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
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.