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).[1] They first occur in non-mammalian cynodonts such as tritylodontids, suggesting that they are a synapomorphy between them and Mammaliformes.

Skeleton of eastern grey kangaroo hind legs. Epipubic bones labeled as 10

Skeleton of red-necked wallaby, centered on the epipubic bones.

Only placentals, and possibly the early mammaliformes Megazostrodon and Erythrotherium, lack them;[2] in thylacines and sparassodonts, they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent.[3][4] 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.[5] Epipubic bones show sexual size dimorphism. [6]

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.[7]

The epipubic bones were first described in 1698 but their functions have remained unresolved. It has been suggested[8] 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.[9] It has been suggested that epipubic bones may constrain asymmetrical gaits, though this appears not to be the case.[10]

Placentals are the only mammal lineage that 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.[11] This however apparently did not prevent large litter sizes; Kayentatherium is now known to have given birth to litters of 38 undeveloped young, a considerably higher number than living monotremes or marsupials.[12] However, vestiges of the epipubic bone may survive in a common placental characteristic, the baculum.[13]