Mesopropithecus
Encyclopedia
Mesopropithecus is an extinct genus
of small to medium-sized lemur
, or strepsirrhine primate
, from Madagascar
that includes three species, M. dolichobrachion, M. globiceps, and M. pithecoides. Together with Palaeopropithecus, Archaeoindris
, and Babakotia
, it is part of the sloth lemur family (Palaeopropithecidae). Once thought to be an indriid
because its skull
is similar to that of living sifaka
s, a recently discovered postcranial skeleton shows Mesopropithecus had longer forelimbs than hindlimbs—a distinctive trait shared by sloth lemurs but not by indriids. However, as it had the shortest forelimb
s of all sloth lemurs, it is thought that Mesopropithecus was more quadrupedal and did not use suspension
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 (folivore
s), but also ate fruit and occasionally seeds. M. globiceps ate a mix of fruits and leaves, as well as a larger quantity of seeds than M. pithecoides. M. dolichobrachion also consumed a mixed diet of fruits and leaves, but analysis of its teeth suggests that it was more of a seed predator
than the other two species.
Although rare, the three species were widely distributed across the island yet allopatric
to each other, with M. dolichobrachion in the north, M. pithecoides in the south and west, and M. 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 extinct subfossil lemur
s, but was still slightly larger than the largest living lemurs. Known only from subfossil
remains, it died out after the arrival of humans on the island, probably due to hunting pressure and habitat destruction
.
within the sloth lemur family
(Palaeopropithecidae), which includes three other genera: Palaeopropithecus, Archaeoindris
, and Babakotia
. This family in turn belongs to the infraorder Lemuriformes, which includes all the Malagasy lemur
s.
Mesopropithecus was named in 1905 by Herbert F. Standing using four skulls found at Ampasambazimba. He noted that the animal had characteristics of both Palaeopropithecus and the living sifaka
s (Propithecus). In 1936, Charles Lamberton defined Neopropithecus globiceps (based on one skull from Tsirave) and N. platyfrons (based on two skulls from Anavoha). He thought that Neopropithecus was a separate, intermediate genus between Mesopropithecus and Propithecus. In 1971, paleoanthropologist
Ian Tattersall
merged N. platyfrons into N. globiceps and Neopropithecus into Mesopropithecus.
Until 1986, Mesopropithecus was only known from cranial
(skull) remains from central and southern Madagascar, and because these are similar to teeth and skulls of living indriid
s, particularly those of Verreaux's Sifaka
(Propithecus verreauxi), Mesopropithecus was often assigned to the family Indriidae. For example, in 1974, Tattersall and Schwartz labeled Mesopropithecus as a sister group
to sifakas. With the discovery of an associated skeleton of M. dolichobrachion near Ankarana in 1986, it became clear that Mesopropithecus shared distinct traits with sloth lemurs. Unlike the indriids, but like the sloth lemurs, they had elongated forelimbs and other adaptations for arboreal suspension
(hanging in trees), linking them most closely to family Paleaeopropithecidae. A comparison of these morphological traits between the sloth lemurs and indriids suggest that Mesopropithecus was the first genus to diverge within the sloth lemur family.
s, but all species were still noticeably larger than all living (extant) lemurs. They ranged in weight from 10 to 14 kg (22 to 30.9 lb). They were also the least specialized of the sloth lemurs, more closely resembling living indriids in both skull and postcranial characteristics. Skull length ranged from 93.4 to 105.5 mm (3.7 to 4.2 in). The dentition
and cranial proportions, however, more closely resembled those of the sifakas. The dental formula of Mesopropithecus was the same as in the other sloth lemur and indriids: either or . Mesopropithecus had a four-toothed toothcomb
, like all indriids and most other sloth lemurs.. It is unclear whether one of the permanent teeth
in the toothcomb is an incisor
or canine
, resulting in the two conflicting dental formulae. Like other sloth lemurs and indriids, Mesopropithecus had rapid tooth development
.
