Epipubic bones are a pair of bones projecting forward from thepelvic bones of modernmarsupials,monotremes and fossil mammals likemultituberculates, and evenbasaleutherians (the ancestors ofplacentals, who lack them).^[1]They first occur in non-mammaliancynodonts such astritylodontids, suggesting that they are asynapomorphy between them andMammaliformes.

They were first described as early as 1698, but to date, their function(s) remain unresolved.^[2] Epipubic bones are often calledmarsupial bones because they support the mother'spouch in modern marsupials ("marsupium" is Latin for "pouch").^[3]

Some writers have suggested^[2]that the epipubic bones are a part of a kinetic link 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 thepectineus muscle, and to the ribs and vertebrae by thepyramidalis,rectus abdominis, andexternal andinternal obliques. According to this hypothesis, the epipubic bones act as levers to stiffen the trunk during locomotion, and aid in breathing.^[4]Others have suggested that epipubic bones may constrain asymmetrical gaits, although this appears not to be the case.^[5]

Only placentals, and possibly the early mammaliformesMegazostrodon andErythrotherium, lack them;^[6]inthylacines andsparassodonts, they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent.^[7][8]

Trichosurus mimicked placentals in shiftinghypaxial muscles attachment sites from the epipubic to thepelvis, losing therespiratory benefits (see below), but otherwise retains large epipubics.^[9]Epipubic bones show sexual size dimorphism.^[10]

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 thatflex the thigh.^[3]

Placentals are the only mammal lineage that lacks epipubic bones, and this absence has been considered to be correlated to the development of theplacenta 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, thebaculum.^[13]

Additionally, the capacity for epipubic bones to prevent long term pregnancies has been debated in a 2022 study onmultituberculates, which proved that they were capable of producing young as developed as those of placentals despite their epipubics.^[14]

Look upepipubic bone in Wiktionary, the free dictionary.

• Evolution of mammals
• Obturator process
• Proximodorsal process
• Pelvic digit

• Marsupial anatomy