Docodonta is an
order of extinct
mammaliaforms
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recen ...
that lived during the
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
, from the
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...
to
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
. They are distinguished from other early
mammaliaforms
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recen ...
by their relatively complex
molar teeth, from which the order gets its name.
Until recently, Docodonta were represented primarily by teeth and jaws found across former
Laurasia
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
, (modern-day
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
,
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, and
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...
). However, recent discoveries in
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
include some exceptionally well preserved, almost complete body fossils.
Description
Skeletal traits
Docodonts have a long and low
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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
(lower jaw), formed primarily by the tooth-bearing
dentary
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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
bone. The dentary connects to the cranium via a joint with the
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone.
In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral co ...
, a connection which is strengthened relative to earlier mammaliaforms. The other bones in the jaw, known as postdentary elements, are still connected to the dentary and lie within a groove (the
postdentary trough) in the rear part of the dentary’s inner edge. Nevertheless, they are very slender, hosting hooked prongs which start to converge towards an oval-shaped area immediately behind the dentary. The
ecotympanic bone, also known as the
angular, fits into a deep slot on the dentary which opens backwards, a characteristic unique to docodonts. The
malleus
The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations fro ...
(also known as the
articular
The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals.
Anatomy
In most vertebrates, the articular bone is connected to two oth ...
) sends down a particularly well-developed prong known as the manubrium, which is sensitive to vibrations. The
incus
The ''incus'' (plural incudes) or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmit ...
(also known as the
quadrate) is still relatively large and rests against the petrosal bone of the braincase, a remnant of a pre-mammalian style jaw joint. In true mammals, the postdentary elements detach fully and shrink further, becoming the
ossicles of the middle ear and embracing a circular
eardrum
In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear
The outer ear, external ear, or auris externa is the extern ...
.
Docodont skulls are generally fairly low, and in general form are similar to other early mammliaforms such as
morganucodonts
Morganucodonta ("Glamorgan teeth") is an extinct order of basal Mammaliaformes, a group including crown-group mammals (Mammalia) and their close relatives. Their remains have been found in Southern Africa, Western Europe, North America, India and ...
. The snout is long and has several
plesiomorphic
In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.
Plesiomorphy, symplesiomorphy, apomorphy, and ...
traits: the paired
nares (nostril holes) are small and separate, and their rear edge is formed by a large
septomaxilla, a bone which is no longer present in mammals. The
nasal bones
The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose.
Eac ...
expand at the back and overlook thick
lacrimals
The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the Orbit (anatomy), orbit. It has two surfaces and four borders. Several ...
. The
frontal
Front may refer to:
Arts, entertainment, and media Films
* ''The Front'' (1943 film), a 1943 Soviet drama film
* ''The Front'', 1976 film
Music
* The Front (band), an American rock band signed to Columbia Records and active in the 1980s and e ...
and
parietal bones of the skull roof are flat and broad, and there is no
postorbital process
The postorbital process is a projection on the frontal bone near the rear upper edge of the eye socket. In many mammals, it reaches down to the zygomatic arch, forming the postorbital bar.
References
See also
* Orbital process
In the human ...
forming the rear rim of the
orbit
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 ...
(eye socket).
The oldest unambiguous fossil evidence of
hair
Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals.
The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...
is found in a well-preserved specimen of the docodont ''
Castorocauda
''Castorocauda'' is an extinct, semi-aquatic, superficially otter-like genus of docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the Yanliao Biota, found in the Daohugou Beds of Inner Mongolia, China dating to the M ...
'', though hair likely evolved much earlier in
synapsids.
Docodonts also see the first occurrence of a mammalian-style saddle-shaped
hyoid
The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical vertebra. ...
complex. ''
Microdocodon'' has a straight, sideways-oriented
basihyal which connects to two pairs of bony structures: the anterior hyoid cornu (a jointed series of rods which snake up to the braincase), and the posterior
thyrohyals (which link to the
thyroid cartilage
The thyroid cartilage is the largest of the nine cartilages that make up the ''laryngeal skeleton'', the cartilage structure in and around the trachea that contains the larynx. It does not completely encircle the larynx (only the cricoid cartilage ...
