progression (Sanson,
1 989).The corollary of a relative increase in the length of the portion of the jaw
that carries the erupted dentary is a reduction in the relative length that is available for application of
occlusive force to the mandible. Similarly a decrease in the relative proportions of the mandible
results in a more distal positioning of the most anterior point of insertion of the occlusal musculature,
a point that is said to be positioned to yield maximum mechanical advantage (Ride,
1 959).If this
change were accompanied by a similar distal movement in the cranial attachments of the major
muscles of mastication, the resulting distal movement of the forces of occlusion along the
premolar/molar row would obviate any need for molar progression. The analysis of the relative
position of the cranial origin of the line of action of the two largest muscles of mastication showed
that whilst that of the masseter moved distally with increase in body size, there was an equal and
opposite, i.e. mesial, movement of the cranial origin of the line of action of the temporalis. Thus whilst there was no change in the relative bulk of masseter and temporalis with body size, compensatory changes in the disposition of the cranial attachments of the major muscles of mastication may result in occlusive forces remaining localised in the same region of the molar row regardless of size. However, it should also be noted that a relative anterior movement of the cranial origin of the line of action of the temporalis muscle must also result in a decrease in the retraction
component (Badoux, 1 979) that is associated with phase I molar movement (Sanson, 1 980). Again, in
view of the more medial disposition of the anterior position of the temporal fossa of the skull and the more lateral disposition of the distal zygomatic arch with respect to the long axis of the lower jaw, relative mesial extension of the temporalis muscle and distal extension of masseter may result in
result in larger animals having greater power in phase 2 movement (Sanson, 1 980) of premolar/molar
rows and in the rotational movement of lower incisors which is discussed below.
Although this study demonstrated a broad resemblance in jaw and tooth structure to that of
M
giganteus
an archetypical grazing type described by Sanson ( 1980), the failure of exposed dentine on anterior and posterior lophs to join along the line of the mid-link with wear is more typical ofWallabia bicolor
a browser /grazer type (Sanson 1 980) and suggests that the link structure is lessprominent, i.e the links are lower in relation to the height of the lophs, in tammars than in
M.
Giganteus.
This morphological difference may reflect the relative importance of larger crushingsurfaces for more efficient mastication of browse (Sanson 1 989) which is known to be consumed on
occasion by tammars (Williamson 1986). Similarly the relatively even progression of wear across the
molar rows demonstrated in this study may reflect the greater depths of occlusion that result from the lower profile of the links.
The pattern of wear on the attrition facets of the procumbent incisors coupled with that of the inner
surface of the upper incisors indicates that, in adult animals, cutting takes place by the thickened enamel (which runs along the labial edge of the attrition facet on each procumbent incisor) being brought into occlusion against the conjoined occlusal surface of the upper incisors and subsequently
rotated infero-Iaterally across this surface. Thus, during jaw closure both procumbent incisors come to
lie with their lateral edges on the medial aspect of the conjoined attrition facet on the upper incisors, i.e. are relatively adducted. When the lower incisors are in this position, the molars are intercuspated
(Hiiemae 1 978) but with the lingual edge of the lower molar row lying more medial to that of the upper molar row. The slight concavity of the conjoined upper incisor attrition facet indicates that the adduction phase of lower incisor movement that follows cutting is accompanied by rotation of each mandible about its longitudinal axis. Moreover, articulation tests demonstrate that these movements simultaneously cause the intercuspated (Hiieme 1 978) lower molar row to be drawn laterally across the upper molar row in phase 2 occlusion described by Sanson ( 1 989). Thus there is close integration between incisor and molar action in adult tammars particularly in respect of the cutting action of incisors and that caused by relative movement of the
links
on upper and lower molars during 'type-G' occlusion (Sanson 1 980). It should be noted that the nature of this integration does not require greater occlusive forces to act on the jaw because the conjoined cutting actions from incisor and molar action occur on lateral rotation and thus are disposed buccal1y and lingually at short distances from the hypomochlion (Badou;x 1 975) which is the longitudinal axis of the jaw. In view of the relatively poor development of pterygoid musculature shown in this study and in other macropodine species (Abbie 1 939) it is probable that this rotation relies on the action of the temporalis and the masseter (Sanson, 1980) muscles. Bio-mechanical studies (BadolLx 1 964; 1 975) indicate that, under conditions where there is relative elevation of the labial edges of the upper premolar/molar rows such as occurs in tammars, the lateral component of any vertically applied occlusive force produces a torsional force that aids the lateral rotation of the lower jaw. Moreover, the magnitude of the torsional force is directly proportional to the length of the jaw.In younger animals with shorter jaws, the extent of rotation of each hemi-jaw is reduced, and the incisor cutting surface is confined to the area between the tips of the procumbent incisors and the buccal surface of the first incisor. With increasing wear of the occlusal aspects of the upper incisors and concurrent rearward extension of the attrition facet on the lower incisors, an increasing amount of abduction and rotation of each mandible is required in order to occlude the opposing incisor surfaces. Thus, as animals age, bite width increases, as does the amplitude of phase 2 movements between upper and lower molars (Sanson 1 980). These age-related differences may be important in respect of dietary specialisation. Older animals may consume larger items of vegetation more efficiently whilst younger animals are constrained to physically smaller items. This observation fits in with that of McArthur and Sanson ( 1988) who showed that the cutting-edge length of molars is short in young