April 19. Fish get jaws

Last month we talked about some early animals that you’d call fish if you saw them: the ostracoderms, a general name for several groups of armored, jawless critters. The Silurian Period was a time of considerable diversification among the fishes. Some of the structures called gill arches evolved into the lower jaw, a movable structure. It’s easy to imagine the advantages of a jaw that could open and close as compared to a relatively solid opening that could take in food but relied on whatever happened to be in front of the animal. Now, fish could bite.

Placoderm

Some of the earliest Silurian fish to develop jaws were primitive sharks, which are cartilaginous animals rather than bony. Their skeletons are made of tough fibers – cartilage – rather than solid bones. That model works, since we have sharks and their relatives today. Another group that developed jaws during the Silurian were the placoderms – they looked a lot like ostracoderms and their name means plate-like skin, but they also had jaws. Placoderms appeared in the late Silurian, and they are extinct today, dieing off at about the end of the Devonian. You really wouldn’t think twice about calling a placoderm a fish – possibly a somewhat weird-looking fish, but certainly a fish. Some of them, especially the group called arthrodires, grew to huge sizes during the Devonian, reaching 10 meters or more in length , more than 30 feet. And they were predators.

Bony fish

The very first bony fish – the group that includes most modern fish, with 28,000 modern species, got started in the late Silurian, about 420 million years ago. In addition to jaws, bony fish have swim bladders, organs they use to maintain their position in the water.  

Fish, of course, have fins. The evolution of fins into legs suitable for walking on land is a huge event in the story of life, which we’ll get into a bit more, but in the meantime I want to recommend, yet again, Your Inner Fish by Neil Shubin – it’s a book and a three-part program on PBS that tells not only this story, but connects that evolution to modern vertebrates including humans.

—Richard I. Gibson

http://en.wikipedia.org/wiki/Evolution_of_fish

Bony fish image by ArthurWeasley via Wikipedia underCreative Commons license 

Placoderm by Nobu Tamura under Creative Commons license