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 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.

Prasopora, with a comment on biostratigraphy

Prasopora, the "gumdrop bryozoan", is one of the most recognizable Ordovician fossils in Minnesota. Museum collections from Minnesota have boxes of the little darlings rattling around together. And I—I hardly ever see the dang things in the field.

Yes, one of these things; a bit more "chocolate kiss" than "gumdrop" but well within morphological variation.

While my competence in many fields is questionable at best, in this case you can be assured I would recognize a Prasopora if I saw one, even if it was years before I learned the stress goes on the second syllable rather than the third. (I have an unerring instinct for putting the stress on the wrong syllable for scientific names I've read but never heard.) No, the real issue here is one of biostratigraphy. My usual stomping grounds cap in the lower third of the Decorah, and Prasopora doesn't really kick in until the middle–upper Decorah. This has been recognized since the days of "The Geology of Minnesota" (Ulrich 1895; Winchell and Ulrich 1897). At that time eight species were recognized (P. affinis, P. conoidea, P. contigua, P. insularis, P. lenticularis, P. oculata, P. selwyni, and P. simulatrix), all of which were restricted to a range extending from the "Fucoid and Phylloporina beds" of the "Black River Group" (roughly Sardeson bed 5, middle–upper Decorah) to the "Fusispira and Nematopora beds" of the "Trenton Group" (as high as Sardeson bed 8, in the Prosser Limestone) (Winchell and Ulrich 1897; approximate correlations after Sloan 1987). None of them are listed in equivalents to Sardeson beds 3 and 4, in the lower Decorah, and only P. conoidea, P. contigua, P. lenticularis, and P. simulatrix were reported from the closest "Fucoid and Phylloporina beds".

Same specimen as above, which has a convenient break showing a partial cross-section; it's not solid all the way through.

Forty years later Stauffer and Thiel (1941) were not quite as dainty in their stratigraphic divisions, simply having a Decorah Shale Member of the Galena Formation and a Spechts Ferry Member of the Platteville Formation (approximately the Carimona Member of the Decorah). The "Spechts Ferry" gets Prasopora grandis, which had been Monticulipora grandis back in 1897, when it had been reported from the Stictoporella bed (lower Sardeson bed 3). P. grandis also appears in S&T's Decorah Shale Member list along with the four "Fucoid and Phylloporina beds" species, Stauffer presumably having stratigraphically higher specimens of P. grandis than W&U. Whether or not grandis pertains to Prasopora has been a matter of some dispute, and it seems to have wandered back to the metaphorical arms of Monticulipora. More importantly, it doesn't look like classic gumdrop Prasopora, instead being "irregularly massive, often tending to become lobate or subramose" (Ulrich 1895). In other words, it's not the kind of thing the typical fossil enthusiast would associate with the genus.

Wee little discoidal Prasopora.

My personal experience with Prasopora is limited to a few pieces in the Valentine box that appear to represent P. conoidea and a small discoidal species, a couple of small discoidal specimens that blur the line between early-stage Prasopora and "less famous bryozoan encrusting the external surface of an inarticulate brachiopod in an aesthetically pleasing Prasopora-like way", and one great honking lopsided hoof of a colony I found a few years ago at a basement excavation. I don't generally attempt to assign species to bryozoan fossils, but P. simulatrix is the only species described by Ulrich (1895) to attain dimensions even vaguely like it, so I'll go with that.

A top view of an unfortunately resolutely three-dimensional object.

And the underside, showing the distinctive layering and a few bits of other things that became part of the structure.

Bonus news: for those of you who've had your fill of gumdrop bryozoans, the spring 2023 edition of the NPS Park Paleontology newsletter is now available.

References

Sloan, R. E. 1987. History of study of the Middle and Late Ordovician rocks of the Upper Mississippi Valley. Pages 3–6 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.

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

Ulrich, E. O. 1895. On Lower Silurian Bryozoa of Minnesota. Pages 96–332 in L. Lesquereux, C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(1). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

Winchell, N. H. and E. O. Ulrich. 1897. The lower Silurian deposits of the Upper Mississippi Province: a correlation of the strata with those in the Cincinnati, Tennessee, New York and Canadian provinces, and the stratigraphic and geographic distribution of the fossils. Pages lxxxiii–cxxix in L. Lesquereux, C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, 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.

Platteville fossils in Cottage Grove

Cottage Grove, Minnesota is not noted as a geological wonderland, although Cottage Grove Ravine Regional Park certainly has its charms if you are willing to put in a little shoe leather. There are also scattered outcrops of the St. Peter Sandstone. However, persistence may be rewarded. Back in November I came across a couple of small fossiliferous blocks in town.

