Abdominal muscle and epipubic bone function during locomotion in Australian possums: Insights to basal mammalian conditions and eutherian‐like tendencies in Trichosurus

Abstract

Mammals have four hypaxial muscle layers that wrap around the abdomen between the pelvis, ribcage, and spine. However, the marsupials have epipubic bones extending anteriorly into the ventral hypaxial layers with two additional muscles extending to the ventral midline and femur. Comparisons of South American marsupials to basal eutherians have shown that all of the abdominal hypaxials are active bilaterally in resting ventilation. However, during locomotion marsupials employ an asymmetrical pattern of activity as the hypaxial muscles form a crosscouplet linkage that uses the epipubic bone as a lever to provide long-axis support of the body between diagonal limb couplets during each step. In basal eutherians, this system shifts off the femur and epipubic bones (which are lost) resulting in a shoulder to pelvis linkage associated with shifts in both the positions and activity patterns of the pectineus and rectus abdominis muscles during locomotion. In this study, we present data on hypaxial function in two species (Pseudocheirus peregrinus and Trichosurus vulpecula) representing the two major radiations of possums in Australia: the Pseudocheiridae (within the Petauroidea) and the Phalangeridae. Patterns of gait, motor activity, and morphology in these two Australian species were compared with previous work to examine the generality of 1) the crosscouplet lever system as the basal condition for the Marsupialia and 2) several traits hypothesized to be common to all mammals (hypaxial tonus during resting ventilation, ventilation to step synchrony during locomotion, and bilateral transversus abdominis activity during locomotor expiration). Our results validate the presence of the crosscouplet pattern and basic epipubic bone lever system in Australian possums and confirm the generality of basal mammalian patterns. However, several novelties discovered in Trichosurus, reveal that it exhibits an evolutionary transition to intermediate eutherian-like morphological and motor patterns paralleling many other unique features of this species.