Reexamining the evolutionary history of the mammalian medial pterygoid muscle

In stem mammals, feeding and hearing are closely linked due to jaw bones being involved in both functions. Osteological changes during the evolutionary separation of these systems are well documented in the fossil record, but changes to soft tissue anatomy are less certain. The medial pterygoid muscle is a jaw adductor that develops medial to Meckel's cartilage, inserting on the mandibular angular process in therians (placentals and marsupials). Likewise, in stem mammaliaforms, the medial pterygoid is often assumed to have passed medial to the ossified Meckel's cartilage, inserting on the dentary "angular" (i.e., pseudangular) process, a plesiomorphic pattern of muscle attachment also assumed to be present in some derived cynodonts. Thus, the traditional interpretation is that the medial pterygoid remained medial to Meckel's cartilage through the evolution from early cynodonts to therians. We highlight a pattern that contrasts with that interpretation: the medial pterygoid inserts lateral (not medial) to Meckel's cartilage in monotremes. Further, fossil evidence indicates that this pattern was also present in stem therian eutriconodontans and spalacotherioids, which lack the pseudangular process. The varying position of the medial pterygoid among mammalian groups suggests that the muscle is either (1) evolutionarily labile in terms of its relative position to Meckel's cartilage or (2) did not insert on the pseudangular process of early cynodonts. We support the latter hypothesis,which indicates that the medial pterygoid did not shift medial to Meckel's cartilage until the complete ear-jaw separation in cladotherians (therians and close relatives), with the shift linked to the evolution of the therian angular process as an insertion site.