The early development of the incisor teeth of Setonix brachyurus (Macropodidae: Marsupialia) with special reference to the prelacteal teeth

Abstract

The development of the incisor teeth has been studied in seven pouch-young specimens of Setonix brachyurus (Macropodidae: Marsupialia). In each half of the upper jaw five incisor teeth develop, the two non-functional incisors occupying the first and fourth positions, and the functional incisors the second, third and fifth positions along the incisor toothrow. Owing to their buccal position, early development and calcification, it is suggested that the non-functional upper incisors are members of a primary series, while the functional incisors are members of a secondary series of teeth. The significance of the alternation between the non-functional and functional incisors is discussed. There is evidence of no more than two separate dental series, the lingual downgrowth by the side of the first functional incisor having been interpreted as representing the free edge of the residual dental lamina. The presence of three separate dental series can only be accounted for if the premolar teeth, more specifically the replacing premolar, are regarded as belonging to a later series than the functional incisors. However, there is no evidence to support this interpretation. The simplest and most acceptable interpretation is that only two separate dental series are present: (1) a primary or deciduous series homologous with the deciduous series of placentals and represented by the non-functional incisors and possibly including the deciduous molariform tooth: (2) A secondary or permanent series homologous with the permanent series of placentals and represented by all the functional teeth which develop anterior to the molar teeth including the replacing premolar. If this interpretation is correct, the term “prelacteal” can no longer be used to describe the homology of the non-functional incisor teeth. Previous studies on the incisor teeth of various members of the Phalangeroidea suggest that a non-functional incisor tooth, originally situated between the first and second functional incisors, has disappeared in Setonix. The lingual edge on the incisal surface of the upper second functional incisor appears to represent the primary edge since it commences calcification before the buccal edge. In each half of the lower jaw three incisor teeth develop, the two non-functional incisors occupying the first and second positions, and the functional incisor the third position along the incisor toothrow. In view of the lingual position of the functional incisor, it is thought to belong to a secondary or permanent series, while the buccal non-functional incisors accordingly belong to a primary or deciduous series of teeth. The homology of the teeth in the lower jaw is similar to that in the upper, there being evidence of no more than two sepate dental series.