. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 90 ANNALS OF THE SOUTH AFRICAN MUSEUM separated from the mandible. It is nevertheless clear that this tooth was reduced in length in the lndarctos-Agriotherium lineage to a degree comparable to that of M2. In /. vireti the M3 is markedly elongated anteroposteriorly and has a pronounced posterior tapering (Crusafont & Kurten 1976). The M3 of/, atticus is similar, but is less elongated, while that of Agriolherium is usually only as long as it is broad. The latter, however, is still Indarctos-\ike i
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. 90 ANNALS OF THE SOUTH AFRICAN MUSEUM separated from the mandible. It is nevertheless clear that this tooth was reduced in length in the lndarctos-Agriotherium lineage to a degree comparable to that of M2. In /. vireti the M3 is markedly elongated anteroposteriorly and has a pronounced posterior tapering (Crusafont & Kurten 1976). The M3 of/, atticus is similar, but is less elongated, while that of Agriolherium is usually only as long as it is broad. The latter, however, is still Indarctos-\ike in having a pro- nounced posterior tapering. The A. africanum M3, L50981, is very like those of the Vienna and Basel /. atticus specimens (Thenius 1959; Helbing 1932) in shape. The reduction of M3 is another development shared by Agriotherium and Thalarctos. Although mention has been made of differences in proportions in certain of the posterior cheek teeth discussed above, this matter warrants more detailed attention, since these differences are directly related to changes in tooth mor- phology and are as significant from a phylogenetic point of view. In addition, metrical data have the advantage of being easily represented diagrammatically and can thus give a visual impression of certain evolutionary changes in the hypothetical Ursavus-Indarctos-Agriotherium lineage. Five Old World species were selected to represent successive stages in this lineage. They are Ursavus primaevus, I. vireti, I. arctoides, I. atticus, and A. africanum. Individual species do not necessarily represent the stock from which the next in the series was derived, but are simply structurally suited to such a role. The lengths and breadths of their posterior cheek teeth are plotted in Figures 38-40. It is clear from these figures that U. primaevus is in an isolated position, whereas there are similarities between /. vireti and /. arctoides on the one II 20 22 24 2« 21 30 32 31 31 40 42 Fig. 38. Leng
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
