Cottage Grove cystoid

I recently paid a return visit to the site in Cottage Grove where I'd previously spotted a few blocks with brachiopods/etc. of the typical Platteville persuasion. While there, a few true outcrops were visible under the snow-free conditions of the so-called winter of December 2023, confirming that the Platteville is indeed at the surface and not just present as lag or buried under a bad toupee of soil and glacial debris. More significant was one of the blocks. See if you can spot what drew my attention:

Any ideas?

How about if we go in closer on the area of interest?

If you answered something along the lines of "the thing near the center that looks kind of like a truncated letter K", you've won! I snapped a couple of pictures thinking it might be echinoderm in origin and moved on. Later, upon reviewing the photos, the rectangular bit below it caught my eye; that definitely looked echinoderm. In fact, there is only one kind of thing it could belong to, as proclaimed by the chevron arrangement of slots on it surface. This is a plate from a rhombiferan cystoid bearing a pore rhomb in a pectinirhomb configuration. ("Of course!" you shout.) (Okay, so I looked up the anatomical terminology). Basically, the slots are vents for the animal's water circulation system.

This rock was obviously a good candidate for further study and photography, so I took some more photos with the hope of doing some taxonomy. Further inspection revealed a couple sharply ridged features similar to the "K", but more weathered.

There's a pretty well-developed one in the upper right, and one that is more poorly exposed near the left side.

When I first noticed the "K", I thought it might be ridges on a crinoid plate, but local crinoid plates don't usually have such sharp ridges. Instead, it turns out that there is a Platteville cystoid that does, Coronocystis durandensis. Coincidentally, this particular cystoid also has pore rhombs that are a good match for the pore rhomb plate on this rock (see photos in Kolata 2021).

Here close-up and with a tiny drip of water applied. There is also a gray rectangular ridge visible near the right border that I suspect to be another plate, but it's not as well-exposed.

I hesitate to make a firm identification from the available material, but it certainly appears that we have Coronocystis or something very similar. Coronocystis is interpreted as a stalked rhombiferan cystoid, unlike its free-thrashing cousin Pleurocystities (which does pore rhombs differently and has softer ridges). I interpret the block as from the Mifflin Member of the Platteville, but I suppose it could be higher. Whatever the exact identification at the genus or species level, this is clearly a rhombiferan cystoid, and the first I've ever seen in the field (and I'm pretty sure the first record from Cottage Grove). Plus, the several bits suggest a disarticulated but fairly associated specimen.

References

Kolata, D. R. 2021. Fossils of the Upper Ordovician Platteville Formation in the upper Midwest USA: an overview. Illinois State Geological Survey, Prairie Research Institute, University of Illinois at Urbana-Champaign, Champaign, Illinois. Bulletin 108.