Reilly Stephen M, McElroy Eric J, White Thomas D, Biknevicius Audrone R, Bennett Michael B
Department of Biological Sciences, Ohio Center for Ecology and Evolutionary Studies, Ohio University, Athens, OH 45701, USA.
J Morphol. 2010 Apr;271(4):438-50. doi: 10.1002/jmor.10808.
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.
哺乳动物有四层轴下肌，它们在骨盆、胸腔和脊柱之间环绕腹部。然而，有袋类动物有耻骨上骨向前延伸到腹侧轴下肌层，还有两块额外的肌肉延伸到腹侧中线和股骨。对南美洲有袋类动物与基础真兽类动物的比较表明，所有腹部轴下肌在静息通气时双侧活动。然而，在运动过程中，有袋类动物采用不对称的活动模式，因为轴下肌形成了一种交叉偶联机制，利用耻骨上骨作为杠杆，在每一步中为对角肢体偶联之间的身体提供长轴支撑。在基础真兽类动物中，这个系统从股骨和耻骨上骨（已消失）转移，导致在运动过程中耻骨肌和腹直肌的位置和活动模式发生变化，从而形成肩部到骨盆的连接。在这项研究中，我们展示了代表澳大利亚负鼠两个主要类群的两个物种（蜜袋鼯和帚尾袋貂）的轴下肌功能数据：袋鼯科（在袋鼯超科内）和袋貂科。将这两个澳大利亚物种的步态、运动活动和形态模式与之前的研究进行比较，以检验以下两点的普遍性：1）交叉偶联杠杆系统作为有袋类动物的基础条件；2）一些假设为所有哺乳动物共有的特征（静息通气时的轴下肌张力、运动时通气与步频同步以及运动呼气时腹横肌的双侧活动）。我们的结果证实了澳大利亚负鼠中交叉偶联模式和基本耻骨上骨杠杆系统的存在，并确认了基础哺乳动物模式的普遍性。然而，在帚尾袋貂中发现的几个新特征表明，它表现出向类似真兽类中间形态和运动模式的进化过渡，这与该物种的许多其他独特特征相似。
