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".

On to our cast of characters. As noted by Sloan and Webers (1987), the local snail taxonomy has not changed much since Ulrich and Scofield (1897). A few species have been sunk or transferred to other genera, but that's about it. In terms of illustrations, all you really need is Ulrich and Scofield (1897). Sloan and Webers (1897) will get you through the modern stratigraphy, because of course *that* has changed. (Show me someone who still calls the Decorah the Black River, and I'll show you a very stubborn 140-year-old geologist.)

The lists below are after the charts in Sloan and Webers (1987), which to a certain extent I had to eyeball. I included all of the species in zones 3, 4, 5, and 6. 3 is included on the grounds that the Carimona Member is now part of the Decorah, but it doesn't make a big difference. It appears that only one species is included that would have otherwise been omitted, Pterotheca attenuata, but it's hard to tell; the charts really should have had lines setting off the zones. Similarly, Zone 6 includes a transition from the Decorah to the Cummingsville outside of St. Paul, but the only species affected is Bucania sublata. Their charts cover a broader area than just Minnesota, so I can't say definitely that all of the species have been found here, but they're all stratigraphically and regionally appropriate.

For convenience, I kept Sloan and Webers' higher-level classifications, except for removing Platyceras depressum altogether as an obvious ringer and adding some text about more recent classifications. I also added some glosses for each entry for quick visual identification. At any rate, identifying the fossils to species is advanced stuff; narrowing things down to a genus can be hard enough! The Paleobiology Database has a page for the Decorah from Sloan and Webers (1987), but it includes not only the Decorah taxa, but also the Platteville taxa (this can be verified with a ruler or other straight edge).

The figures below are derived from my own photos and Ulrich and Scofield (1897); if you're interested in such things, I went through four digital copies before deciding that the one with the best figures for my purposes was the one at the Biodiversity Heritage Library. As with Sloan and Webers (1987), many of the fossils illustrated in Ulrich and Scofield (1897) are not from Minnesota or the Decorah, but from stratigraphic equivalents elsewhere. The 1897 illustrations are included as reference material, and also to represent those forms not represented in the photos. Of course, illustrations do not always match real life, especially when someone is going around adding radial markings to things.

One more complication: most snail fossils in the Decorah appear to be natural casts and molds, particularly internal casts, which are called steinkerns. They form when the shell fills with sediment, the sediment lithifies, and the shell is lost via various chemical processes, leaving the inner cast behind. The upshot is that what a snail has on the outside doesn't always carry over to the inside.

In the lower Decorah, I'd say the most abundant forms are the bellerophont (or bellerophont-like monoplacophoran) Sinuites, high-spired Hormotoma, bulbous coiled "lophospiroid" snails, and conical Clathrospira in about that order, and they have a tendency to show up in the same slabs. Snails are also more like brachiopods than trilobites or bryozoans in that complete specimens can be found loose.

From the same slab as the previous photo, a Hormotoma (H) and a section through a "bellerophont" (B, presumably Sinuites).

Bellerophonts

Bellerophonts are the most common Decorah snails in my experience (if in fact they are snails; the group as a whole has long been controversial, and some or all may be monoplacophorans instead). It may be helpful to consider "bellerophonts" as a general body plan instead of a group: stereotypically they are represented by shells coiled in a flat plane (planispiral) becoming wider toward the aperture. The shape and flaring of the aperture are not always visible, particularly if the example is embedded aperture-down in a slab. Sinuites (also known as Protoworthia in some references, and half the time as a monoplacophoran), which has a moderate amount of flare, is regarded as the common Decorah bellerophont. Indeed, if you see the characteristic arching bellerophont shape in these rocks, it almost always seems to fall under smooth-shelled modestly flared Sinuites, but I confess to harboring a certain reservation: if we're generally seeing steinkerns, i.e., the inner molds, how much of the outer ornamentation can we expect to see? The most unusual is Pterotheca, which I've seen in collections but not in the field, and which looks kind of like a brachiopod.

