With the possible exception of Megazostrodon and Erythrotherium (as well as placental mammals), all mammaliforms possess epipubic bones, a possibly synapomorphy with tritylodontids, which also have them.
The presence of epipubic bones in all mammaliforms except for Megazostrodon and Erythrotherium (and placental mammals) is a possible synapomorphy shared by both mammaliforms and tritylodontids.