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.

Zittelloceras

While on a walk earlier this year, I spotted a Decorah block that I decided to pick up for photography. The initial attraction was the abundance of snails, which are a reliable indicator that pieces of our fossil arthropod friends are also present (if there's only one practical thing you take away from this blog, it's "when you're in the Decorah and see snails, look for trilobites"). This was indeed the case:

Here's the whole block, which rewards a click to embiggen. There is a nice Clathrospira and a lophospire just right of the scale bar, and many smaller snails scattered throughout. You may also pick out the trilobite pygidia.

Here's a pygidium, pointed toward the top of the photo.

A nice pygidium plus a number of other things, including some crinoid columnals, bryozoan fragments, other trilobite bits, and, near the top, a whorl of a snail.

There was also something else: a dark object several millimeters long and broad. It appeared to be a thin-walled flattened tubular object, with a distinct series of ornamented transverse ridges. The ridges showed an alternating pattern of strongly projecting and more subtle, like perforations. Both had little scooped frilling, the same kind of shape as a doodle of stereotypical ocean waves.

The object in question is near center. You may have noticed it in the first photo. The light-colored band near the center is some light prep to see if I could get the matrix out from the groove.

I'd never seen this combination of features before, but I could knock out a lot of things quickly. In fact, I knocked out just about everything, which was a problem. Given the probability I had discovered a completely new phylum is pretty low, all things considered, I figured I'd probably missed something. So, I pulled out my copy of "A Sea Without Fish" (Meyer and Davis 2009) to see if some similar exotica had been found in the well-studied Cincinnatian, as it's only a few million years younger. Then I got excited looking at the figure and description of the machaeridian worm Lepidocoleus. Machaeridia is an extinct group of Paleozoic armored annelid worms, with segments of calcitic plates and a heart-shaped cross-section.

This view, under different lighting, shows the ridges and frills to good effect.

Before I got too excited, I decided to put it up on the Fossil Forum, to see what others might think. The first suggestion was Phragmolites, which was reasonable enough but didn't fit my experience with that snail. The chunk wasn't curved enough, the dark coloration and thin wall were unlike the examples of Phragmolites I'd seen, and the ornamentation of the ridges wasn't a good fit. Then someone came up with the nautiloid Zittelloceras, and provided photos of a form with almost the exact same pattern of frilled ridges found in the Platteville.

An end-on view shows the cross-section, with the thin walls and central crushing.

So, it looks like rather than a worm, it's a nautiloid. Zittelloceras is one of the "arched" nautiloids, not coiled and not a full-on orthocone. Several species are present in the Platteville per Catalani (1987), but none are listed in the Decorah. This is not a particular problem, as the genus is present in younger strata as well, and the Decorah's cephalopod record lags the Platteville. (Note that Zittelloceras is frequently misspelled "Zitteloceras", with one "l", but a look at the original publication, Hyatt 1884, shows the two-l spelling is correct.) I'm sure there are worms out there to be found in the Decorah, but I'll settle for this record of an ornate nautiloid.

Here's one more angle for the road.

References

Catalani, J. A. 1987. Biostratigraphy of the Middle and Late Ordovician cephalopods of the Upper Mississippi Valley area. Pages 187–189 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.

Hyatt, A. 1884. Genera of fossil cephalopods. Proceedings of the Boston Society of Natural History 22:253–338. [some history: The paper is based on a talk presented by Hyatt April 4, 1883, a day before his birthday. There is a note on the first page that there was going to be a monograph in the Memoirs of the Museum of Comparative Zoology, but this did not happen. At any rate it's hard to think of an 80-page paper being "preliminary" to anything!]

Meyer, D. L., and R. A. Davis. 2009. A sea without fish: life in the Ordovician sea of the Cincinnati region. Indiana University Press, Bloomington and Indianapolis, Indiana.

Uŋčí Makhá Park Revisited, Part 2: Further Fossils

We're now up to the fourth entry in a completely unexpected series on the Platteville–Decorah rocks and fossils of Uŋčí Makhá Park. We've already seen the common fossils from the site, so for this go-round I'm focusing on rarities.