• Bucania emmonsi and sublata: non-flaring bellerophont
• Carinaropsis acuta: large broad aperture
• Cyrtolites dialatatus and retrorsus: diamond-shaped aperture (now considered coiled monoplacophorans, but retained here because of their bellerophont body plan)
• Phragmolites obliquus (Conradella of older references): flat-coiled and ornamented, resembling a tiny ammonite
• Pterotheca attenuata: looks strikingly like a flattened triangular brachiopod
• Salpingostoma buelli: large aperture that is relatively evenly sized all the way around
• Sinuites cancellatus and pervolutus: the most common Decorah bellerophont (or bellerophont-like monoplacophoran), a plain genus with moderate apertural flare and no ornamentation or keel, with the spiral coming out of the aperture as if it is swallowing itself
• Tetranota bidorsata, obsoleta, and sexicarinata: broad bellerophont, limited apertural flare

I clipped all of these from Ulrich and Scofield (1897) to represent the species in the above list. The sizes of each crop are eccentric, because I was trying to preserve something of the size relationships of the originals, to give you an idea of relative sizes. The species are: A) Cyrtolites dialatus, plate LXII; B) Cyrtolites retrorsus, plate LXII; C) Sinuites pervolutus, plate LXIII; D) Carinaropsis acuta, plate LXII; E) Salpingostoma buelli, plate LXVII; F) Tetranota sexicarinata, plate LXV; G) Tetranota obsoleta, plate LXV; H) Tetranota bidorsata, plate LXV; I) Bucania emmonsi, plate LXVI; and J) Phragmolites (Conradella of their usage) obliquus, plate LXVII.

Some examples in the wild. A) Three Sinuites in a slab, with the usual view of the curve of the shell; B) Pterotheca attenuata, UMPC GB17280, not the whole thing and in the Platteville instead of the Decorah, but you get the idea (the card mis-classifies it as a brachiopod, which is understandable; sorry, no scale here, but it's about 2.5 cm/1 in across); C) Side and D) apertural views of a large loose Sinuites; E) Two tiny Phragmolites.

Pleurotomariacea

Sloan and Webers (1987)'s pleurotomariaceans are fairly common in the Decorah rocks I've seen. Clathrospira with its conical shape and non-bulging whorls is usually recognizable. I'm calling the  bulbous coiled snails with keels or ridges running along the whorls "lophospiroids", after Lophospira, but there are several genera that share this general shape (Cyclonema, Eunema, Gyronema, Paupospira, Strophostylus), so I'm not committing to one genus. Along with smooth examples of this general shape (Strophostylus?), there seems to be at least two "lophospiroid" species in my own collection: one form with well-developed ridges giving it a screw-like appearance, and one form with much more modest ridges. Alternatively, it may just be steinkerns versus external casts (see K in the next figure below). I've also seen poor examples of the low-coiled varieties, Liospira and/or Raphistoma. More recent classifications find most of these genera and species to be fairly closely related, but the species listed for Lophospira are put in a different group with Gyronema.

• Clathrospira conica and subconica (including Eunema clivosa): the common Decorah pyramidal snail
• Eotomaria supracingulata (also known as Paraliospira supracingulata): ornamented pyramidal verging on low screw-like
• Liospira abrupta (also known as Paraliospira abrupta), americana, angustata, modesta, and vitruvia: low-coiled snail
• Lophospira bicincta, concinnula (also known as Eunema concinnula), oweni (also known as Paupospira oweni), peracuta (also known as Paupospira burginensis), pulchella, serratula, and spironema: ridged bulbous to screw-like snail
• Raphistoma peracutum (also known as Scalites peracutum) and sp.: low-coiled snail

Again, clipped from various plates in Ulrich and Scofield (1897). A) Liospira vitruvia, plate LXIX; B) Eotomaria supracingulata, plate LXIX; C) Raphistoma peracutum, plate LXVIII; D) Liospira angustata, plate LXVIII; E) Clathrospira subconica, plate LXIX; F) Clathrospira conica, plate LXX; G) Lophospira bicincta, plate LXXII; H) Lophospira oweni, plate LXIII; I) Lophospira pulchella, plate LXXIII; J) Lophospira peracuta, plate LXIII; K) Lophospira serratula, plate LXXII (the simpler line drawing on the left is of a steinkern, while the drawing on the right is of an external cast, although how can someone be sure they're the same species?).

