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The Fish, Part II: The Chondrichthyans

© Beverly Eschberger

In my last article, I began talking about the different types of fish. In this article, I want to talk about a group of animals that I find rather fascinating: the chondrichthyans, which include the sharks, skates, and rays.

In "The Fish, Part I," I mentioned that the fish are divided into four groups: the ostracoderms, the placoderms, the chondrichthyans, and the osteichthyans. The chondrichthyan fish are also known as the cartilaginous fish, as their skeletons are made entirely of cartilage, with the exception of their teeth and jaws. Because their skeletons are cartilaginous, fossils of chondrichthyan fish are not as common as those of the bony fish. Many early sharks are known only by their fossilized teeth.

The chondrichthyans are divided into two groups: the Elasmobranchs (the sharks and rays), and the Holocephali. The Holocephali are the chimaeras and a group of mollusk-eating fish that lived during the Paleozoic Era (570 to 230 million years ago).

Modern chimaeras occur in temperate and cold waters, and are commonly known as rat-fish or rabbit-fish. They can grow to a length of 60 to 200 cm (24 to 80 inches), and have crushing tooth plates for eating shellfish.

The chondrichthyans originated from the placoderms, and lost both their armour and their bony skeletons. Modern sharks do retain small amounts of the placoderm armour as placoid scales in their tough skins.

During the Jurassic Period (195 to 140 million years ago), the first modern sharks originated from a group of primitive sharks known as the hybodonts. The hybodonts are very interesting because, in addition to teeth suitable for tearing off chunks of flesh, they also developed flattened back teeth that could be used for crushing mollusks and other hard-shelled sea creatures. It is believed that because they had two kinds of teeth, and could eat more than one type of food, that this enabled the sharks to survive an extiction event at the end of the Permian Period (about 230 million years ago).

Most modern sharks are predators with rows of very sharp teeth. However, the largest extant (still living) shark is the 18 meter (60 foot) whale shark, a gentle giant that feeds on plankton, and tolerates curious humans.

Next week: the Bony Fishes!

My favorite shark is Carcharodon megalodon a large (27 meters, or 90 feet!) extinct shark that resembled our modern great white shark

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