Epipubic bone

Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placental mammals, who lack them). They first occur in non-mammalian cynodonts such as tritylodontids, suggesting that they are a synapomorphy between them and Mammaliformes.

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