''Dermotherium'' is a
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
mammals closely related to the living
colugo
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 (''Galeopterus variegatus'') and the Philippine fly ...
s, a small group of
gliding
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word ''soaring'' is al ...
mammals from Southeast Asia. Two species are recognized: ''D. major'' 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'', "daw ...
of
Thailand
Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is bo ...
, based on a single fragment of the lower jaw, and ''D. chimaera'' from the
Late Oligocene
The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stage ...
of Thailand, known from three fragments of the lower jaw and two isolated upper
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 ...
. In addition, a single isolated upper molar from the
Early Oligocene
The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene Epoch/Series. It spans the time between . It is preceded by the Priabonian Stage (part of the Eocene) and is followed by the Chatti ...
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.
Some features of the teeth differentiate ''Dermotherium'' from both living colugo species, but other features are shared with only one of the two. The third lower
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, whe ...
, lower
canine, and third lower
premolar
The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...
at least are
pectinate or comblike, bearing longitudinal rows of
tines
Tines (; also spelled tynes), prongs or teeth are parallel or branching spikes forming parts of a tool or natural object. They are used to spear, hook, move or otherwise act on other objects. They may be made of metal, wood, bone or other hard, ...
or
cusps, an unusual feature of colugos (the first two lower incisors are unknown in ''Dermotherium''). The fourth lower premolar instead resembles the lower molars. The front part of these teeth, the
trigonid
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 ...
, is broader in ''D. chimaera'' than in ''D. major'', which is known only from the second and third lower molars. The two species also differ in the configuration of the inner back corner of the lower molars. The upper molars are triangular teeth bearing several distinct small cusps, particularly on the second upper molar, and with wrinkled
enamel.
Taxonomy
Colugo
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 (''Galeopterus variegatus'') and the Philippine fly ...
s are a small group of Southeast Asian
gliding
Gliding is a recreational activity and competitive air sport in which pilots fly unpowered aircraft known as gliders or sailplanes using naturally occurring currents of rising air in the atmosphere to remain airborne. The word ''soaring'' is al ...
mammals closely related to the
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 huma ...
s. Their
fossil record
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
is exceptionally poor. Although
Paleogene
The Paleogene ( ; British English, also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period, geologic period and system that spans 43 million years from the end of the Cretaceous Period million yea ...
groups such as the
Plagiomenidae are considered by some to be closely related to colugos, no fossils undoubtedly referable to the living colugo
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
, Cynocephalidae, had been reported until 1992. In that year, Stéphane Ducrocq and colleagues described a jaw fragment from the
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
of Thailand as a new
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
and
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 ...
of colugo, ''Dermotherium major''. In 2000, however, Brian Stafford and Frederick Szalay argued that ''Dermotherium'' might not be a colugo, since the fossil is so poorly preserved that few traits can be unambiguously recognized and some purported dermopteran features of the fossil are also seen in other
placental
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupial
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsup ...
groups. Mary Silcox and colleagues reaffirmed the colugo affinities of ''Dermotherium'' in 2005 on the basis of detailed similarities in
molar morphology.
In 2006, Laurent Marivaux and colleagues described a second species of ''Dermotherium'', ''D. chimaera'', from material from the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
of Thailand. They gave it the
specific name ''chimaera'' (Latin for "chimera") because it shares characters with both the
Philippine colugo (''Cynocephalus volans'') and
Sunda colugo (''Galeopterus variegatus''), the two living colugo species. In addition, they tentatively identified a fossil from the Oligocene of Pakistan as ''Dermotherium chimaera'' and regarded some fossils from the Eocene of Myanmar as indeterminate dermopterans. According to a
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 ...
analysis carried out by Marivaux and colleagues, ''D. chimaera'', ''D. major'', and the Myanmar dermopteran are successive
sister group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
s of the two living colugos.
Description
Known material of ''Dermotherium'' includes a handful of jaw fragments and isolated teeth. ''Dermotherium major'' is known only from a fragment of the left lower jaw bearing the third lower molar (m3) and a poorly preserved second lower molar (m2). 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 several ...
of ''Dermotherium chimaera'' is a lower jaw fragment in which remnants of the
deciduous
In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, aft ...
third lower
premolar
The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...
are visible.
X-ray microtomography
X-ray microtomography, like tomography and X-ray computed tomography, uses X-rays to create cross-sections of a physical object that can be used to recreate a virtual model ( 3D model) without destroying the original object. The prefix ''micro-' ...
reveals the
unerupted lower third
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, whe ...
(i3),
canine (c1), third premolar (p3), and fourth premolar (p4) still inside the jaw. In addition, this species is known from two other jaw fragments, one bearing m1 and m2 and the other bearing m2 and m3, and two isolated molars, an upper first and second molar (M1 and M2). The tentatively referred material of this species from Pakistan includes a single M2.