Despite the similarities, there are several features that distinguish Mesopropithecus skulls from those of living indriids. The skull, including the zygomatic arch, is more robustly built. The temporal line
s 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 rounded braincase, slightly smaller and more convergent orbit
s, more pronounced postorbital constriction
(narrowing of the skull behind the eye sockets), more robust postorbital bar
(bone that encircles the eye socket), a steeper facial angle, more robust and cranially convex zygomatic bone, and a broader, squared snout. The upper incisor
s and canine
s are larger. The more robust mandible
(lower jaw) and mandibular 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, which suggests low visual acuity
. Mesopropithecus and its closest sloth lemur relative, Babakotia, did share a few ancestral traits with indriids, unlike the largest sloth lemurs, Palaeopropithecus and Archaeoindris. These include the aforementioned four-toothed toothcomb, an inflated auditory bulla
(bony structure that encloses part of the middle and inner ear), and a intrabullar ectotympanic ring
(bony ring that holds the eardrum).
While the skull of Mesopropithecus most closely resembles that of modern sifakas, the postcranial skeleton is quite different. Rather than having elongated hindlimbs for leaping
, Mesopropithecus had elongated forelimb
s, suggesting they predominantly used quadrupedal locomotion, slow climbing, with some forelimb and hindlimb suspension. In fact, they were the most quadrupedal of the sloth lemurs, having a intermembral index between 97 and 113, compared to the lower value for indriids and higher values for the other sloth lemurs. (In arboreal primates, an intermembral index of 100 predicts quadrupedalism, higher values predict suspensory behavior, and lower values predict leaping behavior.) Wrist bones found in 1999 further demonstrate that Mesopropithecus was a vertical climber and the most loris
-like of the sloth lemurs. Analysis of a lumbar vertebra of M. dolichobrachion further supported this conclusion.
Our understanding of the morphology of Mesopropithecus has not always been so complete. Until recently, important pieces of the skeleton had not been discovered, including the radius
, ulna
, vertebrae, hand and foot bones, and the pelvis
. In 1936, Alice Carleton mistakenly associated postcranial remains of the Diademed Sifaka
(Propithecus diadema) from Ampasambazimba with Mesopropithecus 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.
) from each other. Subfossil
discoveries indicate that they lived in the same region (sympatric) with other sloth lemurs in the north and center of Madagascar. The subfossil remains of M. globiceps have been found at seven subfossil sites on Madagascar: Anavoha, Ankazoabo-Grotte, Belo sur Mer
, Manombo-Toliara, Taolambiby, Tsiandroina, Tsirave. The subfossil remains of both M. pithecoides and M. dolichobrachion have only been found at one site each, Ampasambazimba and Ankarana respectively.
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. The level of seed predation varies between the three species, with tooth wear indicating that M. dolichobrachion exhibited the greatest level of seed predation within the genus.
, or both.
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...
of small to medium-sized lemur
Lemur
Lemurs are a clade of strepsirrhine primates endemic to the island of Madagascar. They are named after the lemures of Roman mythology due to the ghostly vocalizations, reflective eyes, and the nocturnal habits of some species...
, or strepsirrhine primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
, from Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
that includes three species, M. dolichobrachion, M. globiceps, and M. pithecoides. Together with Palaeopropithecus, Archaeoindris
Archaeoindris
Archaeoindris fontoynonti is an extinct species of Malagasy lemur that was the largest primate to evolve on Madagascar. It weighed about and measured around 1.5m in height, more than a silverback gorilla. Archaeoindris is one of eight known members of the Palaeopropithecinae subfamily...
, and Babakotia
Babakotia
Babakotia is a medium-sized, extinct genus of lemur, or strepsirrhine primate, from Madagascar that contains a single species, Babakotia radofilai. Together with Palaeopropithecus, Archaeoindris, and Mesopropithecus, it forms the family Palaeopropithecidae, commonly known as the sloth lemurs...