). This hyoid system affords greater strength and flexibility than the simple, U-shaped hyoids of earlier
cynodonts
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
. It allows for a narrower and more muscular throat and tongue, which are correlated with uniquely mammalian behaviors such as
suckling.
The structure of the
vertebral column
The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordata, ...
is variable between docodonts, as with many other mammaliaforms. The components of the
atlas
An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth.
Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...
are unfused, attaching to the large and porous
occipital condyles
The occipital condyles are undersurface protuberances of the occipital bone in vertebrates, which function in articulation with the superior facets of the atlas vertebra.
The condyles are oval or reniform (kidney-shaped) in shape, and their anteri ...
of the braincase.
Vertebrae at the base of the tail often have expanded
transverse processes
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...
(rib pedestals), supporting powerful tail musculature.
Most docodonts have gradually shrinking ribs, forming a subdued transition between the
thoracic
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the crea ...
and
lumbar
In tetrapod anatomy, lumbar is an adjective that means ''of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum.''
The lumbar region is sometimes referred to as the lower spine, or as an area of the back i ...
regions of the spine. However, this developmental trait is not universal: ''
Agilodocodon'' lacks lumbar ribs, for example.
The forelimbs and hindlimbs generally have strong muscle attachments, and the
olecranon process
The olecranon (, ), is a large, thick, curved bony eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...
of the
ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
is flexed inwards.
All limb bones except the
tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
lack
epiphyses
The epiphysis () is the rounded end of a long bone, at its joint with adjacent bone(s). Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, including the epiphyseal plate (growth plate). At the join ...
, plate-like ossified cartilage caps which terminate bone growth in adulthood. This suggests that docodont bones continued growing throughout their lifetime, like some other mammaliaforms and early mammals.
The ankle is distinctive, with a downturned
calcaneum
In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock.
...
and a stout
astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
which connects to the tibia via a trochlea (pulley-like joint).
The only known specimen of ''Castorocauda'' has a pointed
spur
A spur is a metal tool designed to be worn in pairs on the heels of riding boots for the purpose of directing a horse or other animal to move forward or laterally while riding. It is usually used to refine the riding aids (commands) and to back ...
on its ankle, similar to defensive structures observed in male
monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
and several other early-branching mammals.
Teeth
![Haldanodon molars](https://upload.wikimedia.org/wikipedia/commons/d/d3/Haldanodon_molars.png)
Compared to other early mammaliaforms, docodont
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 ...
are quite complex. Their structure is defined by a characteristic pattern of conical
cusps, with sharp, concave crests connecting the center of each cusp to adjacent cusps. When seen from below, the upper molars have an overall subtriangular or figure-eight shape, wider (from side to side) than they are long (from front to back). The bulk of the tooth makes up four major cusps: cusps A, C, X, and Y. This overall structure is similar to the
tribosphenic teeth found in true
therian mammals, like modern
marsupials and
placentals
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
. However, homologizing docodont cusps to those of modern mammals has been a controversial and heavily-debated topic. Cusps A and C lie in a row along the labial edge of the tooth (i.e., on the outer side, facing the cheek). Cusp A is located in front of cusp C and is typically the largest cusp in the upper molars. Cusp X lies lingual to cusp A (i.e., positioned inwards, towards the midline of the skull). A distinct wear facet is found on the labial edge of cusp X, extending along the crest leading to cusp A. Cusp Y, a unique feature of docodonts, is positioned directly behind cusp X. Many docodonts have one or two additional cusps (cusps B and E) in front of cusp A. Cusp B is almost always present and is usually shifted slightly labial relative to cusp A. Cusp E, which may be absent in later docodonts, is positioned lingual to cusp B.
