146-65 million years ago
Ancestral therian mammals evolved a number of modifications of their skeletal systems.
Modern placental and marsupial mammals are classified as therians because they give live girth to their young. Kuehneotherium had dental characteristics (such as molar occlusion patterns) of therian mammals. A few species in the Cretaceous seem to predate the split between marsupials and placentals such as Deltatherium (tooth formula 4?,1,3,3-4/1-2,1,3,4-3) and Kielantherium. Some extinct groups left no descendants.
In therian mammals, the occipital condyles were positioned higher on
the skull and the maxillary and mandibular branches of the trigeminal
pass through alisphenoid (through the foramen rotundum and foramen ovale)
as opposed to between alisphenoid and petrosal. The ectopterygoid was
no longer part of the palate, the alisphenoid was expanded, and, after
protoeutherians, a groove existed in the sphenoid for the promontory artery.
The angular process of the dentary was more developed. In primitive therian
mammals, the cusps on cheek teeth were triangular but no longer linear.
Tribosphenic molars had evolved by the Early Cretaceous whose features
included a talonid basin and a protocone.