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 (which includes lemurs and lorisoids), treeshrews, colugos, hyraxes, and some African antelopes. The structures evolved independently in different types of mammals through convergent evolution and varies both in dental composition and structure. In most mammals the comb is formed by a group of teeth with fine spaces between them. The toothcombs in most mammals include incisors only, while in lemuriform primates they include incisors and canine teeth that tilt forward at the front of the lower jaw, followed by a canine-shaped first premolar. The toothcombs of colugos and hyraxes take a different form with the individual incisors being serrated, providing multiple tines per tooth.
The toothcomb is usually used for grooming. While licking the fur clean, the animal will run the toothcomb through the fur to comb it. Fine grooves or striations are usually cut into the teeth during grooming by the hair and may be seen on the sides of the teeth when viewed through a scanning electron microscope. The toothcomb is kept clean by either the tongue or, in the case of lemuriforms, the sublingua, a specialized "under-tongue". The toothcomb can have other functions, such as food procurement and bark gouging. Within lemuriforms, fork-marked lemurs and indriids have more robust toothcombs to support these secondary functions. In some lemurs, such as the aye-aye, the toothcomb has been lost completely and replaced with other specialized dentition.
In lemuriform primates, the toothcomb has been used by scientists in the interpretation of the evolution of lemurs and their kin. They are thought to have evolved from early adapiform primates around the Eocene or earlier. One popular hypothesis is that they evolved from European adapids, but the fossil record suggests that they evolved from an older lineage that migrated to Africa during the Paleocene (66 to 55 mya) and might have evolved from early cercamoniines from Asia. Fossil primates such as Djebelemur , 'Anchomomys' milleri , and Plesiopithecus may have been their closest relatives. The lack of a distinct toothcomb in the fossil record before to 40 mya has created a conflict with molecular clock studies that suggest an older divergence between lemurs and lorisoids, and the existence of a ghost lineage of lemuriform primates in Africa.
The toothcomb, a special morphological arrangement of teeth in the anterior lower jaw, is best known in extant strepsirrhine primates, which include lemurs and lorisoid primates (collectively known as lemuriforms). This homologous structure is a diagnostic character that helps define this clade (related group) of primates. An analogous trait is found in the bald uakari (Cacajao calvus), a type of New World monkey.
Toothcombs can also be found in colugos and treeshrews, both close relatives of primates; however, the structures are different and these are considered to examples of convergent evolution.Likewise, small- or medium-sized African antelopes, such as the impala (Aepyceros melampus), have a similar structure sometimes referred to as the "lateral dental grooming apparatus". Living and extinct hyraxes (hyracoids) also exhibit a toothcomb, although the number of tines in the comb vary throughout the fossil record.
Dating to the Eocene epoch over 50 mya, Chriacus and Thryptacodon —two types of arctocyonids (primitive placental mammals)—also possessed an independently evolved toothcomb.
The toothcomb of most lemuriforms includes six finely spaced teeth, four incisors and two canine teeth that are procumbent (tilt forward) in the front of the mouth.The procumbent lower canine teeth are the same shape as the incisors located between them, but they are more robust and curve upward and inward, more so than the incisors. In the permanent dentition, the canines erupt after the incisors. The crowns of the incisors are also angled in the direction of the forward tilt, and the crowns of both the incisors and canines are elongated and compressed side-to-side. The apical ridge, following along the front edges of the toothcomb teeth, is V-shaped in most lemuriforms, tapering off from the midline. As a result of this dental reconfiguration, the upper and lower incisors do not contact one another, and often the upper incisors are reduced or lost completely.
The French anatomist Henri Marie Ducrotay de Blainville first identified the two lateral teeth of the lemuriform toothcomb as canines in 1840.Canine teeth are normally used to pierce or grasp objects. With modified lower canine teeth, the first lower premolars following the toothcomb are usually shaped like typical canine teeth (caniniform) and assume their function. These premolars are commonly confused with canines. Normally the true canines in the lower jaw sit in front of the upper canines, and in toothcombed primates, the caniniform premolars rest behind it.
The lemuriform toothcomb is kept clean by the sublingua or "under-tongue", a specialized muscular structure that acts like a toothbrush to remove hair and other debris. The sublingua can extend below the end of the tongue and is tipped with keratinized, serrated points that rake between the front teeth.
