Description of the chimaerid jaw and its phylogenetic origins

Anatomical delineation of the holocephalan palatoquadrate has proven to be difficult and, so, has been an extensively debated topic as it relates to t he evolutionary derivation of jaws, modes of jaw suspension, and the interr elationships of the hondrichthyes (Elasmobranchii and Holocephali). Embryol ogical analyses of the chimaerid jaw and cranium are presented to provide a n anatomical description of the palatoquadrate in modern chimaerids, The pa latoquadrate fuses, anteriorly, to the nasal capsule early in development. This marks the first point of contact between the mandibular arch and crani um. Orbitonasal canal foramina delineate the dorsal palatoquadrate margin. The posteriormost margin is marked by fusion of the upper jaw with trabecul ar and parachordal cartilages in the region of the efferent eudobranchial a rtery foramen and by a suborbitally positioned basitrabecular cartilage. Th is basitrabecula generates a subocular shelf as it fuses medially to the pa rachordal cartilage and posteriorly to the postorbital wall and cranial oti c process. The results of these analyses are related to morphological studies of Paleo zoic chondrichthyan fishes, particularly the autodiastylic paraselachians t hat represent morphological intermediates to selachians and holocephalans, The paraselachian basitrabecular, which was mechanically fundamental to sta bilizing the free autodiastylic upper jaw and a hyoid operculum, is shown t o correlate with the suborbital basitrabecular of today's chimaerids. Furth er analyses of both extant and fossil data permit us to conclude that the p rimordial chondrichthyan palatoquadrate did not extend posteriorly to inclu de a palatoquadrate-derived otic process. Rather, the posteriormost extent of this element is primitively found within the limits of the orbit and is demarcated by the highly conserved basitrabecular element. The collective a nalyses support autodiastyly as the ancestral condition from which all fund amental suspensorial states are derived. (C) 1999 Wiley-Liss, Inc.