The content discusses the evolutionary transition from the dual jaw joint of early mammaliaforms, such as Morganucodon, to the mammalian middle ear. Fossils of two Jurassic mammaliaforms provide important insights into this transformation:
A new morganucodontan-like species exhibits previously unknown post-dentary bone morphologies and features suggesting a loss of load-bearing function in its articular-quadrate joint.
A pseudotribosphenic shuotheriid species has a middle ear that approaches the mammalian condition, with features suitable for an exclusively auditory function, although the post-dentary bones are still attached to the dentary.
In both cases, the size reduction of the jaw-joint bones and the medial shift of the quadrate relative to the articular provide evidence of a gradual loss of load-bearing function in the articular-quadrate jaw joint. This is a crucial prerequisite for the detachment of the post-dentary bones from the dentary and the eventual breakdown of Meckel's cartilage during the evolution of mammaliaforms.
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by Fangyuan Mao... at www.nature.com 04-03-2024
https://www.nature.com/articles/s41586-024-07235-0Deeper Inquiries