Among lemurs, the toothcomb is variable in structure.Among indriids (Indriidae), the toothcomb is less procumbent and consists of four teeth instead of six. The indriid toothcomb is more robust and wider, with shorter incisors, wider spaces between the teeth (interdental spaces), and a broader apical ridge. It is unclear whether this four-toothed toothcomb consists of two pairs of incisors or one pair of incisors and one pair of canines. In fork-marked lemurs (Phaner) the toothcomb is more compressed, with significantly reduced interdental spaces. All six teeth are longer, straighter, and form a more continuous apical ridge. In the recently extinct monkey lemurs (Archaeolemuridae) and sloth lemurs (Palaeopropithecidae), the toothcomb was lost and the incisors and canines resumed a typical configuration in the front of the mouth. The aye-aye also lost its toothcomb, replacing it with continually growing (hypselodont) front teeth, similar to the incisors of rodents.
In colugos, the toothcomb has a completely different structure. Instead of individual incisors and canine teeth being finely spaced to act like the teeth of a comb, the biting edge of the four incisors have become serrated with as many as 15 tines each,while the canine acts more like a molar. These serrated incisors are kept clean using the front of the tongue, which is serrated to match the serrations of the incisors. Similarly, the hyracoid toothcomb consists of incisors with multiple tines, called "pectinations". In contrast to the colugos, the size and shape of the tines are more uniform.
The toothcomb of treeshrews is like the lemuriform toothcomb in that it uses interdental spaces to form the comb tines, but only two of its three pairs of lower incisors are included in the toothcomband the canines are also excluded. The lateral two incisors in the toothcomb are generally larger. In the extinct arctocyonids, all six lower incisors were part of the toothcomb. In African antelopes, the toothcomb is strikingly similar to that of lemuriforms in that it consists of two pairs of incisors and a pair of canines.
As a homologous structure in lemuriforms, the toothcomb serves variable biological roles, despite its superficially stereotypic shape and appearance.It is primarily used as a toiletry device or grooming comb. Additionally, some species use their toothcomb for food procurement or to gouge tree bark.
The primary function of the toothcomb, grooming, was first noted by the French naturalist Georges Cuvier in 1829, who pointed out that the ring-tailed lemur (Lemur catta) had lower incisors that "sont de véritables peignes" ("are real combs"). years later, the grooming function was questioned since it was difficult to observe and the interdental spaces were thought to be too small for fur. Observations later showed the teeth were used for that purpose and that immediately after grooming, hair may be found trapped in the teeth, but is removed by the sublingua later.More than 100
In 1981, scanning electron microscopy revealed fine grooves or striations on the teeth in lemuriform toothcombs. These grooves were only found on the sides of the teeth on the concave surfaces between the sides, as well on the back ridge of the teeth. Between 10 and 20 µm wide, these grooves indicate that hair moved repeatedly across the teeth. Inside these grooves were even finer grooves, less than 1 µm, created by abrasion with the cuticular layer of the hair.
Among non-primates, the extinct Chriacus exhibits microscopic groves on its toothcomb,but the Philippine colugo (Cynocephalus volans) does not. The toothcomb of the colugos is generally considered to function as a toothcomb, but due to the lack of striations on the teeth and no documented observations of toothcomb use during oral grooming, its use seems to be limited to food procurement.
In African antelopes, the lateral dental grooming apparatus does not appear to be used during grazing or browsing. Instead, it is used during grooming when the head sweeps upward in a distinctive motion. It is thought to comb the fur and remove ectoparasites.
In lemuriform primates, the toothcomb may also play a secondary role in olfaction, which may account for the size reduction of the poorly studied upper incisors.The toothcomb may provide pressure to stimulate glandular secretions which are then spread through the fur. Furthermore, the size reduction of the upper incisors may create a gap between the teeth (interincisal diastema) that connects the philtrum (a cleft in the middle of the wet nose, or rhinarium) to the vomeronasal organ in the roof of the mouth. This would allow pheromones to be more easily transferred to the vomeronasal organ.
Mouse lemurs (Microcebus), sifakas (Propithecus), and the indri (Indri) use their toothcombs to scoop up fruit pulp.Other small lemuriforms, such as fork-marked lemurs (Phaner), the hairy-eared dwarf lemur (Allocebus), and galagos (particularly the genera Galago and Euoticus ) use their toothcombs to tooth-scrape plant exudates, such as gum and sap. In fork-marked lemurs, the toothcomb is specially adapted to minimize food trapment since the interdental spaces are greatly reduced. The herbivorous colugos in the genus Cynocephalus may also use their toothcomb for food procurement.
