June 3. Archimedes

Today I want to talk about a bryozoan, a colonial animal somewhat like corals but probably more closely related to brachiopods. You may recall that they were the only phylum that, so far as we know, was NOT established before or during the Cambrian explosion but developed later, during the Ordovician. And they do survive to this day.  

During the Mississippian, the warm shallow seas of North America harbored plenty of life, including diverse bryozoans. But the screwiest of all is a group named Archimedes.  

Archimedes was the Greek philosopher who invented the screw as a way to lift water.

Archimedes wortheni

Fossils of the bryozoan genus Archimedes look just like screws. In life, the individual zooids that comprised the colony had lattice-works extending away from the long, spiraling skeletal substrate of the colony, but the fossils usually preserve only that skeletal backbone, and it looks like a screw. The name was first applied by the American geologist David Dale Owen in 1828.

They’re really pretty common, and you find them typically something like one to four inches long, or shorter broken fragments. Screws can be left-handed or right-handed, and so are Archimedes bryozoans. They began and were abundant during the Mississippian and survived about 108 million years, until the mass extinction at the end of the Permian Period.

Archimedes bryozoans also look an awful lot like fusilli pasta, and when you get a lot of geology students together in a place like geology field camp and serve it to them, you can bet that they’ll start calling it Archimedes noodles. I’ve had it dozens of times.

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Two geological birthdays today. James Hutton,, who formulated the theory of uniformitarianism and many of the concepts that laid the groundwork for the modern science of geology, was born June 3, 1726, at Edinburgh, Scotland. Check the posts for March 7 and May 4 for more about him. Lee Suttner, one of my professors of geology at Indiana University, was born June 3, 1939, in Wisconsin. If I’m a decent teacher of geology, I owe it to Lee for sharing his style of asking leading questions, rather than telling. It’s a style I admire and try to emulate when I can. Happy birthday, all.

—Richard I. Gibson

David Dale Owen and the naming of Archimedes

Drawing from an old textbook (public domain)

June 2. Mississippian time

The time span of the Mississippian is about 36 million years. It began with the end of the Devonian about 359 million years ago and it ended about 323 million years ago. As with all the subdivisions of geologic time, there are error bars on these dates, in this case about a half million years, plus or minus, on both ends.  

Mississippian time (from Wikipedia)

In Europe, the Early Carboniferous, the equivalent in time to the Mississippian, is divided into three ages which correspond to three stages in the rock record. The Tournaisian and Viséan are named for rocks in Belgium and the Serpukhovian is from outcrops in Russia. They span from 7 to 17 million years each.

In North America, there are four subdivisions of Mississippian time. From oldest to youngest they are Kinderhookian, Osagean, Meramecian, and Chesterian. These names are used a lot in the literature on Mississippian rocks, and I may use them too – but I will try to avoid too much jargon and I’ll try to always refer any names like that to the part of the period we’re in, and about how many million years ago it was.

As I’ve said before, international agreements are often needed to assign specifics to the breaks in geologic time. Since rocks in one part of the world don’t necessarily record the same events as other parts – or the same event may span some time and may occur at one time here, and another there – because of that, it’s not surprising that we really cannot make an exact, world-wide time scale that applies everywhere. But things do usually work out so that they’re pretty close.

—Richard I. Gibson

June 1. The Carboniferous begins

…or is it the Mississippian?

The next period of the Paleozoic Era is called the Carboniferous, which means carbon-bearing in reference to the coal beds in the upper part of the system in many parts of the world. The name was invented by British geologists William Conybeare and William Phillips in 1822, making it the first to be established of the names we use today for the geologic periods.  

Now we have a nomenclature problem to deal with. In the book I put together in 1994, the month of June corresponds with the Mississippian Period. In the United States, the Carboniferous of Europe is divided into two distinct time spans, the Mississippian and the Pennsylvanian. Technically, in terms of international geologic names, the Mississippian and Pennsylvanian are sub-periods of the Carboniferous, but in part because of long-standing usage, in the United States the two are treated as full-fledged periods of geologic time.

In the U.S., the period takes its names from rocks of this age exposed along the Mississippi River, especially in western Illinois where these rocks are hundreds of feet thick.

For purposes of the original book and for these podcasts, I’m using U.S. nomenclature, so the month of June is the Mississippian and July will be the Pennsylvanian. But technically, they are parts of the Carboniferous Period. Sometimes you’ll see the Mississippian equated with early Carboniferous and the Pennsylvanian with the late Carboniferous. It’s all a matter of human convenience, and in this case, a matter of some notable differences between the rocks of this time period in the U.S. versus those of Europe.

—Richard I. Gibson