The anatomy of mastication in a toothless mammal: Morphological characteristics of the temporomandibular joint in the platypus (Ornithorhynchus anatinus)

The temporomandibular joint (TMJ) is a distinguishing feature of mammals, and in most mammals includes an articular disc that buffers the loads placed on it by mastication. The disc is well developed in mammals with significant lateral masticatory jaw movements but is absent in toothless mammals, including extant monotremes, although histological studies of developing monotremes have shown rudimentary discs that fail to mature. Platypus (Ornithorhynchus anatinus) grind their food between keratinous pads in the maxillae and lower jaws and are the only edentulate mammals that masticate their food. In this study, we characterize the anatomy of the TMJ of the adult platypus to see if we can reconcile the anatomy, including the absence of the articular disc, with the mandibular movements observed in video recordings. We studied the gross anatomy of the maxillofacial region and the microstructure using microcomputed tomography (micro-CT) and histological examination. Platypuses had well-developed masticatory muscles but lacked an articular disc between the mandibular condyle and glenoid fossa. The surface of the glenoid fossa was slightly concave than that of the condylar head was correspondingly slightly convex. The pre- and postglenoid processes were not well developed. Micro-CT showed dense trabecular bone in the anterior part of the condyle, where the lateral pterygoid muscle attached. Histological analysis showed that the surfaces of the condyle and glenoid fossa consisted of dense, avascular and thickened fibrous connective tissue. In addition, well-developed synovial folds were present. These anatomical characteristics are consistent with both anterior and lateral movements of the mandible, while the thick layer of connective tissue substitutes for a disc by absorbing the mechanical stresses associated with mastication. The failure of the disc primordium to develop cannot be attributed to a lack of muscle development, but the distribution of stresses in the toothless platypus jaw is likely to be different from those in a masticating eutherian.