Content area
Full Text
Since the first description of epipubic bones in 1698, their functions and those of the associated abdominal muscles of monotremes and marsupial mammals have remained unresolved. We show that each epipubic bone is part of a kinetic linkage extending from the femur, by way of the pectineus muscle, to the epipubic bone, through the pyramidalis and rectus abdominis muscles on one side of the abdomen, and through the contralateral external and internal oblique muscles to the vertebrae and ribs of the opposite side. This muscle series is activated synchronously as the femur and contralateral forelimb are retracted during the stance phase in locomotion. The epipubic bone acts as a lever that is retracted (depressed) to stiffen the trunk between the diagonal limbs that support the body during each step. This cross-couplet kinetic linkage and the stiffening function of the epipubic bone appear to be the primitive conditions for mammals.
Except for the extant placental mammals (crown-group Eutheria), all mammalian taxa and their immediate cynodont ancestors possess epipubic bones-a pair of bones articulating with the pelvis that project anterolaterally into the hypaxial muscle layers of the abdomen (1). Epipubic bones articulate from the pubic rami with a hingelike synovial joint, and when depressed each bone swings ventrolaterally in the belly wall. They are homologous to the nonarticulating anterior pubic processes of the primitive puboischiatic plate of amniotes (2). Epipubic bones first appear in the Tritylodontidae (a cynodont outgroup to the Mammalia) and are retained as a primitive character in all extinct noneutherian mammals (for which postcranial material is available) and all living monotremes and marsupials (1, 3). Within the Eutheria, they are present in the stem eutherians but have been lost in all extant placental mammals (1, 4).
All mammals have four abdominal hypaxial muscle layers (Fig. 1): two obliquely crossing layers (internal and external obliques), one longitudinal layer (rectus abdominis) and one transverse layer (transversus abdominis, not shown in Fig. 1). Mammals with epipubic bones have an additional hypaxial muscle, the pyramidalis, that extends anteriorly from the medial aspect of the epipubic bones to the linea alba (Fig. 1). Because all of these muscles (which are paired) would act to elevate the belly and the epipubic bones, bilateral activity in all of these muscles...