May 6. The Age of Fishes

The Devonian Period is called the Age of Fishes because fish were abundant then, and they also diversified dramatically. While some of the major groups got started in the late Silurian, it was during the Devonian that they came into their own. 

Devonian fish

The two main subdivisions of fishes that survive today, the cartilaginous fish which include sharks and their relatives, and the bony fish, which is pretty much everything else, became well established during the Devonian. The armored placoderms did well during the Devonian but became extinct at the end of the period. The more primitive jawless fish, which we referred to in March with the general name ostracoderms, declined a lot, presumably in part due to competition from the more efficient predators that had jaws. But jawless fish have survived, and are represented today by hagfish and lampreys, although the exact relationship between them and fossil jawless fish is not completely clear.

As we discussed the other day, we know for sure that fish had invaded fresh-water environments by the Devonian, since their fossils are found in the Old Red Sandstone, whose sediments were laid down by rivers.

The largest fish of the Devonian, the terrors of the seas, were the arthrodires, placoderms that grew to as much as nine meters long – close to 30 feet. Although they survived as a group for nearly 50 million years, they too died out in the extinction near the end of the Devonian Period.

The group of fish that would diversify the most was the lobe-finned or fleshy-finned fishes that we talked about on April 22. Their descendents would become land-dwellers, and we’ll get into that in more detail later in May.

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May 6, 1843, was the birthday of Grove Karl Gilbert, in Rochester, New York. G.K. Gilbert was a geologist with the fledgling U.S. Geological Survey, and did a lot of work in the Rocky Mountains. He’s noted for his work in the Henry Mountains of Utah, and for his studies of Glacial Lake Bonneville, a huge lake of which the Great Salt Lake is a tiny remnant. He was also a pioneer in the study of craters, including Meteor Crater in Arizona, which he thought was of volcanic origin, and the moon, which he interpreted to represent impacts. While that may seem obvious today, the origin of lunar craters was debated well into the 20th century. 

—Richard I. Gibson

Further reading
National Geographic

Drawing by Joseph Smit (1836-1929), from Nebula to Man, 1905 (public domain)