Sunday nautiloids

An interesting fact concerning nautiloids and the Platteville Formation is that the biggest nautiloid shells are often found only partially preserved, because their diameters (up to around 10 in/25 cm) exceeded the depth of the Platteville ooze they sunk into (Webers 1972a, 1972b). This means that when you see a fossil of one of these big Platteville nautiloids where the cross-section is partly round and partly irregular, you can guess that the round part represents what was buried; the rest of it was eroded off. So, while a small nautiloid can be completely buried:

...the big guys aren't so lucky. Take these examples from Sogn, Minnesota:

The more impressive of the two, featuring the part that looks like a nautiloid.

The cross-section of this specimen. Round was down (in the mud), originally.

The cross-section of the other, less impressive nautiloid.

Half a nautiloid is better than none, and for these two nautiloids, it permits a bonus: other shells and organic fragments collected in the eroded nautiloids. Notice the off-white patch near the center of the first cross-section above? Here's a close-up:

This is a small lingulid brachiopod (modern cousin Lingula; see a larger relative from Oklahoma rocks of reasonably similar age to the Platteville here). There are several fossils larger than millimeter-sized chips visible on these nautiloids, and they all appear to be the same kind of lingulids. This is an interesting association, but I couldn't tell you what it means. Here are two of the better-preserved examples:

References:

Webers, G. F. 1972a. Paleoecology of the Cambrian and Ordovician strata of Minnesota. Pages 474–484 in P. K. Sims and G. B. Morey, editors. Geology of Minnesota: a centennial volume. Minnesota Geological Survey, St. Paul, Minnesota.

Webers, G. F. 1972b. Paleoecology of the Ordovician strata of southeastern Minnesota. Pages 25–41 in G. F. Webers and G. S. Austin, editors. Field trip guidebook for Paleozoic and Mesozoic rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook 4.