Epipubic bone

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Epipubic bone

- What are they? A pair of bones that stick forward from the pelvic bones in most marsupials, monotremes, and some extinct mammals. Placental mammals generally lack them.

- Where are they found? Today in marsupials and monotremes, and in many fossil mammals (like multituberculates). Some early mammal relatives may have had them too; placentals do not.

- Why did they exist? They first appear in ancient cynodonts, suggesting they’re an early, shared feature of mammal-like ancestors. They’re often called “marsupial bones” because in living marsupials they help support the mother’s pouch, but that wasn’t necessarily their original function.

- How do they work? They may act as levers that help stiffen the trunk during movement and assist breathing. They’re linked to muscles such as the pectineus, pyramidalis, rectus abdominis, and the external and internal obliques. Some researchers have proposed they constrain certain gaits, but this idea is debated.

- Do they affect pregnancy? Placental mammals lack epipubic bones, which allows more abdominal expansion for longer pregnancies. However, ancient relatives could have large litters, showing the relationship isn’t simple.

- Variation among groups: In thylacines and sparassodonts the bones became cartilaginous or reduced. In some species like Trichosurus (a possum), hip muscle attachments shifted away from the epipubic, reducing respiratory benefits but keeping large epipubic bones.

- Other notes: Vestiges of the epipubic bone may survive in the baculum (the penile bone) in some placentals.

- Recent findings: A 2022 study on multituberculates found they could produce relatively well-developed young despite epipubic bones, challenging the idea that these bones always limited pregnancy length.