200-147 million years ago


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 first toe.