Zittelloceras

While on a walk earlier this year, I spotted a Decorah block that I decided to pick up for photography. The initial attraction was the abundance of snails, which are a reliable indicator that pieces of our fossil arthropod friends are also present (if there's only one practical thing you take away from this blog, it's "when you're in the Decorah and see snails, look for trilobites"). This was indeed the case:

Here's the whole block, which rewards a click to embiggen. There is a nice Clathrospira and a lophospire just right of the scale bar, and many smaller snails scattered throughout. You may also pick out the trilobite pygidia.

Here's a pygidium, pointed toward the top of the photo.

A nice pygidium plus a number of other things, including some crinoid columnals, bryozoan fragments, other trilobite bits, and, near the top, a whorl of a snail.

There was also something else: a dark object several millimeters long and broad. It appeared to be a thin-walled flattened tubular object, with a distinct series of ornamented transverse ridges. The ridges showed an alternating pattern of strongly projecting and more subtle, like perforations. Both had little scooped frilling, the same kind of shape as a doodle of stereotypical ocean waves.

The object in question is near center. You may have noticed it in the first photo. The light-colored band near the center is some light prep to see if I could get the matrix out from the groove.

I'd never seen this combination of features before, but I could knock out a lot of things quickly. In fact, I knocked out just about everything, which was a problem. Given the probability I had discovered a completely new phylum is pretty low, all things considered, I figured I'd probably missed something. So, I pulled out my copy of "A Sea Without Fish" (Meyer and Davis 2009) to see if some similar exotica had been found in the well-studied Cincinnatian, as it's only a few million years younger. Then I got excited looking at the figure and description of the machaeridian worm Lepidocoleus. Machaeridia is an extinct group of Paleozoic armored annelid worms, with segments of calcitic plates and a heart-shaped cross-section.

This view, under different lighting, shows the ridges and frills to good effect.

Before I got too excited, I decided to put it up on the Fossil Forum, to see what others might think. The first suggestion was Phragmolites, which was reasonable enough but didn't fit my experience with that snail. The chunk wasn't curved enough, the dark coloration and thin wall were unlike the examples of Phragmolites I'd seen, and the ornamentation of the ridges wasn't a good fit. Then someone came up with the nautiloid Zittelloceras, and provided photos of a form with almost the exact same pattern of frilled ridges found in the Platteville.

An end-on view shows the cross-section, with the thin walls and central crushing.

So, it looks like rather than a worm, it's a nautiloid. Zittelloceras is one of the "arched" nautiloids, not coiled and not a full-on orthocone. Several species are present in the Platteville per Catalani (1987), but none are listed in the Decorah. This is not a particular problem, as the genus is present in younger strata as well, and the Decorah's cephalopod record lags the Platteville. (Note that Zittelloceras is frequently misspelled "Zitteloceras", with one "l", but a look at the original publication, Hyatt 1884, shows the two-l spelling is correct.) I'm sure there are worms out there to be found in the Decorah, but I'll settle for this record of an ornate nautiloid.

Here's one more angle for the road.

References

Catalani, J. A. 1987. Biostratigraphy of the Middle and Late Ordovician cephalopods of the Upper Mississippi Valley area. Pages 187–189 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Hyatt, A. 1884. Genera of fossil cephalopods. Proceedings of the Boston Society of Natural History 22:253–338. [some history: The paper is based on a talk presented by Hyatt April 4, 1883, a day before his birthday. There is a note on the first page that there was going to be a monograph in the Memoirs of the Museum of Comparative Zoology, but this did not happen. At any rate it's hard to think of an 80-page paper being "preliminary" to anything!]

Meyer, D. L., and R. A. Davis. 2009. A sea without fish: life in the Ordovician sea of the Cincinnati region. Indiana University Press, Bloomington and Indianapolis, Indiana.