Tribosphenida is a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of mammals that includes the ancestor of ''Hypomylos'',

Aegialodontia Aegialodontia is a clade of extinct early mammals, close to the origin of Boreosphenida. The clade includes some of the oldest known tribosphenic taxa, until the discovery of '' Tribactonodon'' from the Berriasian Durlston Formation in 2001, '' A ...

and

Theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

(the last common ancestor of

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s and

placental 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 distinguished ...

s plus all of its descendants). It belongs to the group

Zatheria Cladotheria is a clade (sometimes ranked as a Legion (taxonomy), legion) of mammals. It contains modern therian mammals (marsupials and Placentalia, placentals) and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. ...

. The current definition of Tribosphenida is more or less synonymous with Boreosphenida.

Characteristics

Tribosphenid mammals were originally grouped on the basis of triangular or V-shaped ( tribosphenic) molars. The relationship of the also tribosphenic

australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those ...

ns, a group of mammals from the Jurassic-Cretaceous of the Southern Hemisphere often suggested to be close relatives of living

monotremes Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

, has been questioned, and it has been argued that they developed tribosphenic molars independently from those of "true" tribosphenidans. Some authors have alternatively continued to argue that non-monotreme australosphenidans are in fact true tribosphenidans unrelated to monotremes. "True" unambiguous members of Tribosphenida are placed as part of the clade Boreosphenida, united by characteristics such as the lack of a mesial cingulid and of a triangulated trigonid on the last

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

. They are also united by postcranial features such as the presence of a modern

ear In vertebrates, an ear is the organ that enables hearing and (in mammals) body balance using the vestibular system. In humans, the ear is described as having three parts: the outer ear, the middle ear and the inner ear. The outer ear co ...

(though this too has evolved independently in many other groups, like

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s), modern shoulder blades, and several features of the hindlimb.

Phylogeny

Below is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

showing one hypothesis of mammal relationships based on Rowe (1988) and McKenna and Bell (1997):

Boreosphenida

Boreosphenida (from ''boreas'', "northern wind" and ''sphen'', "wedge") were early

mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

that originated in 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 by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

and had

tribosphenic molar 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 toot ...

s (three-cusped cheek teeth). In boreosphenidans, the mandibular angle is placed posteriorly and the primitive

postdentary trough The postdentary trough is a skeletal feature seen in Mesozoic mammals. It is found on the inside of the lower jaw (dentary), at the back behind the molar teeth. It is the hollow in which the postdentary bones and Meckel's cartilage sit. These bone ...

(hole in the mandible) is absent (in contrast to

Kuehneotheriidae Kuehneotheriidae is an extinct family of mammaliaforms traditionally placed within 'Symmetrodonta', though now generally considered more basal than true symmetrodonts. All members of Kuehneotheriidae which have been found so far are represented o ...

Eupantotheria Pantotheria is an abandoned taxon of Mesozoic mammals. This group is now considered an informal "wastebasket" taxon and has been replaced by Dryolestida as well as other groups. It is sometimes treated as an infraclass and older books refer to ...

, and

Australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those ...

.) They share the tribosphenic molars with the Australosphenida but differ from them by having cingulid cuspules but lacking a continuous mesial cingulid. Boreosphenidans also lack the triangulated trigonid on the last premolar found in Early Cretaceous mammals. They differ from ''

Shuotherium ''Shuotherium'' is a fossil mammaliaform known from Middle Jurassic, Middle-Late Jurassic of the Forest Marble Formation of England,Sigogneau−Russell, D. 1998. Discovery of a Late Jurassic Chinese mammal in the upper Bathonian of England. Compt ...

'' (a monotreme-relative) in having the talonid placed posterior to the trigonid (like in modern tribosphenic mammals) in the lower molars, but upper molars similar to those of ''Shuotherium''. The oldest boreosphenidans are from the

Berriasian In the geological timescale, the Berriasian is an age/ stage of the Early/Lower Cretaceous. It is the oldest subdivision in the entire Cretaceous. It has been taken to span the time between 143.1 ±0.6 Ma and 137.05 ± 0.2 (million years ago) ...

(~145-140 mya). They were restricted to the Northern Hemisphere during the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

, but spread to South America and India during the end of the Cretaceous. An alternative hypothesis is offered by Flannery and colleagues, who view part of australosphenidans as members of Tribosphenida (excluding monotremes and others australosphenidans with ''

Steropodon ''Steropodon'' is a genus of prehistoric platypus-like monotreme, or Oviparity, egg-laying mammal. It contains a single species, ''Steropodon galmani'', that lived about 100.2–96.6 million years ago during the Cretaceous period, from early to m ...

'', which they consider to be an unrelated group), with Barremian-Aptian Australian Bisphopidae as the sister taxon to all Northern Hemisphere Theria (Boreosphenida). On the other hand, the study that cites Flannery's article, given in 2024, maintains '' Ambondro'' as a relative of Monotremata. Implying support for the Australosphenida-Boreosphenida hypothesis. It may also be that both studies are right, consequently Tribosphenida would be a monophyletic group that contains the crown group Mammalia. Australosphenida possible

paraphyly Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

References

External links

* * * * {{Taxonbar, from=Q941624 Mammal taxonomy