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The Fish, Part I

© Beverly Eschberger

In an earlier article, "Coelacanth: The Living Fossil," I talked about fish. Now, I would like to take the opportunity to discuss this interesting group of animals more thoroughly.

Fish are vertebrate animals, meaning that they have backbones. The earliest known vertebrate fossils are fragments of fish fossils found in rocks from the Ordovician Period (500 million years ago). We say that these are the earliest known because these fish were already well-developed and had obviously been around for quite some time before they first appear in the fossil record.

Although fish fossils have been found in rocks from as long ago as the Ordovician Period (500 million years ago) and the Silurian Period (435 million years ago), they do not appear abundantly until the Devonian Period (395 million years ago). In fact, fish fossils are found in such abundance and in such variety, that the Devonian Period is known as "The Age of Fishes."

Extinct and extant ("still living") fish are divided into four main groups: the ostracoderms, placoderms, chondrichthyans, and osteichthyans. All four groups had appeared by the end of the Devonian Period (345 million years ago).

The first group that we find in the fossil record is the ostracoderms. The ostracoderms were rather odd-looking, as they had no mouths. "What? No mouths?" That's right, they had very small mouths which were really just slits at the front of their heads. They sucked in water and food particles through these slits as they scavenged in the mud on the ocean floor.

The first fishes with jaws to be found in the fossil record were the placoderms. They first appear in rocks from the late Silurian Period (about 400 million years ago). The placoderms had a bony covering over their heads and the front part of their bodies with a movable joint between the two sets of armour. This allowed them to rock their head back on their neck and open their mouth very wide. They did not have teeth in their jaws; instead, their armour had jagged edges which worked just like teeth. The hind part of their bodies was not armoured and was not even covered with scales.

Most of the placoderms lived on the ocean floor, and many had bodies that were rather flattened. Some of the placoderms, the arthrodires, grew to be over two meters (six feet) in length and are believed to have been formidable predators.

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