The lower molars are longer than wide. On average, they have seven cusps arranged in two rows. The labial/outer row has the largest cusp, cusp a, which lies between two more cusps. The other major labial cusps are cusp b (a slightly smaller cusp in front of cusp a) and cusp d (a much smaller cusp behind cusp a). The lingual/inner row is shifted backwards (relative to the labial row) and has two large cusps: cusp g (at the front) and cusp c (at the back). Two additional lingual cusps may be present: cusp e and cusp df. Cusp e lies in front of cusp g and is roughly lingual to cusp b. Cusp df (“docodont cuspule f”) lies behind cusp c and is lingual to cusp d. There is some variation in the relative sizes, position, or even presence of some of these cusps, though docodonts in general have a fairly consistent cusp pattern.
A distinct basin, known as a
pseudotalonid, lies in the front part of the tooth, between cusps a, g, and b. When the upper and lower teeth
occlude (fit together), the pseudotalonid acts as a receptacle for cusp Y of the upper molar, which may be termed the "pseudoprotocone". At the same time, cusp b of the lower molar shears into an area labial to cusp Y. Occlusion is completed when the rest of the upper molar slides between adjacent lower molar teeth, letting the rear edge of the preceding lower molar scrape against cusp X. This shearing-and-grinding process is more specialized than in any other early mammaliaform. An interlocking pseudotalonid and pseudoprotocone system references the
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 ...
-and-
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 ...
crushing complex of therian mammals, though this is a case of
convergent evolution
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
, as therian talonids lie at the back of the lower molar rather than the front. Docodont teeth can be considered "pseudotribosphenic" in this regard.
Pseudotribosphenic teeth are also found in
shuotheriids and
australophenidans, though these groups have a pseudotalonid which is positioned further forwards in their lower molars. This is another case of convergent evolution, as skeletal material has revealed that shuotheriids and australosphenidans are true mammals related to modern
monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
. Docodont and shuotheriid teeth are so similar that some genera, namely ''
Itatodon
''Itatodon'' is an extinct genus of primitive mammal known from the Bathonian aged Itat Formation of Russia. The genus is named after the formation, with the species being named after Leonid Petrovich Tatarinov who described the first docodont
fr ...
'' and ''
Paritatodon
''Paritatodon'' is an extinct mammal which existed in Kyrgyzstan and England during the Jurassic period. It was originally the holotype specimen of ''Shuotherium kermacki'', but Martin and Averianov (2010) argued that it resembled the genus ''It ...
'', have been considered members of either group.
Paleoecology
Docodonts and other
Mesozoic
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...
mammals were traditionally thought to have been primarily ground dwelling and
insectivorous, but recent more complete fossils from China have shown this is not the case. ''
Castorocauda
''Castorocauda'' is an extinct, semi-aquatic, superficially otter-like genus of docodont mammaliaforms with one species, ''C. lutrasimilis''. It is part of the Yanliao Biota, found in the Daohugou Beds of Inner Mongolia, China dating to the M ...
''
from the Middle Jurassic of China, and possibly ''
Haldanodon
''Haldanodon'' is an extinct docodont mammaliaform which lived in the Upper Jurassic (Kimmeridgian, about 145 million years ago). Its fossil remains have been found in Portugal, in the well-known fossil locality of Guimarota, which is in the Al ...
''
[Kühne W. G. and Krusat, G. 1972. Legalisierung des Taxon Haldanodon (Mammalia, Docodonta). Neues Jahrbuch für Geologie Monatshefte 1972:300-302][Krusat, G. 1991 Functional morphology of Haldanodon exspectatus (Mammalia, Docodonta) from the Upper Jurassic of Portugal. Fifth Symposium on Mesozoic Terrestrial Ecosystems and Biota.] from the Upper Jurassic of Portugal, were specialised for a semi-aquatic lifestyle. ''Castorocauda'' had a flattened tail and recurved molars, which suggests possible fish or aquatic invertebrate diet.
