Fossil Friday – nautiloid cephalopod

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Because I’m still at the SE GSA conference today’s Fossil Friday post will have to be a short one. Last week Brett and I were in Chicago for the NSTA conference, after which we started driving to Chattanooga for SE GSA. The drive across the intervening states of Indiana, Ohio, and Kentucky took us through some of richest fossil deposits in the world, the Ordovician rocks of the Cincinnati Arch.

Practically every exposure of rocks in this region will include at least some marine invertebrate fossils, so even though it was cold and we were pressed for time we still made a quick stop at a road cut in Kentucky. Ten minutes of searching revealed the fossil shown above, a nice natural cast of a nautiloid shell.

Nautiloids are cephalopods, the group of mollusks that includes squid and octopus. While most modern cephalopods don’t have an external shell, the nautiloids (including the extant chambered nautilus) have a subdivided calcium carbonate shell. While nautiloids are rare today they were much more common and diverse in the past, and during the Ordovician Period were apex predators. But even though they were more common in the Ordovician than they are today, they weren’t that common, and usually make up only a tiny percentage of the fossils in most Ordovician deposits. That makes this a very nice find for a single 10-minute stop.

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