, it is part of the sloth lemur family (Palaeopropithecidae). Once thought to be an indriid
Indriidae
The Indriidae are a family of strepsirrhine primates. They are medium to large sized lemurs with only four teeth in the toothcomb instead of the usual six...
because its skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
is similar to that of living sifaka
Sifaka
Sifakas are a genus of lemur from the family Indriidae within the order Primates. Their name of the family is an onomatopoeia of their characteristic "shi-fak" alarm call. Like all lemurs, they are found only on the island of Madagascar...
s, a recently discovered postcranial skeleton shows Mesopropithecus had longer forelimbs than hindlimbs—a distinctive trait shared by sloth lemurs but not by indriids. However, as it had the shortest forelimb
Forelimb
A forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....
s of all sloth lemurs, it is thought that Mesopropithecus was more quadrupedal and did not use suspension
Suspensory behavior
Suspensory Behavior, exhibited by primates, is a form of arboreal locomotion or a feeding behavior which involves hanging or suspension of the body below or among the branches, rather than moving or sitting on top of the branches...
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 (folivore
Folivore
In zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds. For this reason folivorous animals tend to have long digestive tracts and slow metabolisms....
s), but also ate fruit and occasionally seeds. M. globiceps ate a mix of fruits and leaves, as well as a larger quantity of seeds than M. pithecoides. M. dolichobrachion also consumed a mixed diet of fruits and leaves, but analysis of its teeth suggests that it was more of a seed predator
Seed predation
Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable...
than the other two species.
Although rare, the three species were widely distributed across the island yet allopatric
Allopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
to each other, with M. dolichobrachion in the north, M. pithecoides in the south and west, and M. 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 extinct subfossil lemur
Subfossil lemur
Subfossil lemurs are lemurs from Madagascar that are represented by recent remains dating from nearly 26,000 years ago to approximately 560 years ago. They include both living and extinct species, although the term more frequently refers to the extinct giant lemurs...
s, but was still slightly larger than the largest living lemurs. Known only from subfossil
Subfossil
Subfossil refers to remains whose fossilization process is not complete, either for lack of time or because the conditions in which they were buried were not optimal for fossilization....
remains, it died out after the arrival of humans on the island, probably due to hunting pressure and habitat destruction
Habitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...
.
Classification and phylogeny
Mesopropithecus is a genusGenus
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...
within the sloth lemur family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
(Palaeopropithecidae), which includes three other genera: Palaeopropithecus, Archaeoindris
Archaeoindris
Archaeoindris fontoynonti is an extinct species of Malagasy lemur that was the largest primate to evolve on Madagascar. It weighed about and measured around 1.5m in height, more than a silverback gorilla. Archaeoindris is one of eight known members of the Palaeopropithecinae subfamily...
, and Babakotia
Babakotia
Babakotia is a medium-sized, extinct genus of lemur, or strepsirrhine primate, from Madagascar that contains a single species, Babakotia radofilai. Together with Palaeopropithecus, Archaeoindris, and Mesopropithecus, it forms the family Palaeopropithecidae, commonly known as the sloth lemurs...
. This family in turn belongs to the infraorder Lemuriformes, which includes all the Malagasy lemur
Lemur
Lemurs are a clade of strepsirrhine primates endemic to the island of Madagascar. They are named after the lemures of Roman mythology due to the ghostly vocalizations, reflective eyes, and the nocturnal habits of some species...
s.
Mesopropithecus was named in 1905 by Herbert F. Standing using four skulls found at Ampasambazimba. He noted that the animal had characteristics of both Palaeopropithecus and the living sifaka
Sifaka
Sifakas are a genus of lemur from the family Indriidae within the order Primates. Their name of the family is an onomatopoeia of their characteristic "shi-fak" alarm call. Like all lemurs, they are found only on the island of Madagascar...
s (Propithecus). In 1936, Charles Lamberton defined Neopropithecus globiceps (based on one skull from Tsirave) and N. platyfrons (based on two skulls from Anavoha). He thought that Neopropithecus was a separate, intermediate genus between Mesopropithecus and Propithecus. In 1971, paleoanthropologist
Paleoanthropology
Paleoanthropology, which combines the disciplines of paleontology and physical anthropology, is the study of ancient humans as found in fossil hominid evidence such as petrifacted bones and footprints.-19th century:...
