The modifications of the postcranial
skeleton in early mammals included the loss of lumbar and cervical ribs,
the loss of reptilian shoulder bones, the development of epipubic bones,
and modifications of the leg.
Early mammals also modified their vertebral columns. Large intervertebral
foramina developed between vertebrae in early mammals, perhaps to accommodate
the nerves of a larger brachial plexus. Lumbar ribs were lost in early
mammals and lumbar vertebrae achieved the mammalian pattern of neural
arch angles (with the anterior vertebrae angling posteriorly and the posterior
vertebrae angling anteriorly with transitional vertebrae between).
In the shoulder of early mammals, the procoracoid was excluded from the
glenoid fossa. Monotremes still retain both coracoid bones and the interclavicle,
unlike therian mammals. After the earliest mammals, separate centers of
ossification existed in the epiphyses of the humerus and the ectepicondylar
foramen was lost.
Many Mesozoic mammals, modern monotremes, and modern marsupials, have
small bones extending from the pubis called epipubic bones. Epipubic bones
existed in many primitive mammals (mutlitberculates, eupantotheres, and
even some therapsids) of both sexes. These bones may have served more
for muscle attachment than for a pouch. Although modern placental mammals
lack epipubic bones, at least two early species, Zalambdalestes and Ukhaatherium
(and probably Barunlestes as well) had them. Epipubic bones seem to be
homologous to the bones which exist in the penis and, more rarely, the
clitoris, of many modern placental mammals.
Mesozoic mammals also modified their hips and legs. The obturator foramen
became larger and the pubis was reduced. After the first mammals, separate
centers of ossification formed in the epiphyses during the development
of the femur. Mammals lost the ancestral internal and fourth trochanters
of the femur and developed the linea aspera. Mammals evolved a patella
in the vicinity of the knee. The tarsus of early mammals possessed a centrale
(navicular) and 4 distal tarsals. There was some specialization of the