When brachiopods ruled the Earth

...well, maybe that's an overstatement, but it's a catchier title than "When brachiopods were dominant marine fauna in parts of cratonic North America", right? Our story today goes back to the latter part of the Cambrian, 500 million years ago or so. The Cambrian Explosion had come and gone, the confetti and stray napkins had been disposed of by various wormy things, and in the absence of thumbs to twiddle, there was nothing much to do until the Ordovician Radiation. Many forms of life got bored of waiting and went extinct, or otherwise died out from less frivolous causes, leaving behind a kind of "blah" marine fauna dominated by brachiopods, trilobites, and conodonts. This stretch of time has been called the "Late Cambrian plateau" or, more ominously, a "dead interval".

"We are your masters now! Ha ha!"

In Minnesota and Wisconsin, this kind of fauna is typical of several Upper Cambrian formations, at least when you get fossils (some formations are more cooperative than others). Putting aside the plentiful burrows and trails, there isn't a whole lot else. There's an unusual fauna of snails and monoplacophorans in the "Mill Street Conglomerate"; hyoliths, which as we found out this spring were kind of like brachiopods in a cone, show up from time to time; there's a noted assemblage of graptolites in the St. Lawrence Formation; there are scattered reports of echinoderm plates and bits; and every so often part of an aglaspidid turns up (an aglaspidid was kind of like a trilobite and kind of like a horseshoe crab, but without the staying power of either). Otherwise, it's brachiopods and trilobites.

"Your masters demand a steady supply of tiny filterable organic particles, and a suitable substrate!"

The Cambrian section, in ascending order (oldest to youngest), is composed of the Mount Simon Sandstone, Eau Claire Formation, Wonewoc Sandstone, Tunnel City Group (the old Franconia), St. Lawrence Formation, and Jordan Sandstone. In most of these units, trilobites are common enough, diverse enough, and underwent enough turnover that a detailed trilobite zonation has been devised. In fact, the formations were formerly defined partially on the basis of trilobite zones, which is problematic in practice. You have to know your trilobites pretty well, and you have to be able to find them in whatever outcrop or well core you're studying, which doesn't even get into the issues that arise when the zones don't follow lithological boundaries. Brachiopods aren't quite as well suited for zonation, but they make up for it by being much nicer and more recognizable fossils. The trilobites are often preserved as natural molds or casts in these rocks, but the brachiopods frequently include original shell material. These shells, made up of calcium-phosphatic-chitinous stuff that has proven quite durable, frequently look off-white or red in the Cambrian rocks.

"Sure, you try bein' dead for 500 million years, and see how you look!" (it was more obvious in person, but there is a D-shaped pygidium mold or cast in this rock, with the central axis being the ridged thing at about 99 to 104 mm.)

The brachiopod fossils are pretty small, with whole shells on the order of a few millimeters across, and generally subcircular or ovoid when complete, pointed on one side. Sometimes they look like fingernails in the rock. Although found throughout the local Cambrian rocks, they are particularly abundant in the Eau Claire Formation (the rock in the photo at the end of this post is probably from the Eau Claire Formation) and the Ironton Member of the Wonewoc Sandstone, formerly known as the Ironton Sandstone and several other things. The brachiopods in the photos in this post come from the Ironton Member in Hudson, Wisconsin, although I'm not sure of the exact position because they were float below the outcrop. They come from the "inarticulate" wing of the brachiopods. (The trilobite was higher up, from somewhere in the Tunnel City Group.)

They're kind of pretty like this, almost like agates with their banding.