Alana C. Sharp

“ However, comparisons with other marsupials could shed more light on the jaw movements of Diprotodon. In all extant diprotodonts, jaw movements consist of a vertical Phase I and transverse Phase II stroke ( [27] ) . However, the sequence of activation of the muscles changes considerably between species. Wombats are unique in that the power stroke is restricted to a single transverse phase where only the muscles on the working side are active ( [13] ) . In contrast, macropods and koalas exhibit a vertical phase followed by a transverse phase of varying degrees ( [27] ; [28] ) . ”

“ This study resulted in a 3D digital reconstruction of the jaw adductor musculature of Diprotodon, revealing that the arrangement of muscles is similar to that of kangaroos and that the muscle actions were predominantly vertical. 3D digital muscle reconstructions provide considerable advantages over 2D reconstructions for the visualisation of the spatial arrangement of the individual muscles and the measurement of muscle properties (length, force vectors and volume) . Such digital models can further be used to estimate muscle loads and attachment sites for biomechanical analyses. ”

“ With wear, the enamel gains thickness giving more resistance or grinding-power as the tooth wears down, and then thins again with extreme wear toward the base of the lobe. There is no evidence of molar progression but heavier wear does occur on the anterior molars, so that when the posterior molars erupt, the first and second molars are already quite worn ( [21] ) . Thus, it’s likely that Diprotodon had a mixed diet similar to that of browsing wallabies rather than one dominated by grass like the diet of grazing kangaroos. ”

“ Unlike koalas ( [14] ) , the large inflected angle of the mandible present in Diprotodon, kangaroos and wombats, provides an area of insertion for the medial pterygoid. The medial pterygoid accounts for 11% of the total masticatory muscle mass in Diprotodon and likely worked in combination with the masseter and anterior fibres of temporalis to elevate the mandible, producing a vertical movement. While the medial pterygoid would still provide some transverse force, the reduction of the lateral pterygoid means there was only a small force for transverse movements of the jaw. ”

“ The terminology used here was selected for its consistency with jaw muscle nomenclature used for marsupials ( [12] ; [13] ; [14] ; [15] ; [16] ; [17] ; [18] ) . The masseter muscle group includes three portions (superficial, deep and zygomaticomandibularis) based on their area of origin. Each portion can include multiple layers within it if partial laminae are present. However, in fossil taxa one cannot ascertain the complexity of the muscles, so each portion of the masseter has been modelled as a single muscle. ”

“ W. bicolor has a crushing action at the molars for softer material, and there is no need to replace worn teeth because worn molars retain some useful function. The lower tooth row remains flat so that all teeth occlude in the adults. The molars of Diprotodon are simple bilophodont molars without strong links and the occlusal surface of the upper molars is slightly convex downwards, but not to the high degree seen in Macropus. ”

“ Building the model
The CT data were imported into the image processing software program Mimics 13.1 (Materialise) , where editing of the CT slices took place. Many of the internal cavities, including the extensive sinuses within the fossilised cranium, were filled with inorganic matrix (Fig. 1) . This matrix has approximately the same radiodensity (transparency to the passage of X-rays) as the fossilised bone ”