Ian Tattersall
Ian Tattersall
Ian Tattersall is a paleoanthropologist and a curator at the American Museum of Natural History. Tattersall received his PhD from Yale University in 1971. In addition to human evolution, he has worked extensively with lemurs. He is working with The Templeton Foundation.-Selected publications:* The...
merged N. platyfrons into N. globiceps and Neopropithecus into Mesopropithecus.
Until 1986, Mesopropithecus was only known from cranial
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
(skull) remains from central and southern Madagascar, and because these are similar to teeth and skulls of living indriid
Indriidae
The Indriidae are a family of strepsirrhine primates. They are medium to large sized lemurs with only four teeth in the toothcomb instead of the usual six...
s, particularly those of Verreaux's Sifaka
Verreaux's Sifaka
Verreaux's sifaka , or white sifaka, is a medium sized primate in one of the lemur families, Indriidae. It lives in Madagascar and can be found in a variety of habitats from rainforest to western Madagascar dry deciduous forests and dry and spiny forests...
(Propithecus verreauxi), Mesopropithecus was often assigned to the family Indriidae. For example, in 1974, Tattersall and Schwartz labeled Mesopropithecus as a sister group
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...
to sifakas. With the discovery of an associated skeleton of M. dolichobrachion near Ankarana in 1986, it became clear that Mesopropithecus shared distinct traits with sloth lemurs. Unlike the indriids, but like the sloth lemurs, they had elongated forelimbs and other adaptations for arboreal suspension
Suspensory behavior
Suspensory Behavior, exhibited by primates, is a form of arboreal locomotion or a feeding behavior which involves hanging or suspension of the body below or among the branches, rather than moving or sitting on top of the branches...
(hanging in trees), linking them most closely to family Paleaeopropithecidae. A comparison of these morphological traits between the sloth lemurs and indriids suggest that Mesopropithecus was the first genus to diverge within the sloth lemur family.
Species
Three species are recognized within Mesopropithecus:- M. pithecoides, described in 1905, was the first species to be formally named. Its specific name, pithecoides, derives from the Greek word pithekos, 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. It was a small to medium-sized lemur, weighing approximately 10 kg (22 lb) and having an intermembral index (ratio of limb proportions) of 99. Its skull was similar to that of M. globiceps, but had a broader snout and was more robust, particularly in its sagittalSagittal crestA sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull of many mammalian and reptilian skulls, among others....
and nuchal crests (ridges on the skull for muscle attachments) and massive zygomatic archZygomatic archThe 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...
es (cheekbones). Its skull length averaged 98 mm (3.9 in), ranging from 94 to 103.1 mm (3.7 to 4.1 in). It was predominantly folivorousFolivoreIn zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds. For this reason folivorous animals tend to have long digestive tracts and slow metabolisms....
(leaf-eating), but also consumed some fruit and (rarely) seeds. It was moderately abundant on the high, central plateauCentral Highlands (Madagascar)The Central Highlands, Central High Plateau, or Hauts-Plateaux are a mountainous biogeographical region in central Madagascar. They include the contiguous part of the island's interior above 800 m altitude...
of Madagascar. It shared its range with the larger sloth lemurs, Palaeopropithecus maximus and Archaeoindris fontoynontiiArchaeoindrisArchaeoindris fontoynonti is an extinct species of Malagasy lemur that was the largest primate to evolve on Madagascar. It weighed about and measured around 1.5m in height, more than a silverback gorilla. Archaeoindris is one of eight known members of the Palaeopropithecinae subfamily...
. One sample of its subfossil remains has been radiocarbon datedRadiocarbon datingRadiocarbon 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" ,...
, yielding a date of 570–679 CECommon EraCommon Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...
.
