Grant County Wisconsin- 7/7/23

[Tales From the Shale]

Age: Ordovician: Approx. 454-452 Mya

Formation: Decorah Shale

Location Grant County, Wi

Amidst the frequent outcroppings of Platteville Limestone in Southern Wisconsin, you occasionally encounter the overlying Decorah Shale. From what I read this formation is more frequent north of Grant County, and in Minnesota as well. The outcrop itself is roughly 200 feet in length, and only extends about 6 feet from the ground. Its located in a very remote area, amongst a sea of corn fields. Here are a couple finds from my third trip to this locality.

 

Here is a complete crinoid calyx with associated pinnule filtering appendages. Complete crinoids are much more common in Silurian/Devonian stratum.

 

Next is a hashplate, with a few assorted brachiopods and crinoid fragments.

 

There were also a handful of brachiopods to be found alone as well. All of these are very similar to the Ordovician Lepidocyclus perlamellosa/H. capax. Although these were much smaller in size.

 

A single Lingula inarticulate brachiopod, which was a huge surprise since this was my first one ever.

 

I found a mid sized orthoconic nautiloid. This one is 7 inches (17.7 cm) in length. The approximate width at its widest end is 2 3/4 (6.9 cm). Note the visible septa markings on the specimen. This is an internal mold, so structure is a little different from that of a normal mineralized specimen. Not the largest from this site, as others I've recovered push 25 inches (63.5 cm).

 

The internal siphuncle is exhibited here in cross section:

 

I marked the cross section and direction of the siphuncle here. Phragmocones from this locality only preserve as internal molds without any calcified structure. As a result of poor preservation they can be difficult to spot. With a trained eye, it becomes significantly easier.

 

Here are some additional brachiopods that I dug out from this locality.

 

A fennestellid bryozoan as well. There were a few, but none to write home about unfortunately.

 

My favorite finds at this location are by far the large nautiloids, which have been consistently produced from here. (If only I could find one at 6 feet now ) If anyone has an i.d for that crinoid calyx, I'd be happy to post measurements or more detailed photos. Other than that I hope to run out to Indiana sometime within the month, maybe I'll see some of you out there.

 

-Jonah

[Mark Kmiecik]

Nice finds.