''Plesiopithecus '' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of early

strepsirrhine Strepsirrhini or Strepsirhini (; ) is a suborder of primates that includes the lemuriform primates, which consist of the lemurs of Madagascar, galagos ("bushbabies") and pottos from Africa, and the lorises from India and southeast Asia. Colle ...

primate Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...

from the late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

Anatomy

Originally described from the right

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

(lower jaw), its confusing anatomy resulted in it being classified as an

ape Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...

—its name translates "near ape". ''Plesiopithecus teras'', the only species so far identified, was discovered in 1992 by Elwyn L. Simons in

Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...

at the base of the Jebel Qatrani Formation at

Fayum Faiyum ( ar, الفيوم ' , borrowed from cop, ̀Ⲫⲓⲟⲙ or Ⲫⲓⲱⲙ ' from egy, pꜣ ym "the Sea, Lake") is a city in Middle Egypt. Located southwest of Cairo, in the Faiyum Oasis, it is the capital of the modern Faiyum ...

in quarry L-41. Within two years, a more complete, crushed skull was found, demonstrating that it was a strepsirrhine primate. It is thought to be a close relative of lemuriforms ( extant strepsirrhines), and a sister group to either lorisoids or all lemuriforms. ''Plesiopithecus'' was a medium-sized strepsirrhine primate, with large orbits, a high muzzle, and a skull exhibiting klinorhynchy (a marked angle between the

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 separ ...

and the basicranium, or the lower region of the braincase). Its large orbits indicate that it was nocturnal, and its teeth suggest that it bored holes in wood in search of soft-bodied insects, just like the aye-aye.

Etymology

The

name ''Plesiopithecus'' derives from the

Greek root The English language uses many Greek and Latin roots, stems, and prefixes. These roots are listed alphabetically on three pages: * Greek and Latin roots from A to G * Greek and Latin roots from H to O * Greek and Latin roots from P to Z. Some ...

''plesi-'' (πλησίος, plēsios), meaning "near", and the Greek word πίθηκος (''pithekos''), meaning "trickster", which has historically been used to mean "ape". The

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

name derives from the Greek word τερασ (''teras''), meaning "wonderful or amazing animal".

Evolutionary history

Although ''Plesiopithecus'' is now clearly recognized as being related to lemuriforms ( extant strepsirrhine primates), its

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relations among the fossil primates are unclear. Phylogenetic analyses using 359 morphological traits place it as a sister group of lemuriforms, along with other

African sister taxa, the djebelemurids, which include '' Djebelemur'' and '' 'Anchomomys' milleri''. This Afro-Arabian clade, which includes crown lemuriforms and possibly the disputed azibiids, excludes the

Holarctic The Holarctic realm is a biogeographic realm that comprises the majority of habitats found throughout the continents in the Northern Hemisphere. It corresponds to the floristic Boreal Kingdom. It includes both the Nearctic zoogeographical reg ...

adapiforms and may be descended from an early Asian branch of adapiforms such as a primitive branch of cercamoniines predating '' Donrussellia'' (one of the oldest European adapiforms). A general consensus places ''Plesiopithecus'' at the base of the lemuriform clade, possibly closer to the lemuriforms than the djebelemurids. Due to its combination of the lack of a

toothcomb A toothcomb (also tooth comb or dental comb) is a dental structure found in some mammals, comprising a group of front teeth arranged in a manner that facilitates grooming, similar to a hair comb. The toothcomb occurs in lemuriform primates ( ...

(a specialized dental structure found in most lemuriforms) and a front dentition that is enlarged and procumbent, ''Plesiopithecus'' has been considered a possible relative of the

aye-aye The aye-aye (''Daubentonia madagascariensis'') is a long-fingered lemur, a strepsirrhine primate native to Madagascar with rodent-like teeth that perpetually grow and a special thin middle finger. It is the world's largest nocturnal primate. ...

(''Daubentonia madagascariensis''), suggesting a possible close relationship. This close relationship between ''Plesiopithecus'' and the aye-aye within the

lemur Lemurs ( ) (from Latin ''lemures'' – ghosts or spirits) are wet-nosed primates of the superfamily Lemuroidea (), divided into 8 families and consisting of 15 genera and around 100 existing species. They are endemic to the island of Madagas ...

radiation was recovered by Gunnell et al. (2018), suggesting that lemur evolution involved two dispersal events from Africa to