- M. globiceps was discovered in 1936 and originally classified in its own genus, Neopropithecus. The name globiceps comes from its prominent forehead and derives from the Latin word globus, meaning "ball", and the New LatinNew LatinThe term New Latin, or Neo-Latin, is used to describe the Latin language used in original works created between c. 1500 and c. 1900. Among other uses, Latin during this period was employed in scholarly and scientific publications...
suffix -ceps, meaning "head". Like M. pithecoides, it was a small to medium-sized lemur, weighing approximately 11 kg (24.3 lb) and having an intermembral index of 97. It had the most narrow snout and gracile skeleton of the Mesopropithecus species, similar to but smaller than M. pithecoides, making it more like the living sifakas. 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), ranging from 93.4 to 94.8 mm (3.7 to 3.7 in). It was a mixed feeder, eating fruit, leaves, and a moderate amount of seeds, having a diet similar to that of the living IndriIndriThe indri , also called the babakoto, is one of the largest living lemurs. It is a diurnal tree-dweller related to the sifakas and, like all lemuroids, it is native to Madagascar.- Etymology :...
(Indri indri). Although its forelimbs were more like those of living indriids, its hindlimbs and axial skeletonAxial skeletonThe axial skeleton consists of the 80 bones along the central axis of the human body. It is composed of six parts; the human skull, the ossicles of the middle ear, the hyoid bone of the throat, the rib cage, sternum and the vertebral column...
(skull, spine, and ribs) were more specialized for suspension, as in Palaeopropithecus and Babakotia. It was found in the south and west of Madagascar. Three samples of its subfossil remains have been radiocarbon dated, yielding dates of 354–60 BCE, 58–247 CE, and 245–429 CE.
- M. dolichobrachion was discovered in 1986 and formally described in 1995. It was found in the caves of AnkaranaAnkarana ReserveAnkarana Reserve is a small, partially vegetated plateau in northern Madagascar composed of 150-million-year-old middle Jurassic limestone. With an average annual rainfall of about , the underlying rocks are susceptible to erosion, thereby producing caves and underground rivers—a karst topography...
, northern Madagascar, around the same time that the first remains of Babakotia were unearthed. The species name dolichobrachion is Greek, coming from dolicho- ("long") and brachion ("arm"), and means "long-armed". It was a medium-sized lemur, slightly larger than the other two members of its genus, weighing approximately 14 kg (30.9 lb). It differed significantly from the other two in its limb proportions and its postcraniaPostcraniaPostcrania[p] in zoology and vertebrate paleontology refers to all or part of the skeleton apart from the skull. Frequently, fossil remains, e.g...
l morphology. Most notably, it was the only species in the genus to have forelimbForelimbA forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....
s that were longer than the hindlimbs, due to a substantially longer and more robust humerusHumerusThe humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
(yielding an intermembral index of 113), as well as more curved phalanges (finger and toe bones). For these reasons, it is thought to have been more slothSlothSloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae , part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.They are arboreal residents of the jungles of Central and South...
-like in its use of suspension. This was further supported by a study of a single lumbar vertebra. This vertebra was similar to that of Babakotia in having a moderately reduced, dorsally oriented spinous processSpinous processThe spinous process of a vertebra is directed backward and downward from the junction of the laminae , and serves for the attachment of muscles and ligaments. In animals without an erect stance, the process points upward and may slant forward or backward...
and a transverse processes (plates of bone that protrude from the vertebrae) that points to the side (laterally). The vertebra was intermediate in length when compared with other sloth lemurs, and its laminaeLamina of the vertebral archThe laminæ are two broad plates, extending dorsally and medially from the pedicles, fusing to complete the roof of the vertebral arch.Their upper borders and the lower parts of their anterior surfaces are rough for the attachment of the ligamenta flava....
(two plates of bone that connect to the spinous process) were not as broad as seen in Palaeopropithecus. In M. dolichobrachion, skull length averaged 102 mm (4 in), ranging from 97.8 to 105.5 mm (3.9 to 4.2 in). 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 of cuspsCusp (dentistry)A cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
on the molar teeth). It was a mixed feeder, eating leaves, fruits, and seeds. This species was more of a seed predatorSeed predationSeed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable...
than the other two species, but was not as specialized as closely related Babakotia radofilaiBabakotiaBabakotia is a medium-sized, extinct genus of lemur, or strepsirrhine primate, from Madagascar that contains a single species, Babakotia radofilai. Together with Palaeopropithecus, Archaeoindris, and Mesopropithecus, it forms the family Palaeopropithecidae, commonly known as the sloth lemurs...