Replacement of Fossils

You might think that getting a shell or bone or wood chunk safely buried is the tough part for fossilization, that once something's entombed in sediment it's all smooth sailing. Burial is certainly important, but it's not the end of the story. A lot of things can happen between deposition and exposure. Pore spaces are filled with new minerals. Existing minerals are replaced. Entire structures can be replaced, then lost. These changes all fall under diagenesis. What exactly happens depends on things like the physical and chemical structure of the object in question, temperature and pressure of burial, and the chemical composition of the fluids in the sediment. Denser fossils like teeth are less vulnerable to changes than more porous materials. The form of calcium carbonate known as aragonite is less stable than calcite. Many different minerals and mineraloids can get involved in the fun; for example, there are opalized fossils and pyritized fossils.

Bivalve mold and internal cast (steinkern). Not pictured: bivalve shell.

Because silica and carbonate minerals are so abundant at typical surface and near-surface temperatures and pressures, they are the minerals most frequently involved. In Minnesota, we generally get dolomitization. This is somewhat inconvenient, because dolomitization has a tendency to destroy fossils, and even when it doesn't, it usually leaves behind molds and casts that aren't as crisp as the original. It's a bit like replacing the Venus de Milo or Michelangelo's David with nothing but 2x4 Lego bricks; you'll notice a difference. Dolomitic replacement may give a fossil a quirky sparkly appearance thanks to the dolomite rhombs, but that's about the only plus. Pervasive dolomitization is why many fossils in the Platteville are gray with a sugary appearance: you're actually looking at a natural mold or cast of the original in dolomite.

Sometimes diagenesis gives you exotic, spectacular fossils, and sometimes it gives you dolomite.

Although once in a while you get something to write home about; this is a nautiloid in the Science Museum of Minnesota collections with its internal structures replaced.

I was inspired to write a note about this topic by a different kind of replacement. Someone reviewing one of my work projects commented on a type of replacement seen in some of the fossils, consisting of circular mineralizations. They informed me this was a kind of silicification known as beekite. This immediately twigged my memory banks, because I'd also seen it in photos of fossils from other work projects. Like dolomitization, it's not exactly faithful reproduction, although it can be aesthetically pleasing.

A beekitized (beekitified?) lower Permian brachiopod, central Kansas.

Quick Guide to Fossils at Uŋčí Makhá Park

So I went back to Uŋčí Makhá Park last weekend and spent a couple of hours taking photos of fossils, because it makes such an ideal place to see the upper Platteville fauna. After all, a winter of freezes and thaws may not leave these new exposures looking as nice as they do now. Here's a quick guide to what can be seen there. (Let's see how many photos I can squeeze into one post, and how many species I can misidentify!)

Determining where you are stratigraphically

First of all, I'd just like to reiterate the stratigraphy. Most of the vertical extent is in the Magnolia Member of the Platteville Formation, with the upper part composed of the Carimona Member of the Decorah Shale. I'm thinking more or less the entire extent of the Carimona is exposed, based on thickness; at any rate the next thing up would be the shaly part of the Decorah, and there isn't a trace of it to be seen. I'm suspicious because the difference is just so darn clear, but at this site there is an unmistakable color change between the two units: the Carimona is the upper blue-gray interval and the Magnolia is the light tan-gray interval below. The Deicke K-bentonite is the lower and thicker of the two bentonite gaps in the Carimona. (Note that the Carimona is sometimes supplemented or replaced by landscaping, but this is pretty obvious.) As you walk from south to north, the "floor" goes up stratigraphically, so it's not all one bedding plane but a gently rising series of planes, until by the exit you're close to the color change.

The color change is quite evident here. The Deicke K-bentonite is the cut-in about halfway up the blue-gray Carimona (above the scale bar in the center of the photo).

Here we've gone north, and the floor has risen. The Deicke is still the seam in the middle of the blue-gray rocks.

A fossiliferous Prairie du Chien block

The Prairie du Chien Group is great for stromatolites, but not much for other fossils except for localized occurrences of molds and casts of mollusks; see for example the second photo here or the second photo here. You do get lucky occasionally, though. I did not collect the following block myself, nor was it found in situ, but I would be very, very surprised if it did not originate in the Prairie du Chien Group. The most notable aspect about it is this spiral feature:

What is it? A few thoughts:

If you had to pick a second group to find in the Prairie du Chien after stromatolites, it would be snails, and the Prairie du Chien snails do include some large forms that are flat-coiled or coiled in a low spiral, such as Helicotoma and Rhachopea. They're even of comparable size to this.