The two species of ''Dermotherium'' were about as large as the Philippine colugo and larger than the Sunda colugo and differed from both in several characteristics of the dentition. Not enough is known of the skeleton of ''Dermotherium'' to assess whether the animal already possessed the gliding adaptations of living colugos. The two species are similar in size, but again differ in details of the dentition. In two specimens of ''D. chimaera'', the length and width of the m2 are and respectively; this tooth is long and wide in the only known specimen of ''Dermotherium major''. The Pakistani M2 of ''Dermotherium'' is long and wide, compared to in the only known M2 of ''D. chimaera'' from Thailand.
The lower jaw of ''Dermotherium major'' resembles that of living colugos in the presence of a strong
angular process (a projecting piece of bone at the lower side of the back of the jawbone) and a
retromolar space (a flat space behind the last molars). The
coronoid process (a projecting piece of bone directly behind the molars) rises steeply, with its front wall virtually vertical.
Lower dentition
The i3 of ''Dermotherium chimaera'' is an elongate tooth bearing six tines (narrow, high "fingers" as in a comb) arranged from front to back. The frontmost is larger, the next four are about equal in size, and the sixth is smaller. The number of tines resembles that seen in the Sunda colugo, which has four to seven; the Philippine colugo has three to five. The c1 is also an elongate, narrow tooth; at the front, it is slightly curved towards the midline of the jaw. On the buccal (outer) side of this tooth are six
cusps, of which the third (counting from the front) is the largest. The p3 is similarly elongate and rounded at the front, but it is broader at the back, forming a
talonid
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 ...
(a "heel" of cusps at the back of a
tribosphenic tooth). There are six cusps on the narrow anterior part of the tooth, and the fifth (identified as the
protoconid
Many different terms have been proposed for features of the tooth crown in mammals.
The structures within the molars receive different names according to their position and morphology. This nomenclature was developed by Henry Fairfield Osborn i ...
) is by far the largest. On the buccal side of the talonid is a strong cusp, the
hypoconid
Many different terms have been proposed for features of the tooth crown in mammals.
The structures within the molars receive different names according to their position and morphology. This nomenclature was developed by Henry Fairfield Osborn ...
, with a crest, the
cristid obliqua, descending from it towards the front. A second, smaller cusp, the
hypoconulid, is present on the lingual side of the talonid, connected to the hypoconid by a
postcristid. The
pectinate (comb-like) shape of the anterior teeth is a shared characteristic of the colugos and highly unusual among mammals. ''Dermotherium chimaera'' resembles the Sunda colugo in that the c1 and p3 are also pectinate; in the Philippine colugo, these teeth are not pectinate.
In ''Dermotherium chimaera'', the p4 and m1 through m3 are similar to each other (and unlike the i3, canine, and p3) and appear to form a series of decreasing size from front to back. In ''D. major'', only m2 and m3 are known, but they are similar in morphology and m3 is smaller than m2. In the Sunda colugo, however, the teeth get larger from p4 to m3. The p4 and molars have a distinct
trigonid
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 ...
(a triangular group of cusps at the front of a tribosphenic tooth) and talonid. The trigonid contains strong protoconid and
metaconid
Many different terms have been proposed for features of the tooth crown in mammals.
The structures within the molars receive different names according to their position and morphology. This nomenclature was developed by Henry Fairfield Osborn ...
cusps. The metaconid is stronger than the protoconid in the molars, but it is not clear whether this is the case in the p4. In the living colugos, the protoconid is higher than the metaconid in both p4 and m1. The trigonid is longer in the p4 than in the molars. In ''D. chimaera'', the trigonid is broader than in ''D. major''. A low crest, the
paracristid, descends from the protoconid lingually and towards the front, forming the front margin of the trigonid; there is no distinct cusp (a
paraconid) at the front of the trigonid on the p4, but this cusp is present in the molars.
The talonid contains a hypoconid, hypoconulid, and
entoconid
Many different terms have been proposed for features of the tooth crown in mammals.
The structures within the molars receive different names according to their position and morphology. This nomenclature was developed by Henry Fairfield Osborn i ...
and is much wider than the trigonid because the hypoconid is displaced buccally. A cristid obliqua descends from the hypoconid and reaches the protoconid. Although a crest, the
postmetacristid, descends from the back side of the metaconid, ending in a small cusp, the
metastylid, it is separated from the entoconid by a notch. The living colugos lack such a strong postmetacristid. The hypoconulid is near the back lingual margin of the tooth, behind the entoconid. In ''D. major'', this cusp is further to the back than in both ''D. chimaera'' and the Sunda colugo, while the two cusps are merged in the Philippine colugo. In ''D. chimaera'', a low crest, the
post-hypoconulid cristid, reaches from the hypoconulid to the back lingual corner of the tooth, where a small cuspule, the
distocuspid, is located. A long crest, the postcristid, connects the distocuspid to the hypoconid along the posterior side of the tooth. The Philippine colugo is similar, but in ''D. major'', both the distocuspid and the post-hypoconulid cristid are absent, and the Sunda colugo has a weaker distocuspid and a postcristid that does not reach further lingually than the hypoconulid.