It was thought possible that docodonts had tendencies towards semi-aquatic habits, given their presence in wetland environments, although this could also be explained by the ease with which these environments preserve fossils compared with more terrestrial ones. Recent discoveries of other complete docodontans such as the specialised digging species ''
Docofossor
''Docofossor'' is an extinct mammaliaform (a docodont) from the Jurassic period. Its remains have been recovered in China from 160 million years old rocks. It appears to have been the earliest-known subterranean mammaliaform, with adaptations re ...
'',
and specialised tree-dweller ''
Agilodocodon''
suggest Docodonta were more ecologically diverse than previously suspected. ''Docofossor'' shows many of the same physical traits as the modern day
golden mole
Golden moles are small insectivorous burrowing mammals endemic to Sub-Saharan Africa. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, and other mole-like families, all o ...
, such as wide, shortened digits in the hands for digging.
Classification
The lineage of Docodonta evolved prior to the origin of living mammals:
monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
,
marsupials, and
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 ...
s. In other words, docodonts are outside of the mammalian
crown group
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...
, which only includes animals descended from the
last common ancestor
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
of living mammals. Previously, docodonts were sometimes regarded as belonging to
Mammalia
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
, owing to the complexity of their molars and the fact that they possess a
dentary-squamosal jaw joint. However, modern authors usually limit the term "Mammalia" to the crown group, excluding earlier
mammaliaforms
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recen ...
like the docodonts. Nevertheless, docodonts are still closely related to crown-Mammalia, to a greater extent than many other early mammaliaform groups such as
Morganucodonta and ''
Sinoconodon
''Sinoconodon'' is an extinct genus of mammaliamorphs that appears in the fossil record of the Lufeng Formation of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While sharing many plesiomorphic tra ...
''. Some authors also consider docodonts to lie crownward of the order
Haramiyida,
though most others consider haramiyidans to be closer to mammals than docodonts are.
Docodonts may lie crownward of haramiyidans in
phylogenetic analyses
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 ...
based on
maximum parsimony
In phylogenetics, maximum parsimony is an optimality criterion under which the phylogenetic tree that minimizes the total number of character-state changes (or miminizes the cost of differentially weighted character-state changes) is preferred. ...
, but shift stemward relative to haramiyidans when the same data is put through a
Bayesian analysis
Bayesian inference is a method of statistical inference in which Bayes' theorem is used to update the probability for a hypothesis as more evidence or information becomes available. Bayesian inference is an important technique in statistics, and ...
.
Cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
based on a phylogenetic analysis of Zhou et al. (2019) focusing on a wide range of mammaliamorphs:
Docodont fossils have been recognized since the 1880s, but their relationships and diversity have only recently been well-established.
Monographs
A monograph is a specialist work of writing (in contrast to reference works) or exhibition on a single subject or an aspect of a subject, often by a single author or artist, and usually on a scholarly subject.
In library cataloging, ''monograph ...
by
George Gaylord Simpson
George Gaylord Simpson (June 16, 1902 – October 6, 1984) was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the Modern synthesis (20th century), modern ...
in the 1920s argued that they were specialized "
pantotheres", part of a broad group ancestral to true therian mammals according to their complex molars.
A 1956 paper by
Bryan Patterson instead argued that docodont teeth were impossible to homologize with modern mammals. He drew comparisons to the teeth of ''
Morganucodon
''Morganucodon'' (" Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represe ...
'' and other "
triconodont
Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the p ...
" mammaliaforms, which had fairly simple lower molars with a straight row of large cusps. However, re-evaluations of mammaliaform tooth homology in the late 1990s established that docodonts were not closely related to either morganucodonts or therians.
Instead, they were found to be similar to certain early "
symmetrodonts", a broad and polyphyletic grouping of mammaliaforms with triangular upper molars.
In particular, the closest relatives of Docodonta have been identified as certain Late Triassic "symmetrodonts", such as ''
Delsatia
''Delsatia'' is an early mammaliaform genus that lived during the Late Triassic and has been found in France. The type species, ''D. rhupotopi'', was named in 1997.Sigogneau-Russell D. & Godefroit P., 1997 - A primitive docodont (Mammalia) from ...
