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MESOZOIC ERA

CRETACEOUS PERIOD

146-65 million years ago

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Earlly eutherians evolved a number of modifications of the skeletal system.

Sinuses evolved in some skull bones in eutherian mammals. In protoeutherians such as Kennalestes and all higher eutherians, the temporal bone increased its size as a component of the braincase. In most eutherians after the level of insectivores, outgrowths of bones surrounding the auditory ossicles form an auditory bulla, although there may be variations between groups of mammals as to the contributions that each bone makes to the bulla. The carotid artery was located more medially in eutherians. In eutherians, the orbitosphenoid was reduced and the optic foramen enlarged.


Palatal fenestrae developed in eutherians, the palatine made a smaller contribution to the orbit, the nasal bones became narrower, and the mandibular condyle was no longer lower than the coronoid process. Some of the primitive mammals, even protoeutherians such as Kennalestes, retained a small coronoid bone on the medial side of the lower jaw.


In protoeutherians, the last premolar became semimolariform. Asiorcytes and Ukhaatherium had 5/4 incisors (upper/lower; 5 upper incisors is a primitive trait) as in early marsupials and the last premolar beginning to become molar-like. Kennalestes had 4/3 incisors; adults possessed 4 premolars but infants retained 5.
A number of modifications also evolved in the postcranial skeleton. In the sternum, the manubrium was enlarged. Protoeutherians such as Zalambdalestes lacked separate coracoid bones in the shoulder. In eutherians, the supraspinous fossa is enlarged and located more laterally, the ulna became narrower, and the wrist of protoeutherians was similar to that of the most primitive living eutherians.


In the leg, eutherians reduced the size of the greater trochanter and lesser trochanter. In early protoeutherians, (such as Asioryctes), the astragalus was not dorsal to the calcaneus. This changed in more advanced protoeutherians such as Zalambestes and later eutherians. In eutherians, the digits and metatarsals became more elongate, the foot became more narrow, and the calcaneal contact with fibula was reduced.