Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
Integr Comp Biol. 2011 Aug;51(2):271-81. doi: 10.1093/icb/icr028. Epub 2011 Jun 22.
Movement of the jaw during molar occlusion is determined by the sequence of activity in the adductor muscles and this sequence is one way to define a masticatory motor program. Based on the similarity of molar structure, it is probable that the American opossum and the early Tertiary mammals that gave rise to all Australian marsupials probably shared a common "primitive" masticatory motor program. The distinct and various patterns of movement of the jaw in the major groups of Australian marsupial herbivores (diprotodontids) are achieved by both subtle and substantial shifts in the timing of the primitive sequence. All diprotodonts divide jaw movements during occlusion into a vertical Phase Im and horizontal Phase IIm, but the number of muscles involved and the level of activity associated with each phase varies considerably. In macropodids (potoroos and kangaroos) Phase Im dominates; in wombats Phase IIm dominates and in koalas the two phases are more evenly divided, with a more equal distribution of muscles between them. The motor program of koalas parallels that of some placental ungulates, while both macropodids and wombats have motor programs unique among mammals.
在磨牙咬合期间，下颌的运动由咀嚼肌的活动顺序决定，而这个顺序是定义咀嚼运动程序的一种方式。基于臼齿结构的相似性，美洲负鼠和早期的第三纪哺乳动物，它们是所有澳大利亚有袋类动物的祖先，可能共享一个共同的“原始”咀嚼运动程序。在澳大利亚有袋类草食动物（双门齿目）的主要群体中，下颌运动的明显而多样的模式是通过原始序列的时间上的微妙和实质性变化来实现的。所有的双门齿目动物在咬合期间将下颌运动分为垂直的 I 相和水平的 II 相，但涉及的肌肉数量和与每个相相关的活动水平差异很大。在袋鼠科（袋熊和袋鼠）中，I 相占主导地位；在袋熊科中，II 相占主导地位，而在考拉中，这两个相更加平均地分配，它们之间的肌肉分布更加均匀。考拉的运动程序与一些胎盘有蹄类动物相似，而袋鼠科和袋熊科都有哺乳动物中独特的运动程序。