'' and ''
Woutersia
''Woutersia'' was a Triassic genus of 'symmetrodont' and the only representative of the family Woutersiidae. It was originally classified as a kuehneotheriid, but it has been suggested that it may be related to Docodonta. Remains of ''W. mirabili ...
'' (from the
Norian
The Norian is a division of the Triassic Period. It has the rank of an age (geochronology) or stage (chronostratigraphy). It lasted from ~227 to million years ago. It was preceded by the Carnian and succeeded by the Rhaetian.
Stratigraphic defi ...
-
Rhaetian
The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...
of
France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
) and ''
Tikitherium
''Tikitherium'' is an extinct genus of mammaliaforms from the Late Triassic. It is thought to be an insectivore and a close relative to Docodonta. ''Tikitherium'' refers to Tiki, the village located near the Tiki Formation where the specimen wa ...
'' (from the
Carnian
The Carnian (less commonly, Karnian) is the lowermost stage of the Upper Triassic Series (or earliest age of the Late Triassic Epoch). It lasted from 237 to 227 million years ago (Ma). The Carnian is preceded by the Ladinian and is followed by t ...
of
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
).
These "symmetrodonts" have three major cusps (c, a, and b) set in a triangular arrangement on their lower molars. These cusps would be homologous to cusps c, a, and g in docodonts, which have a similar size and position. ''Tikitherium'' in particular is very similar to docodonts, as its wide upper molars have an apparent lingual cusp (cusp X) with a labial wear facet, though its cusp Y is comparitively underdeveloped. Cusp X is even more prominent in ''Woutersia'', though it lacks a wear facet in that genus.
Unambiguous docodonts are restricted to the
Northern Hemisphere
The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...
, abruptly appearing in the fossil record in the
Middle Jurassic
The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...
. Very few docodonts survived into the
Cretaceous Period
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of t ...
; the youngest known members of the group are ''
Sibirotherium
''Sibirotherium'' is an extinct genus of docodont mammaliaform. It is known from only a single named species, ''Sibirotherium rossicum'', known from jaw fragments and teeth found in the Early Cretaceous (Aptian) aged Ilek Formation
The Ilek F ...
'' and ''
Khorotherium'', from the
Early Cretaceous
The Early Cretaceous ( geochronological name) or the Lower Cretaceous (chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145 Ma to 100.5 Ma.
Geology
Pro ...
of
Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
.
One disputed docodont, ''
Gondtherium
''Gondtherium'' is a genus of extinct mammaliaform from the Kota Formation in India. It was considered a docodontan by those who described it, but it remains unclear if this is the case.
''Gondtherium'' was found in the Kota Formation, which is ...
'', has been described from India, which was previously part of the
Southern Hemisphere continent of
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
.
[Prasad GVR, and Manhas BK. 2007. A new docodont mammal from the Jurassic Kota Formation of India. Palaeontologia electronica, 10.2: 1-11.] However, this identification is not certain, and in recent analyses, ''Gondtherium'' falls outside the docodont family tree, albeit as a close relative to the group.
''
Reigitherium
''Reigitherium'' was a mammal that lived during the Late Cretaceous, in the (Late Campanian-Maastrichtian). Its fossils have been found in the Los Alamitos and the La Colonia Formations of Argentina.
Description
The original specimen of ''Re ...
'', from the
Late Cretaceous
The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...
of
Argentina
Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...
, has previously been described as a docodont, though it is now considered a
meridiolestidan
Meridiolestida is an extinct clade of mammals known from the Cretaceous and Cenozoic of South America and possibly Antarctica. They represented the dominant group of mammals in South America during the Late Cretaceous. Meridiolestidans were morph ...
mammal. Some authors have suggested splitting Docodonta into two families (Simpsonodontidae and Tegotheriidae),
but the
monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...
of these groups (in their widest form) are not found in any other analyses, and therefore not accepted by all mammal palaeontologists.