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

. Alternatively, it may have closer ties to the

(''Daubentonia madagascariensis'') within the lemuroid (

) radiation. The cranial and dental morphology of ''Plesiopithecus'' resemble what is predicted of an aye-aye ancestor, with an arched cranial vault suggesting klinorhynchy (a marked angle between the

and the basicranium, or the lower region of the braincase) similar to that of aye-ayes, significantly enlarged

canines Canine may refer to: Zoology and anatomy * a dog-like Canid animal in the subfamily Caninae ** ''Canis'', a genus including dogs, wolves, coyotes, and jackals ** Dog, the domestic dog * Canine tooth, in mammalian oral anatomy People with the surn ...

and/or

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

s, reduced

molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

and premolars, a high muzzle, and anteriorly placed

orbits In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

. However, it lacked ever-growing incisors and the

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

found in the aye-aye. The increased anterior dentition is peculiar because it has only happened in lemuriforms, and has never been observed in any of the numerous adapiforms. Key to this possible close affinity with the aye-aye is the identity (canine vs. incisor) of the procumbent front teeth of both species, neither of which is definitively known. If they are different between the two species, then the similarities would be due to convergence. If they are the same tooth, then it may suggest a close phylogenetic relation and African origins for the aye-aye. If ''Plesiopithecus'' is an African relative of the aye-aye, it would suggest lemuroid primates colonized

twice. Molecular studies show that the aye-aye diverged early in the

evolutionary history of lemurs Lemurs, primates belonging to the suborder Strepsirrhini which branched off from other primates less than 63 million years ago, evolved on the island of Madagascar, for at least 40 million years. They share some traits with the most basal ...

, followed later by a sudden burst of diversification of the other four lemur families. If ''Plesiopithecus'' is an aye-aye relative, lemurs and aye-ayes would have diverged in Africa and the aye-aye ancestor would have colonized Madagascar separately from the other lemurs, which would have diversified immediately upon arrival to the island. If aye-ayes are more closely related to lemurs than ''Plesiopithecus'', then only one colonization event occurred, with the aye-aye branching off first and then the other lemuroid families diverging several million years later following a major extinction event.

Taxonomy

''Plesiopithecus teras'' was first described in 1992 by paleoanthropologist Elwyn L. Simons. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

, which was found at the base of the Jebel Qatrani Formation at the Egyptian

in quarry L-41 and dated to the latest Eocene, included a right

with intact dentition ranging from the third molar up to the first anterior tooth. Simons acknowledged that the taxonomic interpretation was complicated, though he initially decided to classify under superfamily

Hominoidea Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...

(apes) due to its flat and broad lower molars. Its bizarre and specialized traits made it difficult to classify until the discovery of its skull, reported in 1994, showed it had a

postorbital bar The postorbital bar (or postorbital bone) is a bony arched structure that connects the frontal bone of the skull to the zygomatic arch, which runs laterally around the eye socket. It is a trait that only occurs in mammalian taxa, such as most strep ...

, proving that it was a strepsirrhine primate. ''Plesiopithecus'' was then placed within a new superfamily, Plesiopithecoidea tentatively under "Infraorder

cf. The abbreviation ''cf.'' (short for the la, confer/conferatur, both meaning "compare") is used in writing to refer the reader to other material to make a comparison with the topic being discussed. Style guides recommend that ''cf.'' be used onl ...

Lorisiformes". The superfamily has also been grouped under the infraorder Lemuriformes. As of 2010, ''P. teras'' was represented by a nearly complete and partially crushed cranium and three partial mandibles. Prior to Simons' discovery of the right jaw, one of its molars had been found a few years earlier and incorrectly attributed to an Eocene lorisoid. Several cranial and facial similarities with living lorisoids indicate that it is closely related to lemuriforms (lemurs and lorisoids), although its lack of a

precludes it from being ancestral to either branch of the lemuriform clade. Due to the differences in dental anatomy, ''Plesiopithecus'' is thought to be a sister group to either lorisoids or all living lemuriforms.

Anatomy and physiology

With a skull length of nearly , ''Plesiopithecus'' was a medium-sized strepsirrhine primate. Its skull is marked by a high muzzle, klinorhynchy, and relatively large orbits. It has very large and procumbent upper canines that are straight and compressed on both sides, and have roots that extend deep into the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

(upper jaw). No upper incisors have been found. The unidentified lower front tooth is also enlarged and procumbent. It has three premolars and three molars that decrease in size from front to back. The upper molars are simple and lack a

hypocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

. The lower molars are relatively broad.

Behavior

Its large orbits suggest ''Plesiopithecus'' was nocturnal. Due to cranial and dental similarities with the aye-aye and signs of wear on the tips of its front teeth, it is thought to have bored holes in wood in search of soft-bodied insects, which likely make up its diet.

References

Literature cited

* * * * * * * ** {{Taxonbar, from=Q3392299 Prehistoric primate genera Eocene primates Fossil taxa described in 1992