Tribosphenida
| Tribosphenida Temporal range: Middle Jurassic - Present, 165–0 Ma |
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| Kangaroo with her joey | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Clade: | Zatheria |
| Clade: |
Tribosphenida McKenna, 1975 |
| Clades | |
| Synonyms | |
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Boreosphenida |
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Boreosphenida
Tribosphenida is a group (infralegion) of mammals that includes the ancestor of Hypomylos, Dryolestoidea, 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.
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 monotremes), 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. 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 monotremes), modern shoulder blades, and several features of the hindlimb.
Below is a cladogram from Rowe (1988) and McKenna and Bell (1997) showing one hypothesis of mammal relationships:
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