Cladograms based on phylogenetic analyses focusing on docodont relationships:
Topology of Zhou et al. (2019), based on tooth, cranial, and postcranial traits:
Topology of Panciroli et al. (2021), based on dentary and tooth traits:
Subgroups and genera
*Superfamily †Docodontoidea
**Family †Docodontidae
(Marsh 1887) Simpson 1929
*** †''
Agilodocodon scansorius
''Agilodocodon'' was a genus of shrew-sized docodont from the Middle Jurassic, believed to be the earliest known tree-climbing mammaliaform. It contains one species, ''A. scansorius''.
Appearance
''Agilodocodon'' measured approximately from ...
''
Meng et al. 2015
*** †''
Borealestes
''Borealestes'' was a genus of docodontan from the Middle Jurassic of Britain, first discovered on the Isle of Skye near the village of Elgol. It was the earliest mammaliaform from the Mesozoic found and named in Scotland. A second species and ...
''
Waldman & Savage 1972
**** †''
B. cuillinensis''
Panciroli et al. 2021
**** †''
B. serendipitus''
Waldman & Savage 1972[Waldman, M and Savage, R.J.G 1972 The first Jurassic mammal from Scotland. Journal of the Geological Society of London 128:119-125]
*** †''
Castorocauda lutrasimilis''
Ji et al. 2006
*** †''
Cyrtlatherium canei''
Freeman 1979 sensu Sigogneau-Russell 2001 (
dubious
Doubt is a mental state in which the mind remains suspended between two or more contradictory propositions, unable to be certain of any of them.
Doubt on an emotional level is indecision between belief and disbelief. It may involve uncertainty ...
)
'Simpsonodon_oxfordensis''_
Kermack_et_al._1987.html" ;"title="Simpsonodon_oxfordensis.html" ;"title="'Simpsonodon oxfordensis">'Simpsonodon oxfordensis''
Kermack et al. 1987">Simpsonodon_oxfordensis.html" ;"title="'Simpsonodon oxfordensis">'Simpsonodon oxfordensis''
Kermack et al. 1987*** †''Dobunnodon mussettae'' [''Borealestes mussetti'']
Sigogneau-Russell 2003 sensu Panciroli et al. 2021
*** †''Docodon''
Marsh 1881 [''Dicrocynodon''
Marsh in Osborn, 1888; ''Diplocynodon''
Marsh 1880 non Pomel 1847; ''Ennacodon''
Marsh 1890; ''Enneodon''
Marsh 1887 non Prangner 1845]
**** †''
Docodon, D. apoxys''
Rougier et al. 2014
**** †''
D. victor''
(Marsh 1880) (Marsh 1880); ''Diplocynodon victor'' Marsh 1880">'Dicrocynodon victor'' (Marsh 1880); ''Diplocynodon victor'' Marsh 1880**** †''
D. striatus''
Marsh 1881 isputed**** †''
D. affinis''
(Marsh 1887) Marsh 1887">'Enneodon affinis'' Marsh 1887 isputed**** †''
D. crassus''
(Marsh 1887) Marsh 1887; ''Ennacodon crassus'' (Marsh 1887)">'Enneodon crassus'' Marsh 1887; ''Ennacodon crassus'' (Marsh 1887) isputed**** †''
D. superus''
Simpson 1929 isputed*** †''
Docofossor brachydactylus''
Luo et al. 2015
*** †''
Dsungarodon zuoi''
Pfretzschner et al. 2005 'Acuodulodon''_
Hu,_Meng_&_Clark_2007;_''Acuodulodon_sunae.html" ;"title="Acuodulodon.html" ;"title="'Acuodulodon">'Acuodulodon''
Hu, Meng & Clark 2007; ''Acuodulodon sunae">Acuodulodon.html" ;"title="'Acuodulodon">'Acuodulodon''
