Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals
Stephen M. Reilly,1*
Thomas D. White2
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.
1 Department of Biological Sciences, Ohio
University, Athens, OH 45701, USA.
2 Biology
Department, Buffalo State College, Buffalo, NY 14222, USA.
*
To whom correspondence should be addressed. E-mail:
reilly{at}ohiou.edu
