The Evolutionary Origin and Homologues of the Supracochlear Lamina: a Contribution to our Knowledge of Mammalian Ancestry

In cetaceans, each otic capsule is bound anteriorly to the remainder of the endocranium by an aliform structure called the supracochlear lamina. This lamina has been established here as the homologue of the sphenocochlear commissure of other mammals. Most researchers believe that these two structures arose within mammals. However, a comparative study involving data from both extant and extinct craniates suggests that the supracochlear lamina and the sphenocochlear commissure have a long premammalian history. The conclusion is that the lower of the two pharyngeal segments at the upper end of the mandibular branchial arch first became incorporated in the endocranium, constituting the cartilago trabecularis in the embryos of jawed craniates. The distal, or posterior, extremity of the trabecular cartilage projected outwardly to form the basitrabecular process. This process, which when ossified is called the processus basipterygoideus, functioned as a jaw support. Later, one more jaw support, termed the basivincular, evolved immediately in front of the basipterygoid process. Both these supports for the upper jaw existed in the piscine forerunners of the tetrapods. Subsequently, in the lineages leading to mammals, the basivincular jaw support became the processus alaris to which is attached an upper jaw endoskeletal remnant, called the ala temporalis. The basipterygoid jaw support, however, vanished as a consequence of the evolution of an exoskeletal joint system for the jaws of mammals. Yet the basipterygoid process persisted and as the sphenocochlear commissure or the supracochlear lamina assumed the new function of supporting the cochlear part of the auditory capsule.