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 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 australosphenidans (a group that includes the still extant

monotreme 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 brain ...

s), suggesting that as a synapomorphy this is fundamentally useless as it evolved multiple times among mammals. However, a clade between the aforementioned groups, the "true Tribosphenida" or Boreosphenida, is still identifiable, 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 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 (though this too has evolved independently in many other groups, like

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

Phylogeny

Below is a cladogram 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 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 molars (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, Eupantotheria, and Australosphenida.) 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'' (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 (~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