CROMPTON, A.W.; LIEBERMAN, D.E.; OWERKOWICZ, T.; Harvard Univ., Cambridge, MA; Harvard Univ., Cambridge, MA; Harvard Univ., Cambridge, MA: Control of jaw movements in the hairy-nosed wombat

Strain of the ventral surface of the mandibular symphysis and ramus was recorded synchronously with EMG’s from the principal jaw adductors in three hairy-nosed wombats (Lasiorhinus latifrons). EMG’s were recorded from three additional wombats. Results indicate that control of jaw movement in this marsupial herbivore is fundamentally different from that of placental ungulates. The wombat is characterized by a broad, horizontally orientated and inflected mandibular angle; a tightly sutured mandibular symphysis; heavily worn, ever-growing molars; and isognathic molar rows. Wear facets indicate that molar occlusion is unilateral. Jaw movements during occlusion are controlled entirely by the working side jaw adductors, with no balancing-side activity. Because the insertion of the adductors lies far laterally to the tooth row, unilateral activity of the adductors draws the working side mandibular molars across the maxillary molars, twists the whole mandible around the longitudinal axis of the working side, and separates the molars on the balancing side. The absence of balancing side adductor activity rules out occlusion of the balancing side molars. The direction of principal strains on the ventral surface of the symphysis confirm that the ventral surface of the working side of the lower jaw is inverted and, in contrast to all other mammals, no adductor force is transferred from the balancing to the working side. It is concluded that the unique features of wombat mastication are, in part, related to the retention and enlargement of the inflected angle of the mandible. Placental ungulates have a deep mandibular angle and have evolved a different pattern for the control of jaw movements.