An odontometric study of the maxillary molars in Australian marsupials. - I. The koala (Phascolarctos cinereus)
Ryuji UENO1,3,5, Akira IIMURA2, Shunji YOSHIDA3, Kenji KONDO1, Iwao SATO3, Maciej HENNEBERG4 and Grant C. TOWNSEND5
1Department of Dental Technology, The Nippon Dental University College at Tokyo, 2-3-16 Fujimi, Chiyoda-ku, Tokyo 102-0071, Japan
2Department of Anatomy, Kanagawa Dental College, 82 Inaoka-cho, Yokosuka-shi, Kanagawa 238-8580, Japan
3Department of Anatomy, School of Life Dentistry at Tokyo, The Nippon Dental University, 1-9-20 Fujimi, Chiyoda-ku, Tokyo 102-8159, Japan
4Discipline of Anatomical Sciences, School of Medical Sciences, The University of Adelaide, Medical School North Building, Frome Road, SA 5005, Australia
5School of Dentistry, The University of Adelaide, 233 North Terrace, SA 5005, Australia
Crown dimensions of the maxillary molars were measured in the koala (Phascolarctos cinereus). There were no significant differences in crown diameters between the first and second molars, however the fourth molars were reduced in all crown diameters. The third molar was smaller than the first or second molars in buccolingual crown diameters but there were no significant differences in mesiodistal crown diameters. It is proposed that the similar shapes of the first and second molars are associated with similar types of masticatory activity involving these teeth, The shape of the third molar, which is reduced in size buccolingually, may be linked to the koala’s occlusal function which is characterized by a condylar action that leads to differences in movement between opposing anterior and posterior molar teeth during the occlusal stroke. The fourth molar, the smallest of the molar teeth in crown diameter, erupts significantly later than the other molars, and its reduction may be explained by the terminal and distal reduction theories. It is proposed that the pattern of molar morphology in the koala is associated with both masticatory activity linked to its characteristic occlusal function, as well as reflecting the sequence of tooth emergence.