Sunday, October 29, 2017

Halloween: Platteville in disguise

Let's start with a pair of photos, one of typical Platteville Formation fossils and the same for the Decorah Shale:

Welcome to the Ordovician! Hope you like brachiopods!

...or bryozoans and crinoids

No points for guessing which is which. The first photo shows natural molds and casts in the Platteville, while the second shows the well-shredded fossils of the thin limestone beds of the Decorah. Now try this one:

No peeking!

Despite the preservation, this piece is from the Platteville. Specifically, it comes from a bed about halfway up the Mifflin Member. Somehow this bed managed to escape the worst of the heartbreak of dolomitization, at least in a small area. Little glimpses like this show a richer picture of the Platteville fauna than we get from the natural molds and casts. (The tiny crinoid columnals are an interesting touch, as is the relative scarcity of bryozoans.)

They're in there. It's a proper hash.

The strophomenids might be a little smaller, too.

There are also snails, ostracodes, and some elongate triangular things. Some of these are probably small nautiloids, others might be hyoliths.

The example near the center bottom of the photo is one I suspect is a hyolith.

There are a lot of subtleties to the rocks and faunal assemblages once you start looking!

(Also, having found a decent place to see the Glenwood up close, I've added a couple of photos to the old post.)

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