Upper dentition
The upper molars of ''Dermotherium chimaera'' are triangular in overall shape and much broader than long, with the narrow end of the triangle pointing lingually. The M2 from Pakistan that was tentatively placed in ''D. chimaera'' is slightly smaller than Thai fossils of the species, but otherwise very similar. The crest in front of and behind the major cusps on the buccal side of the tooth, the
paracone
A paracone is a 1960s atmospheric reentry or spaceflight mission abort concept using an inflatable ballistic cone.[metacone
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 ...]
, are well-developed, together forming a long W-shaped ridge. ''D. chimaera'' resembles the Philippine colugo in that the crests behind the paracone and in front of the metacone form an acute angle with each other, so that together they form a V; their shape rather resembles a U in the Sunda colugo. A
cingulum (shelf) is present on the buccal margin of the tooth, but this cingulum is rather weak in the Pakistani fossil. On the M2, smaller cusps, the
paraconule and
metaconule, are present on the lingual sides of the paracone and metacone, but on the M1 the paraconule is missing and the metaconule is small and ridgelike. The small cusps are better developed in the Pakistani M2. A third major cusp, the
protocone
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 ...
, is present on the lingual side of both upper molars. This crest is displaced towards the front in the living colugos, but less so in ''D. chimaera''. Two strong crests descend from the front and back faces of the protocone in a buccal direction. These crests end in small cusps (
protoconules) that are directly lingual to the paracone and metacone. The living colugos lack these protoconules. The
enamel is wrinkled on the flanks of the paracone and metacone and, in the Thai but not the Pakistani specimens, on the lingual side of the protocone. Enamel wrinkling is also seen in some Philippine colugos.
Range and ecology
''Dermotherium major'' was found in a
lignite
Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat ...
pit known as Wai Lek in
Krabi Province, southwestern Thailand. It is part of the
Krabi Basin fauna, which contains at least 40 mammalian genera, mostly
artiodactyl
The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...
s but also including some primates, such as ''
Siamopithecus'', ''
Wailekia'', and ''
Muangthanhinius
''Muangthanhinius'' is a genus of adapiform primate that lived in Asia during 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 Per ...
''. This fauna has been dated to about 33 to 35 million years ago, during 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'', "daw ...
.
Fossils of ''Dermotherium chimaera'' come from
Phetchaburi Province
Phetchaburi ( th, เพชรบุรี, ) or Phet Buri () is one of the western or central provinces (''changwat'') of Thailand. Neighboring provinces are (from north clockwise) Ratchaburi, Samut Songkhram, and Prachuap Khiri Khan. In the w ...
, further to the north in Thailand. The fossil deposits, located in the Cha Prong pit in a coal mine called
Nong Ya Plong, are dated to the
Late Oligocene
The Chattian is, in the geologic timescale, the younger of two ages or upper of two stages of the Oligocene Epoch/Series. It spans the time between . The Chattian is preceded by the Rupelian and is followed by the Aquitanian (the lowest stage ...
. Other mammals found here include the rodent ''
Fallomus ladakhensis'', the rhinoceros
''Diceratherium'' sp. cf. ''D. lamilloquense'', the
carnivora
Carnivora is a Clade, monophyletic order of Placentalia, placental mammals consisting of the most recent common ancestor of all felidae, cat-like and canidae, dog-like animals, and all descendants of that ancestor. Members of this group are f ...
n ''
Chaprongictis phetchaburiensis'', the
lipotyphla
Lipotyphla is a formerly used order (biology), order of mammals, including the members of the order Eulipotyphla (i.e. the solenodons, family Solenodontidae; hedgehogs and gymnures, family Erinaceidae; desmans, moles, and shrew-like moles, family T ...
n ''
Siamosorex debonisi'', and a
pteropodid bat.
Paali Nala, where the M2 tentatively assigned to ''Dermotherium chimaera'' was found, is in the
Bugti Hills of
Balochistan
Balochistan ( ; bal, بلۏچستان; also romanised as Baluchistan and Baluchestan) is a historical region in Western and South Asia, located in the Iranian plateau's far southeast and bordering the Indian Plate and the Arabian Sea coastline. ...
, western Pakistan. The fossil locality, placed
stratigraphically in the
Bugti Member of the
Chitarwata Formation
The Chitarwata Formation is a geological formation in western Pakistan, made up of Oligocene and early Miocene terrestrial fluvial facies. The sediments were deposited in coastal depositional environments (estuarine, strandplain and tidal flats) ...
, is thought to be
Early Oligocene
The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene Epoch/Series. It spans the time between . It is preceded by the Priabonian Stage (part of the Eocene) and is followed by the Chatti ...
in age. The Paali Nala fauna also contains a diverse array of mammals, including many rodents, some primates, and rhinos.
The specializations of dermopterans are such that they are dependent on a forested habitat, and reconstructions of the paleoenvironments at Krabi, Nong Ya Plong, and Paali Nala suggest that all three fossil deposits developed in a humid, tropical forest environment. Their extinction in the Indian subcontinent may have been caused by the development of a drier climate there during the Late Oligocene.
References
Literature cited
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{{Taxonbar, from=Q5262856
Colugos
Eocene mammals
Oligocene mammals
Prehistoric placental genera
Eocene mammals of Asia
Eocene genus first appearances
Chattian genus extinctions
Fossil taxa described in 1992