Indriids such as the sifakas use their toothcombs to gouge bark or dead wood (bark-prising),which is done before scent-marking with the gland on their chest. The more robust structure of their toothcomb is thought to help it withstand the compressive forces experienced during regular bark-prising.
The origins of the lemuriform toothcomb and the clade it characterizes have been the center of considerable debate for more than a century. In 1920, British palaeoanthropologist Wilfrid Le Gros Clark proposed that the toothcomb found in treeshrews (which he believed were primates) was an early version of the dental structure found in lemuriforms. Because he viewed the fossil lorisoids from the Miocene as not having fully developed the modern lemuriform toothcomb, he implied that lemurs and lorisoids had evolved the trait independently. This view was later overturned, and the monophyletic relationship between lemurs and lorisoids is now accepted.
The ancestral condition of the anterior dentition on the lower jaw, based on Eocene primate fossils, suggests that earliest primates had lacked a differentiated toothcomb.Most fossil strepsirrhines lacked the stereotypic lemuriform toothcomb. Collectively, early strepsirrhine primates are known as adapiforms. Adapiforms are considered to be a paraphyletic group (containing many but not all of the descendants of the last common ancestor of the group's members) because the lemuriforms are assumed to have evolved from one of several groups of adapiforms. In terms of ecology, the evolution of the toothcomb is assumed to have required a folivorous (leaf-eating) diet among the ancestral adapiform population, since that would select for reduced incisors, which would serve as an exaptation (a trait with adaptive value for something other than what it was originally selected for), which could then be used for personal or social grooming. However, the inclusion of the canines into the toothcomb must have required exceptional conditions, since large lemuriforms have secondarily modified caniniform premolars to substitute for the loss.
A popular hypothesis about the origins of the lemuriform clade is that they evolved from European adapiforms known as adapids.In some adapids, the crests of the lower incisors and canines align to form functional cropping unit, and the American paleontologist Philip D. Gingerich has suggested this foreshadowed the development of the lemuriform toothcomb. However, no lemuriform toothcomb has been found in the fossil record of the Eocene, and the European adapid lower jaws from that time did not resemble the derived state seen in lemuriforms.
Lemuriforms are currently thought to have evolved in Africa, and the earliest known strepsirrhine primates from Africa are azibiids from the early Eocene, mya). Stem lemuriforms, including Djebelemur and 'Anchomomys' milleri , have been found in Africa and date from 50 to 48 mya and were very distinct from European adapiforms. However, they lack a toothcomb. These stem lemuriforms suggest an early common ancestry with cercamoniines from outside of Europe. Based on large, procumbent lower teeth, Plesiopithecus , a fossil primate found in late Eocene deposits at the Fayum Depression in Egypt, is thought to be most closely related to lemuriforms. Together, Djebelemur, ‘Anchomomys’ milleri, and Plesiopithecus are considered to be sister taxa (the closest relatives) of lemuriform primates.which likely descended from a very early colonization of the Afro-Arabian land mass in the Paleocene (66 to 55
Although stem lemuriforms like Djebelemur may have been contemporaneous with related toothcombed primates around 50 to 48 mya, the sparse African fossil record suggests toothcomb differentiation occurred around 52 to 40 mya according to the French paleoanthropologist Marc Godinot. This would conflict with the molecular clock estimates by evolutionary anthropologist Anne Yoder and others, which predict lemur–lorisoid divergence dating between 61 and 90.8 mya.
In 2001, the discovery of Bugtilemur , a fossil primate from Pakistan dating to the Oligocene and initially thought to be a cheirogaleid lemur, further challenged the theory of lemur origins;however, it was later shown to be a type of adapiform primate and not a lemur.
The minimum paleontological estimate for the divergence of lemurs and lorisoids nearly doubled when additional discoveries were made in northern Egypt during the 2000s of a stem galagid ( Saharagalago ) and a stem or crown lorisoid ( Karanisia ) dating to 37 and 40 mya respectively. Karanisia is the oldest fossil primate to exhibit a distinct lemuriform toothcomb. This, as well as studies of other African adapiforms like ‘Anchomomys’ milleri, suggests a more ancient ghost lineage for lemuriforms in Africa.
