CROMPTON, A W; LIEBERMAN, D; Harvard University; Harvard University: Motor control of mastication in mammals: Marsupials and Placentals compared

Studies on the neuro-muscular control of jaw movements in mammals, until now, have been almost entirely based, with one exception, namely the American opossum, on placental mammals. For this study electromyograms of the major adductor muscles were recorded during feeding in seven different Australian marsupials (ring-tailed possums, Tasmanian devils, long-nosed potoroos, Tammar wallabies, red kangaroos, Southern hairy-nosed wombats and koalas). Their masticatory motor patterns were compared with published information on those of placental mammals and data from new recordings on the American opossum and the domestic goat. Although the motor pattern of primitive placentals and marsupials appears to be identical, it has been modified in fundamentally in different ways in herbivorous placentals and marsupials. In both the working side jaw is drawn transversely during the power stroke. In all herbivorous placentals this movement is effected by a complex pattern of overlapping activity in both the working and balancing side adductors. In herbivorous marsupials the muscles on one side effect transverse movements only. In macropods the working side jaw rotates around its longitudinal axis and this provides the major component of transverse movement. The mechanisms for moving the jaw transversely in herbivorous marsupials appears to be related to the increase in size and division of the medial pterygoid muscle into two distinct parts. Koalas lack an inflected angle and divided medial pterygoid and have a distinct motor pattern