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)

May 5. Devonian brachiopods

Brachiopods reached their peak of diversity during the Devonian. Brachiopods, you recall, are shelled animals that superficially resemble clams, but they are their own phylum. Pretty much the first fossil collecting field trips I went on as a college geology student turned up lots of Devonian brachiopods.

At some places, brachiopods were so abundant that their shells made up small reefs. One of the most famous exposures of a Devonian reef is the Falls of the Ohio River, at Clarksville, Indiana. Brachiopods are common there, but it’s fundamentally a coral reef, possibly the most diverse mass of Devonian corals in the world. It also contains the usual suspects in a reef – crinoids, bryozoans, trilobites, mollusks, and more, as well as fish bones. The limestone containing the reef dates to Middle Devonian time, about 385 million years ago, a time when North America was near the equator but south of it. Today, the Falls of the Ohio is an Indiana State Park. 

Some of the most common Devonian brachiopods belong to the genus Spirifer. They actually range from the Ordovician to the Jurassic Period, but I can tell you from personal experience that they’re really common in Devonian rocks in the Michigan-Indiana-Ohio-Ontario region. Spirifers have a long hinge line, the place where the two shells join together. Often, the hinge line is the widest part of the shell, and this gives them a characteristic wing-like shape. Their name, spirifer, means spiral-bearing, and it comes from a tiny coil within the animal that was the support structure for the lophophores, the fleshy, tentacle-like things that brachiopods used to feed. The spiral structure is often preserved even though it’s tiny, because it was probably made of calcite just as the shell was.

There were at least 40 families of spirifers and dozens of species, but they are all extinct today.

Another group of brachs that was common during the Devonian was the productids. These brachiopods are sometimes called spiny brachiopods for the long projections that grew on their shells. In some species, the spines were as long as or longer than the shells themselves. It seems that they might have used the spines for stability, but a 2013 paper on the subject does not rule out the possibility that they might have used the spines as some kind of sensory organ.

—Richard I. Gibson

Links:
Falls of the Ohio
Falls of the Ohio - brachiopods
Mucrospirifer
Spines
Photo by Richard Gibson

May 4. The Great Unconformity

At Siccar Point on the east coast of Scotland, there’s a dramatic angular unconformity, an erosional gap representing a break between different packages of rock. Relatively flat-lying rocks of the Devonian Old Red Sandstone lie over tilted rocks of Early Silurian age that stand nearly vertical beneath the sandstone. The Old Red Sandstone, which we discussed yesterday, was deposited in a terrestrial environment, in basins within the vast Caledonian Mountain Ranges. The Silurian rocks at Siccar Point contain graptolites and were deposited in the deep sea. So the unconformity represents not only a 70-million-year break in the rock record, but also a dramatic change in the depositional environment for the rocks, from marine to terrestrial. Not to mention the tectonic activity that tilted the Silurian beds and eroded them off before the Old Red Sandstone was deposited.

Hutton's Unconformity at Siccar Point (photo by Dave Souza)

James Hutton, the geologist who first clearly expressed the concept of uniformitarianism, the idea that processes active today acted in the past over vast spans of time, used his observations at Siccar Point to develop his ideas of deep time, the idea that the earth was very old, and that processes like erosion operated in the past.

This outcrop is often called “Hutton’s Unconformity” because of its role in leading Hutton to his conclusions that form some of the most fundamental concepts of geology.

Hutton visited many locations where the unconformity is exposed, but Siccar Point is probably the most famous. He traveled there in 1788 with his friend John Playfair, who later recalled

On us who saw these phenomenon for the first time the impression will not easily be forgotten...We felt necessarily carried back to a time when the schistus [by which he means the Silurian rocks] on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of the supercontinent ocean... The mind seemed to grow giddy by looking so far back into the abyss of time; and whilst we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much further reason may sometimes go than imagination may venture to follow.

—John Playfair (1805) Transactions of the Royal Society of Edinburgh, vol. V, pt. III.

Hutton presented his Theory of the Earth to the Royal Society seven years later, in 1795.

The angular unconformity at Siccar Point and throughout Scotland represents the Caledonian Orogeny. The collision between Laurentia, North America, which included northern Scotland, and Baltica or Europe, which included southern Scotland, squeezed the preexisting rocks so that they were tilted from horizontal to nearly vertical, and then lifted up above sea level into the Caledonian Mountains where erosion began. The erosion produced the sediments that were deposited millions of years later as the Old Red Sandstone, on the erosion surface represented by Hutton’s unconformity.

Today, Siccar Point is something of a holy grail among geologists, since it represents the place that Hutton saw as the final vindication of his theory. And that helped change significantly the way we think about the earth.

—Richard I. Gibson

Further reading
Making of an angular unconformity
Hutton’s Unconformity – Arran


Photo by dave souza at Wikipedia, used under Creative Commons license.