Saturday, October 29, 2005

Tales From the Crypt: It's not just dirt, part I

Some of you may drive past something like this every day. To you, it might just seem like so much dirt near the side of the road, but to me it is a great deal more. The tan material shown in the photo is part of a Pleistocene (Ice Age) river terrace. How do I know it's Pleistocene? Well, perhaps that's a subject for another post. But suffice to say that in the Driftless Area (the area of Wisconsin, Minnesota, Iowa and Illinois that were not covered by the last glacial advance) contains buried treasure.

Treasure for someone looking for fossil land snails, at least. The material, complete with the remains of critters like land snails (even beetles) fills the river valleys as it washes in from the ridges and slopes above. These older river sediments are sometimes preserved in terraces like that pictured above. If one were to take a sample of that material and wash it through a series of sieves (down to .425mm) the remaining fraction might include fossils like those shown below:

What you see are the remains of land snails picked out of that very sediment shown in the top photo. Ostensibly, these critters washed into the valley from the ridges and hills above, but to what degree and how much downstream transport played a role is part of my Ph.D. dissertation.

Specimens in the above sample include: Discus shimeki (the larger helicoid shells). D. shimeki is known only as a pleistocene fossil in this area - modern populations are found at high elevations to the west. Columella alticola, another pleistocene fossil sometimes referred to as C. columella alticola, is one of the pupillid forms along with Vertigo modesta (yet another pleistocene fossil). Columella can be told apart from Vertigo quickly by looking at the body whorl as compared to the pentultimate whorl. The pentultimate whorl on Vertigo appears wider than the body whorl. There's one more pupillid hiding in that photo that I haven't keyed out yet. There is also some type of succineid, but without soft tissue, there's no hope of getting it down to species. The little round ball at the lower right is probably a snail egg.

Given how pristine these shells are (and the large number of intact eggs) it's not likely they were transported very far. I have other samples that do seem to have traveled. Perhaps I'll show some pictures of a well-traveled shell at some point. Just remember - to a Pleistocene paleontologist, it's always more than just dirt.