Epipubic bone

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 thylacines and sparassodonts, they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent.Trichosurus mimicked placentals in shifting hypaxial muscles attachment sites from the epipubic to the pelvis, losing the respiratory benefits (see below), but otherwise retains large epipubics.

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, and aid in breathing. It has been suggested that epipubic bones may constrain asymmetrical gaits, though this appears to not be the case.

...
Wikipedia