Abstract
The novel mammalian jaw joint, known in humans as the temporomandibular joint or TMJ, is cushioned by a fibrocartilage disc. This disc is secondarily absent in therian mammals that have lost their dentition, such as giant anteaters and some baleen whales. The disc is also absent in all monotremes. However, it is not known if the absence in monotremes is secondary to the loss of dentition, or if it is an ancestral absence. We use museum held platypus and echidna histological sections to demonstrate that the developing monotreme jaw joint forms a disc primordium that fails to mature and become separated from the mandibular condyle. We then show that monotreme developmental anatomy is similar to that observed in transgenic mouse mutants with reduced cranial musculature. We therefore suggest that the absence of the disc on monotremes is a consequence of the changes in jaw musculature associated with the loss of adult teeth. Taken together, these data indicate that the ancestors of extant monotremes likely had a jaw joint disc, and that the disc evolved in the last common ancestor of all mammals.
Highlights
The temporomandibular joint (TMJ) is the one of the most used joints in the body, articulating the upper and lower jaw in mammals
In order to discriminate between these two scenarios, we have examined the development of the TMJ in monotremes and made comparison with mouse developmental models where muscle development is perturbed
If the TMJ disc is a therian novelty, no evidence of a disc would be expected in extant monotremes during development of the TMJ
Summary
The temporomandibular joint (TMJ) is the one of the most used joints in the body, articulating the upper and lower jaw in mammals. A fibrous articular disc sits between the skeletal elements of the joint and acts as a cushion. TMJ development occurs by the coming together of two membranous bones: the condylar process of the dentary bone in the mandible and the squamosal bone in the skull. The articular disc sits between the two within a synovial capsule. The TMJ articulates the jaw in all mammals and is referred to as the squamosal dentary joint (SDJ) in those mammals without a fused temporal bone. In non-mammals the upper and lower jaw articulate via the endochondral quadrate and articular, known as the primary jaw joint (Wilson and Tucker, 2004).
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