Hu, Meng & Clark 2007; ''Acuodulodon sunae''
Hu, Meng & Clark 2007]
*** †''Gondtherium, Gondtherium dattai''
Prasad & Manhas 2007 isputed*** †''
Haldanodon exspectatus''
Kühne & Krusat 1972 sensu Sigoneau-Russell 2003
*** †''
Hutegotherium yaomingi''
Averianov et al. 2010
*** †''
Itatodon tatarinovi''
Lopatin & Averianov 2005 shuotheriid.html"_;"title="Shuotheriidae.html"_;"title="isputed,_possibly_a_Shuotheriidae">shuotheriid">Shuotheriidae.html"_;"title="isputed,_possibly_a_Shuotheriidae">shuotheriidref_name=":8"_/>
***_†''
shuotheriid.html"_;"title="Shuotheriidae.html"_;"title="isputed,_possibly_a_Shuotheriidae">shuotheriid">Shuotheriidae.html"_;"title="isputed,_possibly_a_Shuotheriidae">shuotheriidref_name=":8"_/>
***_†''Khorotherium">Khorotherium_yakutensis''_
Averianov_et_al._2018Alexander_Averianov;_Thomas_Martin;_Alexey_Lopatin;_Pavel_Skutschas;_Rico_Schellhorn;_Petr_Kolosov;_Dmitry_Vitenko_(2018)._"A_high-latitude_fauna_of_mid-Mesozoic_mammals_from_Yakutia,_Russia"._PLoS_ONE._13_(7):_e0199983._.
***_†''
shuotheriid.html"_;"title="Shuotheriidae.html"_;"title="isputed,_possibly_a_Shuotheriidae">shuotheriid">Shuotheriidae.html"_;"title="isputed,_possibly_a_Shuotheriidae">shuotheriidref_name=":8"_/>
***_†''Khorotherium">Khorotherium_yakutensis''_
Averianov_et_al._2018Alexander_Averianov;_Thomas_Martin;_Alexey_Lopatin;_Pavel_Skutschas;_Rico_Schellhorn;_Petr_Kolosov;_Dmitry_Vitenko_(2018)._"A_high-latitude_fauna_of_mid-Mesozoic_mammals_from_Yakutia,_Russia"._PLoS_ONE._13_(7):_e0199983._.
***_†''Krusatodon">Krusatodon_kirtlingtonensis
_
''Krusatodon''_is_a_genus_of_extinct__docodont_mammaliaform_from_the_Middle_Jurassic_of_the_United_Kingdom._It_is_known_from_the_Forest_Marble_Formation,_Kirtlington_
Kirtlington_is_a_village_and_Civil_parishes_in_England,_civil_parish_in_Oxf_...
''_
Sigogneau-Russell_2003
***_†''Microdocodon.html" ;"title="Krusatodon.html" "title="Khorotherium.html" ;"title="Shuotheriidae">shuotheriid.html" ;"title="Shuotheriidae.html" ;"title="isputed, possibly a Shuotheriidae">shuotheriid">Shuotheriidae.html" ;"title="isputed, possibly a Shuotheriidae">shuotheriidref name=":8" />
*** †''Khorotherium">Khorotherium yakutensis''
Averianov et al. 2018[Alexander Averianov; Thomas Martin; Alexey Lopatin; Pavel Skutschas; Rico Schellhorn; Petr Kolosov; Dmitry Vitenko (2018). "A high-latitude fauna of mid-Mesozoic mammals from Yakutia, Russia". PLoS ONE. 13 (7): e0199983. .]
*** †''Krusatodon">Krusatodon kirtlingtonensis
''Krusatodon'' is a genus of extinct docodont mammaliaform from the Middle Jurassic of the United Kingdom. It is known from the Forest Marble Formation, Kirtlington
Kirtlington is a village and Civil parishes in England, civil parish in Oxf ...
**** †''Peraiocynodon, P. inexpectatus''
ref name="Butler1939">Butler PM. 1939
In Proceedings of the Zoological Society of London, 109:329-356). Oxford, UK: Blackwell Publishing Ltd.
**** †''