Mechanis and the Evolution of the Synapsid Jaw

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Abstract
A mathematical model of the mechanical arrangement of the lower jaw and the external adductor muscle is constructed which shows that it is unlikely that a coronoid process was initially developed by selection for a higher moment arm. We believe instead that selection for a posteriorly directed line of muscle action was the primary cause for the initial development of this process, and that such selection occurred in conjunction with the predatory habits of the advanced sphenacodontids. In sphenacodontids, the height of the coronoid process was raised past a threshold and eventually allowed selection for a higher moment arm by further rise in the process to become effective. Cynodont evolution is characterized by extensive development of the coronoid process which then migrated posteriorly. The model shows that these are understandable events. Other events in cynodonts include the development of the masseter muscle, first as an extension of the external adductor, and later with an added superficial component acting anterodorsally. Selection for the development of the masseter muscle and for the rise of jaw articulation in advanced cynodonts may have involved the use of the cheek teeth in rudimentary chewing. The technique of solving functional and evolutionary problems by using mathematical models to explore possible evolutionary pathways has been used previously in coiled shells, in tooth replacement, and in a few other situations. This technique promises to be used more often in the future.

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