Some trace fossils spiral, but we can see that the width of the spiral passage increases as it goes out, and animals do not generally increase in girth like that over a short distance unless they handle matter like the Incredible Hulk, so that's not what we have.

Finally, although we usually think of early Paleozoic nautiloids as straight-shelled, there were a few coiled forms, including one known from the Prairie du Chien: Eurystomites. We can see that this spiral seems to separate from the inner coil near the end, and conveniently enough, Eurystomites also gets lax at the end of its coil. However, there's no clear indication of chambers.

What do I think? Without being able to see the entire fossil, you could make an argument that this is an internal cast (steinkern) or external cast of a snail (in which the outer whorl shows separation either because we're looking at the inside or because the plane of exposure isn't low enough to show that the whorl is actually just flaring), or a poor-quality external cast of a Eurystomites or similar nautiloid (can't be internal, because there's no evidence of chambers).

Operating on the principle that where you find one body fossil in the Prairie du Chien, you might find more, I examined the rest of the block, which turned out to have several more easily interpreted molds of much smaller and more obvious snails:

Low-coiled snail, coiling into the block.

A more elongate form; the shell may be slightly sculptured, but relatively coarse mineralization in the mold makes it hard to tell. A similar but less easily photographed example looked much smoother.

A pair of shell molds exposed at an awkward angle.

New Pages: Geologic Time Scale and Classification Diagrams

So far the year has been quiet (well, up until Friday), so I thought I'd address a couple of nagging things, which I've done by adding two pages. The first page is a geologic time scale figure, from one I worked up for National Park Service reports. It's the kind of thing I thought would be handy to have on hand here for reference, instead of having a link to an external site. It'll be updated from time to time as dates are refined. The Quaternary is rather cramped, although I don't feel particularly apologetic for shortchanging Homo sapiens.

The second page is a sort of "map" to the classifications used in the various sheets over at The Compact Thescelosaurus, made with classic ASCII cladograms. (I contemplated drafting them in other ways, but none of them were as amenable to updating.) I thought this would be useful for visualizing the mess of classification columns. The process also forced me to look at the positions of a few clades, as you may have noticed from the updates sheet.

It's not connected to either topic, but I've also added a paragraph of new information to the post on the "Kweichow sauropod" after coming across a mention of it in Averianov and Sues (2017).

Finally, so as not to leave the post without an image, here's one attached to a quick story:

Back in spring 2001 I was on a field trip to the Badlands/Black Hills area of southwestern South Dakota. We were stopped along a road near Deadwood for lunch. One of the professors said something to the effect that "there are fossils in this formation." I looked down at the chunks of rock at my feet and said "You mean like this?"

Yeah, like this.

From my notes the source is the Whitewood Formation (or Dolomite, or Limestone), fittingly enough an (surprise, surprise) Upper Ordovician unit. More on its cephalopods can be found in Miller and Furnish (1937).

References

Averianov, A., and H.-D. Sues. 2017. Review of Cretaceous sauropod dinosaurs from central Asia. Cretaceous Research 69:184–197. doi:10.1016/j.cretres.2016.09.006.

Miller, A. K., and W. M. Furnish. 1937. Ordovician cephalopods from the Black Hills, South Dakota. Journal of Paleontology 11(7):535–551.

Fun with nautiloids: an essay in futility?

Or: "What good is a seven-foot nautiloid?"

In most reconstructions of marine Paleozoic life, large orthoconic (straight-shelled nautiloids) get two jobs: they are either large menacing objects in the water, usually seizing some unfortunate trilobite; or they are parked on the seafloor, again often engaged with a trilobite. The general takeaway is that a giant nautiloid was a voracious predator. Trilobites presumably are selected as the prey because so few other animals that lived during the heyday of the giant nautiloids both moved under their own power and were bigger than a few inches, making them about the only group worthy of the honor of artistic predation (although we can guess that a not-insignificant part of a giant predatory nautiloid's diet would be smaller nautiloids). Another, less scientific reason for having a nautiloid capture a trilobite is that otherwise Paleozoic marine reconstructions would be a whole lot of small filter-feeding animals not going anywhere, which doesn't make for dramatic art.

