Tribosphenida

Tribosphenida is a clade of mammals that includes the ancestor of Hypomylos, Aegialodontia and Theria (the last common ancestor of marsupials and placentals plus all of its descendants). It belongs to the group Zatheria. 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 australosphenidans, a group of mammals from the Jurassic-Cretaceous of the Southern Hemisphere often suggested to be close relatives of living monotremes, 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. 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.