. M. dolichobrachion was rare and shared its range with two other sloth lemurs, Babakotia radofilai and Palaeopropithecus maximus. It was the most distinct member of its genus and was geographically restricted to the extreme north of the island.
Anatomy and physiology
The genus Mesopropithecus includes some of the smallest of the recently extinct subfossil lemurSubfossil lemur
Subfossil lemurs are lemurs from Madagascar that are represented by recent remains dating from nearly 26,000 years ago to approximately 560 years ago. They include both living and extinct species, although the term more frequently refers to the extinct giant lemurs...
s, but all species were still noticeably larger than all living (extant) lemurs. They ranged in weight from 10 to 14 kg (22 to 30.9 lb). They were also the least specialized of the sloth lemurs, more closely resembling living indriids in both skull and postcranial characteristics. Skull length ranged from 93.4 to 105.5 mm (3.7 to 4.2 in). The dentition
Dentition
Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, the characteristic arrangement, kind, and number of teeth in a given species at a given age...
and cranial proportions, however, more closely resembled those of the sifakas. The dental formula of Mesopropithecus was the same as in the other sloth lemur and indriids: either or . Mesopropithecus had a four-toothed toothcomb
Toothcomb
A toothcomb is an anatomical structure found in strepsirrhine primates, which includes lemurs, lorises and galagos. A toothcomb consists of long, flat forward-angled teeth, and includes the lower incisors and the canine teeth...
, like all indriids and most other sloth lemurs.. It is unclear whether one of the permanent teeth
Permanent teeth
Permanent teeth are the second set of teeth formed in humans. There are thirty-two permanent teeth, consisting of six maxillary and six mandibular molars, four maxillary and four mandibular premolars, two maxillary and two mandibular canines, four maxillary and four mandibular incisors.The first...
in the toothcomb is an incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
or canine
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...
, resulting in the two conflicting dental formulae. Like other sloth lemurs and indriids, Mesopropithecus had rapid tooth development
Tooth development
Tooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. Although many diverse species have teeth, non-human tooth development is largely the same as in humans...
.
Despite the similarities, there are several features that distinguish Mesopropithecus skulls from those of living indriids. The skull, including the zygomatic arch, is more robustly built. The temporal line
Temporal line
Crossing the middle of the parietal bone in an arched direction are two curved lines, the superior and inferior temporal lines; the former gives attachment to the temporal fascia, and the latter indicates the upper limit of the muscular origin of the Temporalis.-External links:*...
s 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 rounded braincase, slightly smaller and more convergent orbit
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
s, more pronounced postorbital constriction
Post-Orbital Constriction
In physical anthropology, post-orbital constriction is a narrowing of the cranium just behind the eye sockets , in primates — including primitive hominids. This constriction is very noticeable in non-human primates, slightly less so in Australopithecines, even less in Homo erectus and the...
(narrowing of the skull behind the eye sockets), more robust postorbital bar
Postorbital bar
The postorbital bar is a bone which runs around the eyesocket of strepsirrhine primates. This is in contrast to the higher primates, haplorrhine, which have evolved fully enclosed sockets to protect their eyes....
(bone that encircles the eye socket), a steeper facial angle, more robust and cranially convex zygomatic bone, and a broader, squared snout. 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 canine
Canine tooth
In mammalian oral anatomy, the canine teeth, also called cuspids, dogteeth, fangs, or eye teeth, are relatively long, pointed teeth...
s are larger. The more robust mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
(lower jaw) and mandibular symphysis
Symphysis menti
The external surface of the mandible is marked in the median line by a faint ridge, indicating the symphysis menti, mandibular symphysis, or line of junction of the two pieces of which the bone is composed at an early period of life....
(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, which suggests low visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
. Mesopropithecus and its closest sloth lemur relative, Babakotia, did share a few ancestral traits with indriids, unlike the largest sloth lemurs, Palaeopropithecus and Archaeoindris. These include the aforementioned four-toothed toothcomb, an inflated auditory bulla
Auditory bulla
The auditory bulla is a hollow bony structure on the ventral, posterior portion of the skull of placental mammals that encloses parts of the middle and inner ear. In most species, it is formed by the tympanic part of the temporal bone.In extant primates, the structure is found in tarsiers,...
