C H A P T E R 5 Gnathostome Fishes The two groups of living gnathostome fishes are the Chondrichthyes or cartilaginous fishes, and the Osteichthyes or bony fishes. Both groups may have evolved in separate but parallel fashion from placoderm ancestors and are the survivors of hundreds of millions of years of evolution from more ancient forms. | Linzey Vertebrate Biology 5. Gnathostome Fishes I Text The McGraw-Hill Companies 2003 CHAPTER 5 Gnathostome Fishes INTRODUCTION The two groups of living gnathostome jawed fishes are the Chondrichthyes or cartilaginous fishes sharks skates rays and ratfishes and the Osteichthyes or bony fishes Fig. . Both groups may have evolved in separate but parallel fashion from placoderm ancestors and are the survivors of hundreds of millions of years of evolution from more ancient forms. Fishes are the most diverse group of vertebrates with approximately 26 000 species of bony and cartilaginous fishes extant in the world today Bond 1996 . EVOLUTION The evolution of the major groups of hagfishes lampreys and gnathostome fishes and their relationships to each other to the amphibians and to amniotes are shown in Fig. . In Fig. is presented a cladogram showing probable relationships among the major groups of fishes. Because taxonomy is constantly undergoing refinement and change the relationships depicted in this cladogram along with others used in this text are subject to considerable controversy and differences of opinion among researchers see Supplemental Readings at end of chapter . Evolution ofJaws The development of hinged jaws from the most anterior pair of primitive pharyngeal arches see discussion on page 99 of this chapter was one of the most important events in vertebrate evolution. Jaws permitted the capture and ingestion of a much wider array of food than was available to the jawless ostracoderms and they also permitted the development of predatory lifestyles. Fish with jaws could selectively capture more food and occupy more niches than ostracoderms and thus were more likely to survive and leave offspring. They FIGURE a Dogfish shark External anatomy of a dogfish shark Chondrichthyes and b largemouth bass Osteichthyes . Anterior dorsal fin b Largemouth bass Linzey Vertebrate Biology 5. Gnathostome Fishes I Text The McGraw-Hill Companies 2003 .
