Fossil Friday – Isorthoceras sociale

IMG_4043During the Mastodons of Unusual Size road trip, Brett and I made several stops to collect fossils for WSC. Since I was 20 years old, I’ve made several visits to the small, well-known (to paleontologists) roadcut in Graf, Iowa (the photo below was from a trip I made there in 2007):DSC09279

The rocks at Graf are from the Late Ordovician Maquoketa Formation, about 445 million years old. A number of different types of marine invertebrate fossils are found here, but perhaps the most prominent is the cephalopod Isorthoceras sociale.

Cephalopoda is the group of mollusks that includes modern squid, octopus, and cuttlefish. Another group of modern cephalopods are the nautiloids, such as the chambered nautilus:

Nautilus pompilius

Unlike other living cephalopods, nautiloids have an external shell made of the mineral aragonite. The shell in modern nautiloids is shaped like a coiled cone, and is divided internally into chambers:

Nautilus interior

While nautiloids are relatively uncommon today, for hundreds of millions of years they were among the most abundant animals in the ocean, and it was during the Ordovician Period that they first became widespread. While the living nautilus has a coiled shell, nautiloid shell forms were more diverse in the past. Many of the early nautiloids had straight, conical shells. At Graf, there are several rock layers that are made up almost entirely of the shells of the small straight-shelled nautiloid Isorthoceras, at least four of which are visible in the photo at the top of the page. Here are some reconstructions of what Isorthoceras may have looked like:

Isorthoceras sociale

Most of the shells in any given layer at Graf are aligned in the same direction, so from certain angles the shells are mostly seen in cross section:


The shells also sometime erode out of the rock, and can be found as isolated fossils:


During our short stop at Graf, Brett and I filled a 5-gallon bucket with rocks containing Isorthoceras and other Ordovician marine fossils which will be added to the WSC collections.


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