Cartilaginous fishes, as their name implies, have a skeleton that is made up of cartilage instead of bone (in contrast to the bony fish, whose skeletons are made up of bone). Cartilage is tough and flexible so it provides plenty of structural support to enable cartilaginous fishes to grow to considerable size. Other characteristics of cartilaginous fishes is that they have jaws, paired fins, paired nostrils and a two-chambered heart.
Cartilaginous fishes have a tough skin that is covered with tooth-like scales called denticles. Denticle scales are similar to vertebrate teeth in structure. At the core of a dentical scale lies a pulp cavity that receives blood flow for nourishment. The pulp cavity is capped with a cone-shaped layer of dentine. The denticle sits on top of a basal plate which overlies the dermis. Each denticle is covered with an enamel-like substance.
Most cartilaginous fishes live in marine habitats all their lives, but a few species of sharks and rays live in freshwater during all or part of their lives. Cartilaginous fishes are carnivorous and most species feed on live prey. There are some species that feed on the remains of dead animals and still others that are filter feeders.
The largest living cartilaginous fish is the whale shark (about 30 feet long and 10 tons). The largest known cartilaginous fish ever to have lived is Megalodon (about 70 feet long and 50-100 tons). Other large cartilaginous fish include the manta ray (about 30 feet long) and the basking shark (about 40 feet long and 19 tons).
Small cartilaginous fishes include the short-nose electric ray (about 4 inches long and weighs 1 pound), the starry skate (about 30 inches long), the pale catshark (about 8 inches long) and the dwarf lantern shark (about 7 inches long).
Cartilaginous fish are divided into two subgroups, the sharks, rays and skates and the chimeras.
Cartilaginous fishes first appear in the fossil record about 420 million years ago during the Devonian Period.
The earliest known cartilaginous fishes were ancient sharks that were descended from bony-skeleton placoderms. These primitive sharks are older than the dinosaurs. They swam in the world’s oceans 420 million years ago, 200 million years before the first dinosaurs appeared on land. Fossil evidence for sharks is plentiful but consists mostly of tiny remnants of the former fish—teeth, scales, fin spines, bits of calcified vertebra, fragments of cranium. Extensive skeletal remains of sharks are missing—cartilage does not fossilize like true bone.
By piecing together what shark remains do exist, scientists have uncovered a diverse and deep ancestry. Sharks of the past include ancient creatures such as Cladoselache and Ctenacanths. These early sharks were followed by Stethacanthus and Falcatus, creatures that lived during the Carboniferous Period, in a window of time referred to as the “Golden Age of Sharks”, when shark diversity blossomed to include 45 families.
During the Jurassic Period, there was Hybodus, Mcmurdodus, Paleospinax and eventually the Neoselachians. The Jurassic Period also saw the emergence of the first batoids: the skates and rays. Later came the filter feeding sharks and rays, the hammerhead sharks, and the lamnoid sharks (great white shark, megamouth shark, basking shark, sandtiger, and others).