Uŋčí Makhá Park 2024: another winter, more echinoderms

Two years after opening, Uŋčí Makhá Park can be considered a paleontological gem in the Twin Cities. With its Magnolia Member bedding planes, side cuts through the Magnolia and Carimona, easy access, and lack of vehicle traffic, it's nigh-on perfect for getting in touch with St. Paul as it was about 454–453 million years ago. It's kind of like our own Carnegie Quarry wall, except it's tiny marine invertebrates rather than dinosaurs, it probably wasn't planned, and you can walk right out over it. It's always fun to get to spend time there for work, and like last year, I got the opportunity to assist with a training session for Mississippi National River & Recreation Area seasonals there. Then, of course, I just had to make a quick return trip later to follow up on some things we'd seen.

It had been raining much of the previous three days, so the water level in the excavated channel was higher than I'd ever seen it, precluding access from some areas. On the other hand, a constant mist was keeping the surfaces of the rocks and fossils wet, which can bring out things that aren't as visible when dry; it was an even trade, I'd say.

Fossils just look a little different wet, y'know?

The winter, such as it was, had almost no snow and little precipitation of any kind, but the temperature was still usually below freezing, and there was still groundwater to freeze and thaw things. The Carimona is still weathering faster than the Platteville. Fossils are flaking off in places, but on the other hand new fossils are being exposed and weathering is enhancing the exposure of others. Generally I've had a hard time relocating fossils between visits, but that's more a matter of the "Where's Waldo"-like nature of the bedding planes. Here's a couple that we saw before:

The annulated nautiloid; compare to 2022. By checking Kolata (2021) I think this is Monomuchites rather than Anaspyroceras, as the annulations are more closely spaced and a bit crisper.

The big knobbly burrow; compare to 2023. This one looks like it's suffering more, although some of that could be sediment deposits obscuring the features.

Horn corals

Something I noticed this time is that horn corals seem to show up only in the lower part of the Magnolia here.

Spot the three corals! (Bonus: and the small partially dissected orthocone.)

Brachiopods

There are of course too many brachiopods here to photograph them all, but here are a few interesting occurrences.

Here are a few nice brachs, including strophomenids, the internal cast of a Hesperorthis (they remind me of Reese's peanut butter cups due to the fluting), and a stroph on the right that has some pyritization going on. Then there's that oddly shaped dark fossil (probably a chunk of a trilobite).

Taphonomy! Here's an odd little patch of bedding plane exposure where the shells are not laying flat, but are stacked sideways.

It's hard to be sure with the incomplete preservation and exposure, but the ribbed brach on the left looks different from other ribbed brachs we've seen here; I suspect Doleroides. (Note also a couple Hesperorthis, the favorite non-stroph brachiopod of this site.)

Bivalves

Bivalves are starting to become easier to find, probably due to improving search image.

In this case, the bivalve has become invisible, but we can still tell it was there.

What? Another ex-bivalve? Don't you have any bivalves that are still there?

It's not a bulbous Vanuexmia-type, but it'll do. Note the optical illusion; this is actually concave, not convex.

Nautiloids

It was a good day for spotting nautiloids, even if most of them were partial molds and casts of small orthocones.

Like this happy little fellow (actually, this one probably has the best preservation).

There was some variety, though:

Only a little bit is exposed, but this is clearly more in the Cameroceras/Endoceras mold.

This orthocone falls somewhere between the previous two in size. It's pretty vanilla, probably something in the Cartersoceras/Proteoceras vein.

This one has a subtle bulge partway through. Several Platteville nautiloids do that, but the slender build and relatively thick chambers suggest we're dealing with Whitfieldoceras mumiaforme.

Nautiloid diversity here is fairly significant, around 8 or so taxa at minimum: at least one and probably two annulated forms (Anaspyroceras and Monomuchites), a spiral form (Plectoceras), a fat curved form (Beloitoceras), a bulging form (Whitfieldoceras), at least one fairly large unornamented orthocone, at least one great big orthocone, and at least one finger-sized orthocone.

Gastropods

One day I'll have to guesstimate the snail diversity here, although it's hard with the preservation we get here.

This may look like nautiloid chambers, but note the way each "chamber" bulges. This is actually a high-spired snail, probably something like a large Hormotoma.

No particular reason except I liked it. You've got a Lophospira-type screw snail on the left and a cross-section through a spiral galaxy on the right.

Trilobites

Because of their habit of generally being smallish and only leaving being small parts of the whole, trilobites can be hard to spot; I often only find them after the fact in photos of other things. There are exceptions, though.

Hey! Another chunk of an Isotelus!

Ostracodes

Here's a better Eoleperditia than before. (Again with the optical illusions—this is concave; note the other shadows.)

Echinoderms

The stars this time were the echinoderms. The most exciting find was made by one of the participants, who observed these little objects in the Carimona:

The group of small equant light-colored objects arranged in an array.

There's really only one thing this could be: part of the arm of a small sea star such as Hudsonaster (compare to the photos here). This is not something you see every day!

The Carimona is good for crinoids, too, with lots of columnals.

Lots of columnals; where's the rest of the crinoid?

The Platteville here is less echinoderm-rich, mostly with just the odd columnal here and there, but we did have the partial stem in 2023, and I saw this when I returned after the session this year:

This set of arms is just barely at the surface. I wish the calyx was there!

Trace fossils

Under the overcast, damp conditions, the burrows really popped. A lot of them are dark, on the order of a finger in thickness, and have a wrinkly texture; I feel even more strongly now that these were lined burrows and the lining collapsed. For convenience, I've been thinking of them as basically Palaeosynapta.

Here's a pretty representative example.

In general the Platteville burrows here are dark against the lighter yellow-gray rock, whereas the Carimona burrows are about the same color as the surrounding medium to dark gray rock, or a bit lighter.

When you know where you're going.

When you realize you forgot something.

Others

Of course, there were a few things I haven't identified.

It's the Carimona Brown Thing again!

Well, it's in the Platteville, and it appears to have some kind of internal space, and the preservation looks similar to Platteville echinoderms, and it looks kind of chambered but isn't when you zoom in... I dunno...

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.