Maisey, John G. Division of Paleontology, American Museum of Natural History, New York, New York.
- Chondrichthyan fossils
- Modern sharks
- Phylogeny (evolutionary history)
- Links to Primary Literature
- Additional Readings
Sharks belong to a group of fishes known as the chondrichthyans (“cartilaginous fishes,” referring to their unossified internal skeleton). This group also includes the rays and chimaeras (ratfish). The popular perception of sharks as primitive relatives of bony fishes (osteichthyans) is undeserved because both have equal antiquity, and traces of bonelike tissue in some fossils suggest that early sharks evolved from bony ancestors. Modern and extinct chondrichthyans uniquely possess a type of endoskeletal mineralization known as tessellated calcified cartilage. In addition, modern sharks are anatomically advanced over earlier ones in the structure of their cranium, jaws, gill arches, fin skeletons, vertebral column, teeth, skin denticles (small toothlike projections), and fin spines. Today, sharks are among the top modern marine predators, and the fossil record reveals that they have held this distinction for more than 400 million years, despite occasional challenges from Paleozoic armored placoderms and lobe-finned fishes, Mesozoic marine reptiles, and modern marine mammals.
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