RELATIONSHIPS OF THE CHIMAERIFORMES AND THE BASAL RADIATION OF THE CHONDRICHTHYES

The origin and early evolution of the cartilaginous fishes (Chondricht hyes) has been the subject of considerably more debate than of data. T he two modern groups, Chimaeriformes and Elasmobranchii, differ so rad ically in morphology that in the past they have often been considered unrelated - descended from some remote and unknown common ancestor. Th e current consensus promotes the Chimaeriformes and Elasmobranchii as sister taxa of the Class Chondrichthyes which are linked by an assembl age of Palaeozoic fossil taxa, but no taxonomic or phylogenetic scheme has been accepted for the Class. Of the two groups, the Chimaeriforme s is the less understood. The few species of Chimaeriformes existing t oday are enigmatic, principally deeper-water fish that are not readily accessible for study. In the past the fossil record of both groups ha s been relatively scanty, primarily due to the poor potential for skel etal fossilization, and so has provided little useful input into funda mental discussions of vertebrate diversification. However, these situa tions are changing. Chimaerids are increasingly becoming the subject o f renewed biological and limited fisheries interests. Regarding extinc t chondrichthyans, the last 30 or so years have entailed discoveries o f new fossils that illuminate our view of Palaeozoic life and are elic iting dramatic changes in our understanding of these early fishes, the ir relations, and the origins of jawed conditions. Morphological exami nation of fossil chondrichthyans indicates that the plesiomorphous sta te of the gnathostome suspensorium is autodiastylic and that complex l abial cartilages are primitive and likely to have been critical to the mechanical architecture of the first jaws. Analysis of cranial morpho logy, cranial proportions, the phyletic and developmental history of c alcified tissues, and postcranial data including the evolution of the prepelvic tenaculum are now feasible. Cumulatively, when the results o f these analyses are subject to cladistical evaluation, the result is one predominant cladogram supporting two monophyletic subclasses: the Elasmobranchii and the Euchondrocephali. The latter subclass contains a monophyletic group of holocephalans including the Cochliodontomorpha , and within this taxon, the restricted Chimaeriformes. Alternative cl adograms of the non-holocephalan Euchondrocephali are dependent upon w hether whole-body or cranial characters alone are employed in the anal ysis, or the additive or non-additive treatment of characters. Otherwi se, only the discovery and description of additional members of this d iverse assemblage are expected to alter these patterns of associations .