(bony structure that encloses part of the middle and inner ear), and a intrabullar ectotympanic ring
Ectotympanic ring
The ectotympanic ring, or tympanicum, is a bony ring that holds the eardrum, or tympanic membrane. Its position and attachment to the skull vary within mammals, and particularly primates, and can be either intrabullar or extrabullar....
(bony ring that holds the eardrum).
While the skull of Mesopropithecus most closely resembles that of modern sifakas, the postcranial skeleton is quite different. Rather than having elongated hindlimbs for leaping
Jumping
Jumping or leaping is a form of locomotion or movement in which an organism or non-living mechanical system propels itself through the air along a ballistic trajectory...
, Mesopropithecus had elongated forelimb
Forelimb
A forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....
s, suggesting they predominantly used quadrupedal locomotion, slow climbing, with some forelimb and hindlimb suspension. In fact, they were the most quadrupedal of the sloth lemurs, having a intermembral index between 97 and 113, compared to the lower value for indriids and higher values for the other sloth lemurs. (In arboreal primates, an intermembral index of 100 predicts quadrupedalism, higher values predict suspensory behavior, and lower values predict leaping behavior.) Wrist bones found in 1999 further demonstrate that Mesopropithecus was a vertical climber and the most loris
Loris
Loris is the common name for the strepsirrhine primates of the subfamily Lorisinae in family Lorisidae. Loris is one genus in this subfamily and includes the slender lorises, while Nycticebus is the genus for the slow lorises....
-like of the sloth lemurs. Analysis of a lumbar vertebra of M. dolichobrachion further supported this conclusion.
Our understanding of the morphology of Mesopropithecus has not always been so complete. Until recently, important pieces of the skeleton had not been discovered, including the radius
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
, ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
, vertebrae, hand and foot bones, and the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
. In 1936, Alice Carleton mistakenly associated postcranial remains of the Diademed Sifaka
Diademed Sifaka
The diademed sifaka , or diademed simpona, is an endangered species of sifaka, one of the lemurs endemic to certain rainforests in eastern Madagascar. This species is one of the world's largest living lemurs, with a total adult length of approximately 105 centimetres , half of which is tail...
(Propithecus diadema) from Ampasambazimba with Mesopropithecus 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.
Distribution and ecology
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, although they appear to have been geographically separated (allopatricAllopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
) from each other. Subfossil
Subfossil
Subfossil refers to remains whose fossilization process is not complete, either for lack of time or because the conditions in which they were buried were not optimal for fossilization....
discoveries indicate that they lived in the same region (sympatric) with other sloth lemurs in the north and center of Madagascar. The subfossil remains of M. globiceps have been found at seven subfossil sites on Madagascar: Anavoha, Ankazoabo-Grotte, Belo sur Mer
Belo sur Mer
Belo sur Mer is a town and commune in Madagascar. It belongs to the district of Morondava, which is a part of Menabe Region. The population of the commune was estimated to be approximately 8,000 in 2001 commune census....
, Manombo-Toliara, Taolambiby, Tsiandroina, Tsirave. The subfossil remains of both M. pithecoides and M. dolichobrachion have only been found at one site each, Ampasambazimba and Ankarana respectively.
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. The level of seed predation varies between the three species, with tooth wear indicating that M. dolichobrachion exhibited the greatest level of seed predation within the genus.
Extinction
Because Mesopropithecus died out relatively recently and is only known from subfossil remains, it is considered to be a modern form of Malagasy lemur. It may have been among the last subfossil lemurs to go extinct, possibly surviving until 500 years ago, although radiocarbon dating places the most recent remains at 570–679 CE for a M. pithecoides from Ampasambazimba. The arrival of humans roughly 2,000 years ago is thought to have sparked the decline of Mesopropithecus through hunting, habitat destructionHabitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...
, or both.