A few fossils (click to see larger): A) A small chunk with a Clathrospira (C) and "lophospiroid" (L; there is a ridge around the largest whorl not apparent at this angle); this piece also has a partial bellerophont on the other side; B) A "lophospiroid" steinkern (with the mystery object above and to the right); C) A loose "lophospiroid" of a more screw-like configuration; D) The small curled specimen at the arrow appears to be a low-coiled snail along the lines of Liospira or Raphistoma; E) A loose Clathrospira.

Trochonematacea

• Trochonema umbilicatum and retrorsum: ornamented pyramidal to screw-like snail, fairly low coiling (not as snowman-like as "lophospiroids")

Trochonema umbilicatum, plate LXXVII (Ulrich and Scofield 1897); T. retrorsum (same plate) looks about the same, except a bit smaller and with more defined ridging.

Platyceratacea

I've removed Platyceras depressum, included in this group in Sloan and Webers (1987), because it is a monoplacophoran now classified under Helcionopsis. The other two genera look something like Lophospira wearing corduroy. Cyclonema is an exception to the steinkern rule: shells of this genus were calcitic instead of aragonitic and better resisted dissolution (Meyer and Davis 2009). They also are known for a tendency to show up attached to crinoid calyces. This has been interpreted as the snails feeding on the crinoids' feces (Meyer and Davis 2009).

• Cyclonema varicosum: ridged bulbous snail
• Gyronema pulchellum and semicarinatum: convergence of ridged, bulbous, conical, and screw-like; this genus is actually more closely related to Lophospira

Illustrations from Ulrich and Scofield (1897), all from plate LXXVIII. A) Cyclonema varicosum; B) Gyronema semicarinatum; C) Gyronema pulchellum.

Microdomatacea

I have some smooth bulbous snail shells I've tentatively assigned to Strophostylus, differing from the ornamented "lophospiroids" with evident ridges running around the whorls, but they could just be steinkerns of ornamented shells. More recent classifications put this genus more closely related to Cyclonema.

• Strophostylus textilis: smooth bulbous snail

A) A smooth bulbous snail I'm attributing to Strophostylus; B) Illustrations of Strophostylus textilis from Ulrich and Scofield (1897), plate LXXXII.

Murchisoniacea

Hormotoma is one of the most common and recognizable of the Decorah snails; it's the one that looks like a stack of beads.

• Hormotoma gracilis: high-spired snail

A) Hormotoma in matrix; B) Three loose Hormotoma. As you can see, they aren't the biggest snails around. I omitted the Ulrich and Scofield (1897) illustrations because they show about the same things.

Subulitacea

I can't say I've run across either of these genera in the field, although I have seen some from the Decorah in the University of Minnesota collections. Fusispira is often considered a subgenus of Subulites.

• Fusispira spicula and vittata: high-spired to scroll-like snail
• Subulites sp.: scroll snail

There are a lot of subulitaceans illustrated in Ulrich and Scofield (1897), but only a few seemed applicable. A) Fusispira schucherti (plate LXXX) was not one of the species mentioned by Sloan and Webers (1987), but is stratigraphically appropriate. B) Subulites sp., plate LXXXI.

References

McKee, E. D. 1938. The environment and history of the Toroweap and Kaibab formations of northern Arizona and southern Utah. Carnegie Institution of Washington Publication 492.

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.

Sloan, R. E., and G. F. Webers. 1987. Stratigraphic ranges of Middle and Late Ordovician Gastropoda and Monoplacophora of Minnesota. Pages 183–186 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.

Ulrich, E. O., and W. H. Scofield. 1897. The Lower Silurian Gastropoda of Minnesota. Pages 813–1081 in E. Ulrich, 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.