Rockford, Part 2: Fossils (exclusive of brachiopods)

Before we get into the festivities, I've recently written an article for the online magazine Agate, about identifying common Paleozoic fossils of Minnesota. It's a compact summary that covers the most abundant groups, so if you're looking for something like that, go have a look!

In our previous post we had a look at the geology of the Fossil & Prairie Park Preserve of Floyd County, Iowa, also known as the Rockford site. For this post I'm going to briefly detail the fossils I collected, with the exception of the brachiopods, which will get a post of their own. For most of the non-brachiopods, I didn't get too far into the weeds on taxonomy, because many of the groups don't lend themselves to simple eye-checks for genera and species. Horn corals and bryozoans, for example, usually require thin sections, and crinoid columnals are generally only diagnostic of the presence of crinoids. I did, though, have recourse to Fenton and Fenton (1924) and other peoples' identifications to get some ideas.

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.

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.

Phragmolites

I could have titled this "That One Snail", because that's what Phragmolites seems to have become in my circle of acquaintances; for whatever reason, we remember having seen this distinctive fossil but can't remember what it's called. This is where a common name, something like "sculpted ring snail", might come in handy. If you haven't seen them, they could easily be mistaken for a tiny coiled cephalopod, or some kind of worm tube or crinoid columnal if the specimen is not exposed clearly. They range in size from less than a centimeter to about an inch across and are flat-coiled (planispiral), with the outer coils wider than the inner coils and partially covering them, and a distinct but small keel. There are strong markings on the coils that resemble the sutures of a nautiloid or ammonite.

These little fellas.

The other side of the slab

Here we have the opposite side of a big ol' rock.

Decorah gastropods (and some things that look like gastropods)

The snails* of the Decorah Shale are a lesser component of the fauna than bryozoans, brachiopods, or crinoids. My personal experience is that snails are uncommon except for certain beds, which feature abundant and diverse snails. One of these beds is perhaps 20 ft (6 m) above the top of the Carimona in St. Paul; this bed produced the plate in the "Equatorial Minnesota" box near the top of the page. For whatever reason, this bed also hosts abundant trilobite pieces, particularly of Eomonorachus. There's probably a facies thing going on, such that the original depositional environment was favorable to snails and trilobites; it's not quite as stark as, say, McKee (1938)'s mollusk and open marine facies of the Kaibab Formation (the very durable rock at the top of the Grand Canyon stack), but there's certainly some kind of difference. It might be a carbonate thing; both the underlying Platteville and overlying Cummingsville/Prosser, which have more limestone and dolomite, also have more diverse mollusks than the muddy Decorah (Sloan and Webers 1987). On the other hand, the relatively limited diversity and abundance of snails, combined with most genera having visually distinctive appearances, make it possible to summarize them in a reasonably brief guide.

*and things that look a whole lot like snails, and things that people argue about, such as Sinuites

Several snails in a small section of a slab: C for Clathrospira, S for Sinuites, and L for "lophospiroid".

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?

Sunday morning Decorah fossils

I've recently made a small collection of Decorah Shale pieces from spoil piles at a construction site. Most of them will go to other people and groups for use in education, but while I've got them I'm certainly going to take the opportunity to photograph them. Incidentally, construction can be a good source of fossils in the Twin Cities, if you don't mind disruption of the original stratigraphic context (which tends to happen anyway with the Decorah around here). Of course, as always, you'll want to ask for permission, and it's advisable to make collections when someone is working there, so you aren't mistaken for a trespasser or other nefarious sort.

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.)

Snails (and not-snails?)

I am posting this on May 24, 2015, which also happens to be Bob Dylan's 74th birthday. I considered titling this "A Hard Shell's A-Gonna Crawl", but even I have standards. "Shell" doesn't sound a bit like "rain".