When I was an undergraduate Andy Moore and I were teaching assistants for Carleton’s paleontology course. This gave us the opportunity to organize and lead our first field trip, to look at Ordovician and Silurian fossils in northeastern Iowa. The highlight of the trip was a roadcut near Graf, Iowa. On our return trip from Wyoming last week Tim and I stopped in Graf to collect specimens for VMNH’s upcoming exhibit on the Ordovician Period.
From the road Graf is a rather unexceptional-looking outcrop. It’s only a few hundred yards long, and the tailings slope is often overgrown with weeds in the spring and summer:
Most of the 10 meters or so exposed at Graf are shales from the Elgin Member of the Maquoketa Formation (Witzke and Heathcote, 1997), which is in the Upper Ordovician. In this small section there is a remarkable variety of fossils. The massive upper beds are mostly dolostones and limestones, and several of them are coquinas (a rock that is composed mostly of fossils):
The remarkable thing about these is that the fossils are nautiloid cephalopods, Isorthoceras sociale. The nautiloids are a group of mollusks that includes squid, octopus, cuttlefish, and the chambered nautilus. Only a few species of nautiloids survive today, but they were extremely diverse during much of the Paleozoic, and in the Ordovician they were the dominant marine predators. Even with the abundance of nautiloids in the Ordovician, however, there are not many deposits where you can collect hundreds of them in just a few minutes!
As remarkable as the nautiloid beds are, they aren’t the only surprising thing at Graf. The lower part of the section includes a series of thin, black-to-grey, phosphatic shales, that once again are packed with fossils. But this time the fossils are much more diverse, and tiny. In fact, these fossils have often been referred to as the “depauperate fauna” or the “diminutive fauna”, because few of the shells reach even one centimeter in length.
Some of the most common fossils in this fauna are graptolites. These are colonial hemichordates; in some taxa, the colonies floated near the surface, suspended beneath a floatation bag. They’re almost always preserved as dark lines on the rock that almost look like they were drawn on with a pencil. What’s unusual about the Graf graptolites is that many of them are preserved in three dimensions, such as in this specimen of Orthograptus:
Here is another common and unusual fossil found in the “diminutive zone”:
A good deal of controversy has surrounded these tiny tubes. They were originally described by James, 1890, as calcareous worm tubes and given the name Coleolus? iowaensis (the type specimen is from Graf). In 1970, Bretsky and Bermingham reinterpreted them as scaphopods (tusk shells), curved tube-shaped mollusks, and referred them to the scaphopod genus Plagioglypta as P. iowaensis. This was a major referral, since there are almost no records of Ordovician scaphopods; the oldest unquestioned scaphopods are from the Carboniferous, nearly 100 million years later. In 2004, Yochelson questioned this interpretation, and sent the species back to Coleolus? as a worm tube, although reading Yochelson’s paper it sounds as if even he wasn’t completely sure about its status. Then, in 2006 Peel showed that it’s likely that many different mollusk lineages were converging on the overall scaphopod shape in the early Paleozoic, so that there could be several similar-looking but unrelated mollusks with tube-shaped shells. So, where does that leave our tiny shells from Graf? It’s not clear; they could be one of the earliest records of scaphopods, they could be an unrelated group of tube-shaped mollusks, or they may, in fact, be worm tubes.
I still use several of the specimens I collected on that field trip in my teaching. I have always wanted to go back… Glad you did!