The first thing to do was to figure out the stratigraphy. The rocks were not in situ, but they were not heavily worn, either; in other words, they hadn't fallen off a glacier. The rocks of the area included outcrops of the St. Peter, and initially I wondered if I'd stumbled on a stray carbonate lens in the upper part of the formation. A rather more likely identity, though, is as erosional lag from the Platteville. In fact, the rocks look not unlike the occasional poorly dolomitized intervals I've seen in the Mifflin, and the fossils are certainly appropriate.

A (relatively) large brachiopod shell, among numerous teeny-tiny crinoid columnals and other fragments.

They include a mix of the BBC (brachiopods, bryozoans, and crinoids). There is also one object that looks like a whorl of a small Phragmolites sticking out. A couple of notable features include the small size of the fossils, particularly the crinoids; and the density of small fossil fragments and elements.

A different relatively large brachiopod shell among numerous teeny-tiny crinoid columnals and other fragments; note some nice bryozoan fragments on the left.

A hash of tiny fragments.

See what looks like tire tread near the center? That's what reminds me of Phragmolites.

Before we sign off for the year, I'd like to note that for some reason, over the past few weeks the "Fossil Felids of the National Park Service" post has been attracting a few dozen views a week. (Considering I don't do any external promotion and have reduced my social media presence from "negligible" to "zero", this is a lot!) I have no idea where this is coming from (probably picked up by a group somewhere that doesn't register in stats) or why it should have attracted interest (well, felines, I suppose, or maybe Pleistocene megafauna or predators in general), but I'm not complaining! Someone who actually knows something about cats could probably make an interesting and well-attended series like "Your Friends The Titanosaurs".

Bryozoan Overload

Sometimes you look at a slab, and you notice one special thing about it. "That's a nice Isotelus hypostome." "Neat strophs." "Look at that Phycodes!" In this case, it's "Gee, that's a lot of bryozoans!"

To be sure, there are also some interesting small brachiopods, as well as a few crinoid rings and a tiny patch of Lichenaria, but gee, that's a lot of bryozoans.

(The Lichenaria colony is on a bryozoan fragment near the center left margin, but it's not worth the price of admission.)

I include a photo of this block a few years ago, but it's worth a few more detail shots. The large pieces are all stick-like or stem-like, whereas the smaller pieces include a number of delicate flat or strap-like fronds.

Branching straps plus a few different brachiopods.

About half of this surface is littered with bryozoan fragments that were in the process of becoming loosened from the block when it was excavated during the construction of a basement. Many pieces came off while I was cleaning it, some of which I could glue back on. (Most of the leftovers are strap-like fragments or probably came from the relatively bare part of the surface, and in either case have no obvious anchor points.) Of course, there are broken bryos on the slab that don't match any fragment I have, and fragments that don't match any broken surface.

Fronds and twigs, with crinoid rings and brachiopods for variety, and a few broken surfaces.

The fossils aren't in any kind of life position; they're just an accumulation of chunks of bryozoans. Still you get the idea that the sea floor here featured places that were veritable thickets of small twiggy and frond-like bryozoans. To all you time travelers: probably not recommended for bare feet.

It's bryozoans almost all the way through, as well.

Stictoporellina cribrosa, saltine bryozoan

I've never put much emphasis on identifying bryozoans beyond their basic forms. There are, though, a few which are quite distinctive and worth differentiating. One that most fossil enthusiasts in Minnesota will be familiar with is the "gumdrop" or "Hershey kiss" Prasopora. Another notable example is Stictoporellina cribrosa, which can be identified as the "saltine" bryozoan if you don't mind extending the food comparison. S. cribrosa is the Decorah bryozoan known for broad thin colonies with perforations ("fenestrules").

As pictured here. If you enlarge it, you'll be able to more clearly see the tiny pores that were the living spaces of the bryozoan animals.

The other side of the slab

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

Concerning an Isotelus and its acquaintances

Here we have a big ol' rock.

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.

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.

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.

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.)

Bryozoans

Bryozoans make up one of the "B"s of the Decorah BBC (brachiopods, bryozoans, and crinoids), and may be the most abundant; I have seen rocks that are more or less bryozoan hash. Bryozoans are known colloquially as "moss animals", which is fair enough in terms of some of them encrusting things in the manner of carpets of moss, and also being animals. They are often compared to corals, but are quite different. A coral animal is a sack; food goes in and waste goes out the same passage. Bryozoans have a one-way flow with two holes. Coral animals are much larger than bryozoan animals; for example, for the Ordovician fossils of the Twin Cities, coral apertures typically measure several mm across, while the apertures in bryozoan colonies are sub-mm in scale. Finally, bryozoans have never gone in for the algal symbiont trick like corals have done. Not having symbionts, bryozoans must rely on filter-feeding, which the individual bryozoan animals do via a structure called a lophophore, a sort of ring of tentacles around the mouth. This structure is also found in some soft-bodied marine critters with very poor fossil records and the other "B" of the BBC, the brachiopods, showing that they were related, making up a group called the Lophophorata. Various extinct groups known from "worm tubes", like our old friends the cornulitids, may also have been lophophorates.