The selective pressure that shaped the original lemuriform toothcomb has been a topic of considerable debate since the 1970s. Evidence can be seen as supporting a grooming function, food procurement function, or both.In the early 1900s, there was less debate. Grooming was seen as the primary function since primates lack the claws needed to adequately comb the fur, although prosimian primates (strepsirrhines and tarsiers) possess at least one grooming claw on each foot to compensate. Grooming—in the form of fur-combing—is generally considered the primary function and original role of the lemuriform toothcomb, and subsequent changes in morphology across multiple lineages have altered its function and obscured its original function.
The hypothesis that the toothcomb evolved for food procurement was based on observations of recent lemuriform taxa, such as cheirogaleid lemurs (particularly fork-marked lemurs and the hairy-eared dwarf lemur) and galagos, which demonstrate tooth-scraping of plant exudates, as well as sifakas, which practice bark-prising. Each of these were considered "primitive" forms among the living strepsirrhines, suggesting the first lemuriforms exhibited similar behaviors.Also, strong selective pressure from feeding ecology placed on the anterior dentition was emphasized, based on the specialized upper anterior dentition seen in the recently extinct koala lemurs (Megaladapis). If feeding ecology could have such profound effects on the shape of the anterior dentition, then convergent evolution might explain the similarities seen between the compressed lower incisors of the lemuriform toothcomb and the exudate feeding adaptations in the genus Callithrix (a type of marmoset).
In contrast, the grooming hypothesis emphasized that all lemuriforms use their toothcombs for grooming, and long, thin teeth are poorly suited for the mechanical stress of gouging and exudate feeding.Also the interdental spaces seen in most lemuriforms favor fur combing and would also promote bacterial growth and tooth decay if used for exudate feeding. Supporting this, reduced interdental spacing is found in exudate feeding lemuriforms. Furthermore, the canine included in the toothcomb provides additional interdental spacing for fur combing. Even the behavior of young lemuriforms suggests that grooming plays a more important role in the use of the toothcomb than food procurement.
Colugos are arboreal gliding mammals that are native to Southeast Asia. Their closest evolutionary relatives are primates. There are just two living species of colugos: the Sunda flying lemur and the Philippine flying lemur. These two species make up the entire family Cynocephalidae and order Dermoptera. They are the most capable gliders of all gliding mammals. A fur-covered membrane, called patagium, connects to the face, paws, and tail. This enables them to glide in the air for distances of up to 200 meters between trees. They are also known as cobegos or flying lemurs. However, they are not actually lemurs, though they resemble them, and like colugos, some lemur species are nocturnal.
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. Collectively they are referred to as strepsirrhines. Also belonging to the suborder are the extinct adapiform primates that thrived during the Eocene in Europe, North America, and Asia, but disappeared from most of the Northern Hemisphere as the climate cooled. Adapiforms are sometimes referred to as being "lemur-like", although the diversity of both lemurs and adapiforms does not support this comparison.
Prosimians are a group of primates that includes all living and extinct strepsirrhines, as well as the haplorhine tarsiers and their extinct relatives, the omomyiforms, i.e. all primates excluding the simians. They are considered to have characteristics that are more "primitive" than those of simians.
Lemurs are mammals of the order Primates, divided into 8 families and consisting of 15 genera and around 100 existing species. They are native only to the island of Madagascar. Most existing lemurs are small, have a pointed snout, large eyes, and a long tail. They chiefly live in trees (arboreal), and are active at night (nocturnal).
Lemuriformes is an infraorder of primate that falls under the suborder Strepsirrhini. It includes the lemurs of Madagascar, as well as the galagos and lorisids of Africa and Asia, although a popular alternative taxonomy places the lorisoids in their own infraorder, Lorisiformes.
Notharctus tenebrosus was an early primate from the early Eocene, some 54–38 million years ago. Its fossil was found by Ferdinand V. Hayden in 1870 in southwestern Wyoming. When first found, Notharctus tenebrosus was thought to be a small pachyderm due to the concentration of pachyderm fossils in the area. However, after Walter W. Granger's discovery of a nearly complete skeleton, also in Wyoming, it was firmly established as a primate. Notharctus tenebrosus most resembles modern-day lemurs, although they are not directly related.
Lorisoidea is a superfamily of nocturnal primates found throughout Africa and Asia. Members include the galagos and the lorisids. As strepsirrhines, lorisoids are related to the lemurs of Madagascar and are sometimes included in the infraorder Lemuriformes, although they are also sometimes placed in their own infraorder, LorisiformesGregory, 1915.
