The Find

A weekend or so back I traveled down to Whitewater State Park to participate in their Fossil Field Trip event. As part of this, visitors go to a site off of the park where they can hunt for fossils (participants can keep two). It was very well presented and attended! The site is in the Cummingsville Formation, which is the limestone that follows the Decorah Shale. (Actually, it's a bit more complicated than that: much of the Cummingsville Formation in Olmsted County is roughly the offshore equivalent of the more nearshore Decorah of the Twin Cities, so where we were is probably equivalent to a horizon in the upper half of the Decorah quarry walls at the Brickyard.) Unlike the distinctive green-gray Decorah, the Cummingsville presents as a tan unit. It's a shaly limestone at the site, so it doesn't quite have the strength of a pure limestone, but it does better than the Decorah.

Weathering Cummingsville at the site.

The Cummingsville is not as fossiliferous as the Decorah, even accounting for my different search image, and the site is a known fossil site, so the going was slow at the beginning. Then I found this:

Well, I did do a little prep work and applied water before taking this picture, and the rock is about palm-sized all told, but you get the idea.

I'd been hoping to find a receptaculitid, because those are common in the Cummingsville and I haven't found one, but this was something better. Even in an unprepared state it was obviously an echinoderm based on the plates, but what kind? A crinoid calyx? Something else?

If you know what the arrows are pointing to...

The prep work exposed the little vent-like features, meaning it was some kind of rhombiferan cystoid. The cleaning also made it more obvious where the plates met, which can be kind of confusing when there are all these ridges to mislead the eye.

A handy diagram! The dotted lines are where the plate edges become obscure or are lost.

If you don't know what it is, check this out:

Figure 1 from Parsley (1970), with red lines and dots added by me. Original image CC BY-NC-SA 3.0.

Yup, this is a Pleurocystites, or at least a good-sized chunk of one. Either P. squamosus (Parsley 1970) or P. strimplei (Brower 1999) would be appropriate for the Cummingsville; there's not really enough there to tell between them, but I'm not complaining! It's fun to be in the right place at the right time with the right knowledge and experience.

References

Brower, J. C. 1999. A new pleurocystitid rhombiferan echinoderm from the Middle Ordovician Galena Group of northern Iowa and southern Minnesota. Journal of Paleontology 73(1): 129–153. 

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Parsley, R. L. 1970. Revision of the North American Pleurocystitidae (Rhombifera-Cystoidea). Bulletins of American Paleontology 58(260): 135–213. 

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.

Release the robotic Pleurocystites!

Speaking of paleobiology, here's a fun story that crossed my desk recently...

If you've ever seen a fossil of Pleurocystites or its ilk, you've probably wondered what the heck it was doing in life (and perhaps how disturbing it may have looked while doing it). What once was confined to the realm of speculation now takes a step, or more accurately a kind of thrash, into the 21st century with the "Rhombot", a robotic pleurocystitid detailed in Desatnik et al. (2023).

The rhombots use the basic form of a pleurocystitid: central capsular body (theca), two short appendages (brachioles) at one end, one long appendage (stem) at the other end. The body plan is scaled-up in comparison to the real thing, and, of course, it is made of various artificial materials (many of which are not rigid, hence the "soft robotics" tag) rather than pleurocystitid stuff.

Pleurocystitids, thrashing their way from Ordovician seafloors to the modern day via the rhombot (Figure 1, Desatnik et al. 2023) (CC BY-NC-ND-4.0).

Granted, a rhombot is not a direct replica of a Pleurocystites. Assuming the living thing did indeed use its stem for propulsion, what can we gather from a robotic equivalent? First off, the rhombot was much more efficient with the stem moving it from behind rather than pulling it along. (This seems like a common-sense conclusion on first principles, so it's good to see it confirmed.) The stem also was more effective when used with a stiff sweep, rather than working with a sinuous motion. Finally, there was an optimal length for the stem (Desatnik et al. 2023). I encourage you to go to the linked article and see the movies of rhombots in action; they are not the most graceful robots, but pleurocystitids were probably not the most graceful animals.

References

Desatnik, R., Z. J. Patterson, P. Gorzelak, S. Zamorad, P. LeDuca, and C. Majidi. 2023. Soft robotics informs how an early echinoderm moved. Proceedings of the National Academy of Sciences 120(46):e2306580120. doi: https://doi.org/10.1073/pnas.2306580120.

Platteville sea stars and cystoids

It's been a while since I've featured something Ordovician, so here are some photos of some uncommon Platteville Formation echinoderms. I took them a few years back, when I wasn't quite as good at this as I've become. These specimens all come from the Platteville of the Twin Cities, and are in the collections of the Science Museum of Minnesota. In fact, the first example has since been moved out to the exhibits, which is good because it certainly deserves the visibility. It is a specimen of a small stocky early sea star identified as Hudsonaster (Protopalaeaster in older references, not to be confused with Promopalaeaster, another Ordovician sea star but of more conventional appearance).

