How Did the Maxilla Get Rid from the Palatoquadrate: On the Evolutionary Origin of the Coronoid Process in Neopterygians

The acquisition of the coronoid process of the mandible was one of the key-innovations appeared during the transition from the palaeopterygian (“palaeoniscoid”) to neopterygian levels of organization. Presence of this process is commonly acknowledged to be a synapomorphy of Neopterygii. However, the homology and evolutionary origin of the coronoid process remain absolutely obscure. The present article is an attempt to resolve this conundrum based on comparison of the mandibular arch development in Amia with that in Acipenser as a proxy of the palaeopterygian bauplan. Presumably, palaeopterygians possessed the quadrato-maxillar process of the palatoquadrate to which the caudal end of the maxilla was attached. During the palaeopterygians-to-neopterygians transition this process was translocated to the lower jaw to become the cartilaginous coronoid process. As a result, the maxilla became free from the palatoquadrate.