Advanced jawed fishes II: Chondrichthyes
The lineages of bony fishes can be traced with fair certainty back to the Silurian. Their success is evidenced by the diversity of forms found throughout the late Paleozoic and Mesozoic, and because of the overwhelming dominance of teleosts today. However, another group of fishes also arose during the early Paleozoic that followed a very different course of development and that also radiated in the Mesozoic and is well represented today. These are the Chondrichthyes (“cartilaginous fishes”), a group that rapidly specialized as marine predators. By the Carboniferous, sharks made up as much as 60% of the species of fishes in some shallow tropical habitats (Lund 1990).
Although traditionally thought of as “primitive” because of their cartilaginous skeleton, it turns out that many of the characters of modern Chondrichthyes are secondarily derived and represent specializations for a very different, parallel mode of life in water. As with the sarcopterygian and actinopterygian divergence among the bony fishes, two major subclasses of chondrichthyans – the Holocephali and the Elasmobranchii – also developed. The two groups are united by several synapomorphies, chief among which are a prismatic type of calcifi cation of endoskeletal cartilage and the presence of pelvic claspers in males (Grogan & Lund 2004).
The common ancestor of the two groups remains to be discovered, and many “sharklike” fossils do not fit well into known groups, or are the subject of debate. Our knowledge of chondrichthyan phylogeny is constrained by the availability of fossil skeletal material; by its nature, cartilage does not fossilize readily and hence our ideas concerning many basal groups rest on incomplete specimens. Accordingly, interrelationships among the Chondrichthyes are, once again, the subject of considerable discussion. Fortunately, the last few decades have seen an upsurge in discoveries, clarifying if not solving many earlier points of contention but leaving others unresolved (again, see Nelson 2006 for a review).