Poster presentation 12th International Mammalogical Congress

Absence of an Articular Disk in the Platypus Temporomandibular Joint (#652)

Katsuhiko Hayashi 1 , Hiroyuki Akiyama 1 , Stewart Nicol 2
  1. Jikei University School of Medicine, Tokyo, Japan
  2. Biological Sciences, University of Tasmania, Hobart, Tasmania , Australia

In the vast majority of mammals, the temporomandibular joint comprises an articular disk between the mandibular condyle and the mandibular fossa. However, the temporomandibular articular disk has been reported to be absent in two monotremes, the platypus and the short-beaked echidna. The aim of this study was to investigate the morphological characteristics of the temporomandibular joint in the platypus by anatomical, histological and radiological examination. Two fresh frozen platypus corpses were obtained from the School of Zoology, University of Tasmania. The right maxillofacial regions of the corpses were dissected, and the morphology of the temporomandibular joints was carefully observed by gross anatomical examination. Then, the maxillofacial region was removed and fixed with buffered formaldehyde solution. Focusing on the trabecular bone in the condyle, the left temporomandibular joints were examined by microcomputed tomography. Thereafter, the left temporomandibular joints were histologically examined. In all cases, absence of the articular disk was morphologically confirmed. Both dense and fine trabecular bone were observed in the loaded area of the condyle. Histologically, well-developed synovial membrane was present in both anterior and posterior parts of the articular cavity. We reconfirmed the absence of the articular disk in the temporomandibular joint of the platypus, first reported in 1900. Well-developed synovial membrane close to the articular surface and dense trabecular bone in the condyle might together play a role in masticatory absorption and loading of the platypus temporomandibular joint.