Nautiloids and trilobites: natural enemies forever locked in combat. Also pictured: many snails.

Identifying invertebrate fossils

Pop quiz! (don't worry, it's not for credit)

Romance *and* brachiopods

Here we have an assortment of fossils, tastefully arranged in a holiday-appropriate setting. They're all the typical local Ordovician stuff, but many Paleozoic shallow marine formations will have a lot of the same general things. What are they, and how can you tell?

Mea culpa and Moabosaurus

I apologize for having been light on the whole "Minnesota" and "invertebrates" part of the blog for this year. Having been doing this for a few years now, the low-hanging fruit is picked, and of course the winter is not the best time to be out and about in the rocks, even if "winter" came with quotation marks instead of snow this year. I'm currently on a short trip to Reston, Virginia, to do some work at the USGS, but I thought I'd at least try to put in something relevant for those topics. Then, of course, there’s a sauropod.

Sunday nautiloids

An interesting fact concerning nautiloids and the Platteville Formation is that the biggest nautiloid shells are often found only partially preserved, because their diameters (up to around 10 in/25 cm) exceeded the depth of the Platteville ooze they sunk into (Webers 1972a, 1972b). This means that when you see a fossil of one of these big Platteville nautiloids where the cross-section is partly round and partly irregular, you can guess that the round part represents what was buried; the rest of it was eroded off. So, while a small nautiloid can be completely buried:

...the big guys aren't so lucky. Take these examples from Sogn, Minnesota:

The more impressive of the two, featuring the part that looks like a nautiloid.

The cross-section of this specimen. Round was down (in the mud), originally.

The cross-section of the other, less impressive nautiloid.

Half a nautiloid is better than none, and for these two nautiloids, it permits a bonus: other shells and organic fragments collected in the eroded nautiloids. Notice the off-white patch near the center of the first cross-section above? Here's a close-up:

This is a small lingulid brachiopod (modern cousin Lingula; see a larger relative from Oklahoma rocks of reasonably similar age to the Platteville here). There are several fossils larger than millimeter-sized chips visible on these nautiloids, and they all appear to be the same kind of lingulids. This is an interesting association, but I couldn't tell you what it means. Here are two of the better-preserved examples:

References:

Webers, G. F. 1972a. Paleoecology of the Cambrian and Ordovician strata of Minnesota. Pages 474–484 in P. K. Sims and G. B. Morey, editors. Geology of Minnesota: a centennial volume. Minnesota Geological Survey, St. Paul, Minnesota.

Webers, G. F. 1972b. Paleoecology of the Ordovician strata of southeastern Minnesota. Pages 25–41 in G. F. Webers and G. S. Austin, editors. Field trip guidebook for Paleozoic and Mesozoic rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook 4.

And the state fossil of Minnesota is...

...nothing. Despite the map on "List of U.S. state fossils" [note, 2018/2/22: this has since been fixed], Minnesota does not have an official state fossil. There was a push to draft the giant beaver into this position, but the rodent fell short. The immediate political reason for its failure can be grasped from its scientific name: Castoroides ohioensis. Should the state fossil of Minnesota refer to another state? Certainly not!

Mostly bryos and burrows

It's still a little early to be out and about in the metro rocks and expect good luck (for one thing, the Decorah is still in winter-spring-transition mud mode) or consistent weather, but we're getting close, and sooner than usual. Here's one seasonal photo and then a few interesting pieces I hadn't featured before. These are all photos of Decorah Shale fossils from the St. Paul side of the river.

Lower Decorah a couple of weeks ago, after having been relatively warm and dry for several days. These two fragments were covered with half-burrows (imagine a tubular burrow split along its long axis), and several other pieces from the same area and probably a similar stratigraphic level had the same kind of abundant half-burrows.

