Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials and most non-placental fossil mammals: multituberculates, monotremes, and even basal eutherians (the ancestors of placental mammals). They first occur in tritylodontids, suggesting that they are a synapomorphy between them and Mammaliformes. Only placentals, and possibly the early mammaliformes Megazostrodon and Erythrotherium, lack them.
In modern marsupials the epipubic bones are often called "marsupial bones" because they support the mother's pouch ("marsupium" is Latin for "pouch"), but their presence on other groups of mammals indicates that this was not their original function, which some researchers think was to assist locomotion by supporting some of the muscles that flex the thigh.
The epipubic bones were first described in 1698 but their functions have remained unresolved. It has been suggested that they form part of a kinetic linkage stretching from the femur on one side to the ribs on the opposite side. This linkage is formed by a series of muscles: each epipubic bone is connected to the femur by the pectineus muscle, and to the ribs and vertebrae by the pyramidalis, rectus abdominis, and external and internal obliques. According to this hypothesis, the epipubic bones act as levers to stiffen the trunk during locomotion.
Placentals are the only mammal lineage which lacks epipubic bones, and this absence has been considered to be correlated to the development of the placenta itself; epipubic bones stiffen the torso, preventing the expansion necessary for prolonged pregnancy. However, vestiges of the epipubic bone may survive in a common placental characteristic, the baculum.
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