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.