Here it is, with scale artfully copied from elsewhere in the cropped photo and placed in a more convenient location (mm divisions, one full cm included). A crinoid columnal is visible in the upper left corner.

This was not a very large animal, only about an inch across or so. It's not from the former Johnson Street Quarry, the fabled lost storehouse of Platteville echinoderms in the Twin Cities. Actually, none of the three specimens in this post are from that site. It's possible they're from the Hidden Falls Member, as the Johnson Street specimens were, but I don't have stratigraphic information lower than formation for these specimens, two of which were found on loose slabs.

Another view, taken at closer range and thus a bit sharper, but with no scale.

Hudsonaster was not the only sea star in the Platteville. Urasterella was a larger, more lanky sea star, although since we're talking about the Ordovician of Minnesota, "larger" still isn't enormous by our standards. (It's still pretty big for a non-cephalopod of the Platteville, though.) The specimen is not as complete as the Hudsonaster, but there is one nearly complete arm and most of two others, with the missing sections partially recorded by natural molds.

And I've rather brilliantly arranged to have my reversible transparent ruler wrong-side-out.

From a quick glance of the exteriors, neither Hudsonaster or Urasterella are all that different from sea stars you might see today. Going from the familiar to "what-is-that-and-what-happened-to-it", we have a fossil identified as the rhombiferan cystoid Pleurocystites. Because only one side is visible, I'm not entirely sure if it is Pleurocystites or a close relative (the very similar genus Amecystis is also known from this interval in the Upper Midwest per Kolata et al. 1987), but it's a reasonable identification.

With quarter for scale; the block is also peppered with partial crinoids.

It somewhat resembles a deflated balloon on a string, or some kind of odd fish, and again is not very large. Rhombiferan cystoids were stalked ancestrally, but these pleurocystitids seem to have been using theirs for something else, perhaps on the order of a flagellum. It's not apparent in this specimen, but at the opposite end from the "stalk" were two long appendages that looked like antennae but were actually part of the feeding apparatus (only the bases are visible here). A few formations up, in the lower Prosser Formation, a mass-death assemblage in southern Minnesota yielded dozens of Pleurocystites (Sloan and DesAutels 1987), so whatever they were doing, they were at least briefly successful at it.

Strangely enough, members of a completely different group of echinoderms, the solutan "carpoids", hit upon an astonishingly similar body plan at almost the same time (Kolata et al. 1977). The Prosser bed also includes specimens of these solutan doppelgängers (Sloan and DesAutels 1987). A quick way to tell the two types apart is the "stalk": pleurocystitid "cystoids" have a simple "stalk" of stacked pieces, but the lookalike solutans have complex, "braided" "stalks". (Also, pleurocystitids have two appendages opposite the "stalk" and the lookalikes have one, but those aren't always as easy to see.) The Science Museum has one of these solutan lookalikes, Dendrocystis, on display. The fad for echinoderms that looked like deflated balloons with antennae did not last for either version, with both lineages going extinct long before the end of the Paleozoic.

References

Kolata, D. R., H. L. Strimple, and C. O. Levorson. 1977. Revision of the Ordovician capoid family Iowacystidae. Palaeontology 20(3): 529–557.

Kolata, D. R., J. C. Brower, and T. J. Frest. 1987. Upper Mississippi valley Champlainian and Cincinnatian echinoderms. Pages 179–181 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.

Sloan, R. E., and D. A. DesAutels. 1987. The Wagner Quarry cystoid bed: a study in Prosser (Sherwood) paleoecology. Pages 60–62 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.

All of the other echinoderms

If you thought that echinoderms have a variety of seemingly unrelated body plans today (sea stars, crinoids, sea urchins, sea cucumbers...), you should have seen them during the Paleozoic, when several now-extinct classes populated the oceans. The group with probably the most recognition are the blastoids, or sea buds, which had stalks like crinoids but with a nut-like structure instead of a cup-like structure as the business end. The rocks of the Twin Cities area are not known to have produced blastoids, but they have produced rare examples of four other groups not including the crinoids we saw before. They are: asteroids (sea stars), rhombiferan cystoids, edrioasteroids, and stylophorans (a.k.a. carpoids, a.k.a. homalozoans). Three of these groups are extinct, and it doesn't take much to guess which. This diversity of echinoderms is not particularly unusual; Cincinnatian rocks have all the same classes as well as brittle stars and cyclocystoids (extinct and known mostly from their ring-like outer structures), and the Bromide Formation of Oklahoma, of comparable age, outdoes the Minnesota rocks handily. But for the luck of the cosmic draw, any of these extinct groups might be around today.