Fork-marked lemurs or fork-crowned lemurs are strepsirrhine primates; the four species comprise the genus Phaner. Like all lemurs, they are native to Madagascar, where they are found only in the west, north, and east sides of the island. They are named for the two black stripes which run up from the eyes, converge on the top of the head, and run down the back as a single black stripe. They were originally placed in the genus Lemur in 1839, later moved between the genera Cheirogaleus and Microcebus, and given their own genus in 1870 by John Edward Gray. Only one species was recognized, until three subspecies described in 1991 were promoted to species status in 2001. New species may yet be identified, particularly in northeast Madagascar.
Altiatlasius is an extinct genus of mammal, which may have been the oldest known primate, dating to the Late Paleocene from Morocco. The only species, Altiatlasius koulchii, was described in 1990.
Bugtilemur is an extinct genus of Strepsirhine primate belonging to the adapiform family Ekgmowechashalidae.It is represented by only one species, B. mathesoni, which was found in the Chitarwata Formation of Pakistan.
Algeripithecus is an extinct genus of early fossil primate, weighing approximately 65 to 85 grams. Fossils have been found in Algeria dating from 50 to 46 million years ago.
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 primates, and thus are often confused as being ancestral to modern monkeys, apes, and humans. Instead, they merely resemble ancestral primates.
Dermotherium is a genus of fossil mammals closely related to the living colugos, a small group of gliding mammals from Southeast Asia. Two species are recognized: D. major from the Late Eocene of Thailand, based on a single fragment of the lower jaw, and D. chimaera from the Late Oligocene of Thailand, known from three fragments of the lower jaw and two isolated upper molars. In addition, a single isolated upper molar from the Early Oligocene of Pakistan has been tentatively assigned to D. chimaera. All sites where fossils of Dermotherium have been found were probably forested environments and the fossil species were probably forest dwellers like living colugos, but whether they had the gliding adaptations of the living species is unknown.
The sublingua ("under-tongue") is a muscular secondary tongue found below the primary tongue in tarsiers and living strepsirrhine primates, which includes lemurs and lorisoids. Although it is most fully developed in these primates, similar structures can be found in some other mammals, such as marsupials, treeshrews, and colugos. This "second tongue" lacks taste buds, and in lemuriforms, it is thought to be used to remove hair and other debris from the toothcomb, a specialized dental structure used to comb the fur during oral grooming.
Azibiidae is an extinct family of fossil primate from the late early or early middle Eocene from the Glib Zegdou Formation in the Gour Lazib area of Algeria. They are thought to be related to the living toothcombed primates, the lemurs and lorisoids, although paleoanthropologists such as Marc Godinot have argued that they may be early simians. It includes the genera Azibius and Algeripithecus, the latter of which was originally considered the oldest known simian, not a strepsirrhine.
Azibius is an extinct genus of fossil primate from the late early or early middle Eocene from the Glib Zegdou Formation in the Gour Lazib area of Algeria. They are thought to be related to the living toothcombed primates, the lemurs and lorisoids, although paleoanthropologists such as Marc Godinot have argued that they may be early simians. Originally described as a type of plesiadapiform, its fragmentary remains have been interpreted as a hyopsodontid, an adapid, and a macroscelidid. Less fragmentary remains discovered between 2003 and 2009 demonstrated a close relationship between Azibius and Algeripithecus, a fossil primate once thought to be the oldest known simian. Descriptions of the talus in 2011 have helped to strengthen support for the strepsirrhine status of Azibius and Algeripithecus, which would indicate that the evolutionary history of lemurs and their kin is rooted in Africa.
Djebelemur is an extinct genus of early strepsirrhine primate from the late early or early middle Eocene period from the Chambi locality in Tunisia. Although they probably lacked a toothcomb, a specialized dental structure found in living lemuriforms, they are thought to be a related stem group. The one recognized species, Djebelemur martinezi, was very small, approximately 100 g (3.5 oz).
Djebelemuridae is an extinct family of early strepsirrhine primates from Africa. It consists of five genera. The organisms in this family were exceptionally small, and were insectivores. It is predicted that this family existed from early to late Eocene, they lacked a teeth comb and were able to fully rotate their heads. It is also predicted that this family was a pivotal point for primate evolution, and that they were the cause for the adaption of a tooth comb.
Plesiopithecus is an extinct genus of early strepsirrhine primate from the late Eocene.
Sivaladapis is a genus of adapiform primate that lived in Asia during the middle Miocene.
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