Tribosphenida is a group (infralegion) 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 in pal ...

and

Theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. Ch ...

(the last common ancestor of

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...

s 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 plus all of its descendants). Its current definition is more or less synonymous with Boreosphenida.

Characteristics

Tribosphenid mammals were originally grouped on the basis of triangular or V-shaped ( tribosphenic) molars. Since then, other unrelated mammal groups have been found to have tribosphenic molars, such as the

australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are thought to have acquired their tribosphenic molars independently from those of Tribosphenid ...

ns (a group that includes the still extant monotremes), suggesting that as a

synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

this is fundamentally useless as it evolved multiple times among mammals. However, a clade between the aforementioned groups, the "true Tribosphenida" or

Boreosphenida Tribosphenida is a group (infralegion) of mammals that includes the ancestor of ''Hypomylos'', Aegialodontia and Theria (the last common ancestor of marsupials and placentals plus all of its descendants). Its current definition is more or less sy ...

, is still identifiable, united by characteristics such as the lack of a mesial

cingulid A cingulid is a term used when describing teeth, it refers to a ridge that runs around the base of the crown of a lower tooth (the equivalent on the upper teeth is the cingulum). The presence or absence of a cingulid is often a diagnostic featur ...

and of a triangulated trigonid on the last

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 ...

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

ear An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...

(though this too has evolved independently in many other groups, like monotremes), modern shoulder blades, and several features of the hindlimb.

Phylogeny

Below is a

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 ...

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

Boreosphenida

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

mammals 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 or ...

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 as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

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 to ...

s (three-cusped cheek teeth). In boreosphenidans, the mandibular angle is placed posteriorly and the primitive postdentary trough (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. They are thought to have acquired their tribosphenic molars independently from those of Tribosphenid ...

.) They share the tribosphenic molars with the Australosphenida but differ from them by having

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 mammal known from 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. Comptes Rendus de l’Acad ...

'' (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 145.0 ± 4.0 Ma and 139.8 ± 3.0 Ma (million years ago ...

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

, but spread to South America and India during the end of the Cretaceous.

References

External links

MESOZOIC MAMMALS; Stem zatherians, zatherians & Peramuridae, an internet directory

THERIA И MARSUPIONTA
(in Russian)
Infralegion Tribosphenida - Hierarchy - The Taxonomicon
{{Taxonbar, from=Q941624 Mammal taxonomy