Musculoskeletal anatomy and nomenclature of the mammalian epipubic bones

Despite the well-established anatomy nomenclature for the marsupial skeleton, there are no names for the epipubic bone structures. Epipubic bones are paired bones articulating with the pubis and projecting cranially in the ventral body wall, present on the pelvic girdle of cynodonts, monotremes and marsupials. These bones were commonly thought to be related to pouch support in marsupials and more recently associated with locomotion. The parts of the epipubic bones have not been named and this has impeded proper morphological analysis. We analyzed the epipubic bones of 302 skeletons comprising American and Australian marsupials, as well as 27 monotreme skeletons, and dissected 10 marsupials for myological attachments analysis. We suggest the following nomenclature for the epipubic bone structures: crest for the cranial end, shaft for the body of the bone, lateral tubercle and the medial articular process. Some markings on the epipubic bone include the oblique line, pertaining to the attachment of external abdominal oblique muscle from the opposite side. The pyramidalis line is the suggested nomenclature for the pyramidalis muscle attachment and the inguinal ligament line for the inguinal ligament attachment. Regarding myology and attachments, based on dissections and review of the literature, the muscles pyramidalis, pectineus, external and internal abdominal oblique, transversus abdominis and rectus abdominis and the structures linea alba, linea semilunaris and the inguinal ligament are connected to the epipubic bone. As has been previously noted, anatomically, epipubic bones are so named due to their position (epi-above, pubic-pubis), and the same applies to structures such as the "epipubic process" or "epipubic cartilage" in amphibians and reptiles. While testing epipubic bone homology in vertebrates is beyond the scope of this work, we believe that using "epipubic bones" or epipubic cartilage/process as standardized terms for the structures found in the most cranial part of the superior ramus of the pubis would facilitate better anatomical communication. This should be valid for other similar terms, such as "epipubes" or "prepubis", that might occur in the literature in relation to this same physiographic position, and it should also be named as epipubic. We believe that this nomenclature will help in future morphologic studies.