Gonioceras: when a nautiloid is also a shovel-flounder

In a previous post, we were briefly introduced to the local nautiloids, which for the most part inhabited straight, gently tapered shells. Plectoceras coils, but otherwise the local squidlings are reasonably predictable. There is one other notable exception, though: Gonioceras is one of the oddest-looking nautiloids you can run across. Picture the blade of a shovel: it's pointed, it's flat, it's got a ridge at the broad end that turns into a socket for the handle. Now, let's modify it a bit. First, trim the blade into a triangle. Now, chop off the fitting for the handle where it goes beyond the blade. Next, take the rest of the fitting section and extend it down to the tip, and make it hollow, so you've got a tube running the length of the blade. Taper it, and keep the underside of the blade flat. Finally, put an appropriately sized nautilus body in the tube. This is essentially what a Gonioceras looked like. One other thing: the septa (the divisions between chambers) are arranged in sine-wave-like curves, concave behind the aperture and convex on the flanges. The illustration below includes the central portion and one of the flanges of a shell, with the aperture toward the top, and should make things more clear:

Gonioceras occidentale from Illinois, plate LVII of Clarke (1897). This specimen shows part of the wedge-like flaring of the shell and the curved septa.

Of course, in the field, you're liable to just find a chunk with those distinctive wavy septa:

Gonioceras in the wild, skooshed flat and otherwise worse for wear, from the lower to middle Mifflin Member of the Platteville Formation.

Although Gonioceras certainly cannot be accused of simply doing what the other nautiloids were doing, its innovative structure does not appear to have caught on. In Minnesota, Gonioceras is primarily a Platteville Formation concern (Stauffer and Thiel 1941; Catalini 1987). It is generally thought of as a bottom-feeder, sometimes interpreted as a crawler but perhaps analogous to a flounder (Vickers Rich et al. 1996). Like other nautiloids, its propulsion would have moved it primarily backward, so the pointed end was the leading end of the animal when it had to get going.

References:

Catalani, J. A. 1987. Biostratigraphy of the Middle and Late Ordovician cephalopods of the Upper Mississippi Valley area. Pages 187–189 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.

Clarke, J. M. 1897. The Lower Silurian Cephalopoda of Minnesota. Pages 760–812 in Ulrich, E., W. Scofield, J. Clarke, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Vickers Rich, P., T. H. Rich, M. A. Fenton, and C. L. Fenton. 1996. The fossil book: a record of prehistoric life (corrected republication of 2nd edition). Dover Publications, Inc., Mineola, New York.

Thanksgiving Leftovers

You're probably pretty busy this weekend. How about something light, like some photos? These all come from a few site visits over October and November, taking advantage of the very pleasant autumn weather conditions in the Twin Cities metro.

This and the next photo come from the U.S. Route 10 roadcuts, in the Shakopee Formation (Prairie du Chien Group). All of the little stone rainbows are small domed stromatolites. At very close range, you can distinguish between layers that are "crystalline", so to speak, representing minerals deposited by the microbes, and layers of sand (a grain or two thick). There is a band populated by these small stromatolites about as thick as the area photographed here that extends for at least a few tens of meters. (I do not recommend casual visits along this busy road; the couple of times I've stopped have been Sunday mornings.)

Nautiloids: cephalopod overlords of the Ordovician

The cephalopods include a wide variety of tentacled friends. Today, they are represented by a few species of Nautilus and Allonautilus, and an array of squids, octopuses, and cuttlefish grouped under the subclass Coleoidea. Aside from the nautiloids, with their chambered shells, the modern cephalopods are not great for making fossils. The external shell fell out of cephalopod fashion coincidentally not long after the Cretaceous–Paleocene extinction event eliminated almost all of the practitioners, and although there is a history of internal shells in coleoids, those have been reduced or lost altogether by the smart modern coleoid. They still have more resistant beaks, feeding organs (the radula), and tentacle hooks, with some preservation potential, but otherwise you're looking at a lot of soft tissue. What this means is that most of the groups of cephalopods we see today have wimpy fossil records. On the other hand, we know of a great diversity of extinct shelled cephalopods, from three major lineages. The most famous are the ammonites, best known for coiled forms. They appeared by the middle Paleozoic and left the scene at the end of the Cretaceous. Going extinct at the same time, but apparently only extending back to the Triassic, are the belemnites, a subgroup of the coleoids represented by their bullet-shaped internal shells. Finally, there are abundant extinct forms, primarily from the Paleozoic, lumped together as "nautiloids". These early nautiloids had the run of things from the Ordovician to the Devonian, following which they declined until the lineage that includes modern nautilids was the last branch remaining unpruned at the end of the Mesozoic.