicating thatM. dolichobrachion exhibited the greatest level of seed predation within the genus.[14]
BecauseMesopropithecus died out relatively recently and is only known from subfossil remains, it is considered to be a modern form of Malagasy lemur.[12] It may have been among the last subfossil lemurs to go extinct, possibly surviving until 500 years ago,[1][30] although radiocarbon dating places the most recent remains at 570–679 CE for aM. pithecoides from Ampasambazimba.[3][30] The arrival of humans roughly 2,000 years ago is thought to have sparked the decline ofMesopropithecus through hunting,habitat destruction, or both.[1]
^abStanding, H.F. (1905). "Rapport sur des ossements sub-fossiles provenant d'Ampasambazimba" [Report on subfossil bones from Ampasambazimba].Bulletin de l'Académie malgache (in French).4:95–100.
^abcdefghijkSimons, E.L.; Godfrey, L.R.; Jungers, W.L.; Chatrath, P.S.; Ravaoarisoa, J. (1995). "A new species ofMesopropithecus (Primates, Palaeopropithecidae) from Northern Madagascar".International Journal of Primatology.15 (5):653–682.doi:10.1007/BF02735287.S2CID21431569.
^Lamberton, C. (1936). "Nouveaux lémuriens fossiles du groupe des Propithèques et l'intérêt de leur découverte" [New fossil lemurs of the groupPropithecus and the significance of their discovery].Bulletin du Muséum National d'Histoire Naturelle. 2 (in French).8:370–373.
