The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, April 19, 2014

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.


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


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