Rice and Hedbloom (1987) indicate the following would be appropriate for the lower Decorah, based on the Sardeson collection:
(Cheirurine cheirurids are probably present, because they are recorded above and below)
(Illaenus sp. is probably present, because it was recorded both above and below)
My own record?
Undetermined trilobites, perhaps two or three taxa
One or two cheirurines, perhaps Ceraurus sp. and Gabriceraurus sp.
Calyptaulax sp.? (potentially upper Decorah)
(also Calymenidae: Flexicalymene sp. near the Decorah/Cummingsville contact)
These determinations work out suspiciously well versus Rice and Hedblom (1987), except for the cheirurines, but I have no doubt that they are identified to the correct subfamily. Enough of this, though: on to the photos!
Anatomy and Provenance NotesAs discussed back in the day, trilobites grew by molting their exoskeletons, so most of what we see as fossils are discards. You're usually either dealing with the back end or the front end; the segments of the thorax usually broke up and became part of the background fossil fragments. The back end is easy to deal with: it's one unit, called the pygidium (plural pygidia). The front end is a bit more complex, because there are several pieces. If you get the whole head, you have a cephalon. Usually, parts of the cheeks (the free cheeks, or librigenae) are missing, because they would break off when the trilobite was molting. If the free cheeks are missing, you've got a cranidium. If the fixed cheeks are also gone, you've got the central part of the head, called the glabella. Check here if you need more cephalon anatomy.
In the interest of avoiding repetition, all but four of the specimens figured come from the lower third to lower half of the Decorah Shale of St. Paul. The exceptions are in Figure 2A and 2B (unknown level of the Decorah at the Brickyard), Figure 3B (Platteville of Minneapolis, in situ), and Figure 4A (near the Decorah/Cummingsville contact, southeastern Minnesota). (It should be noted that my sample should not be biased to any particular trilobites, because I usually only found them as secondary fossils in slabs after collecting).
Pterygometopidae: EomonorachinaeMost of the handful of cranial bits I've collected or seen pertain to pterygometopid trilobites (Figure 1), which were fairly "vanilla" as trilobites go. I attribute most of them to Eomonorachus intermedius, because A) it's known as the most prolific producer of pygidia in the Decorah, so the front end ought to be in there as well, and B) they're consistent with Eomonorachus glabella. They aren't as strongly pustular as some Eomonorachus, but pustules are present. The largest part of the glabella is a rounded and curved diamond-shaped object at the anterior end, with a pair of laterally projecting lobes behind; the lobes immediately behind the "diamond" are much larger than the next lobes.
From an unknown level at the Brickyard, I have a small piece with similar a fossil including eyes and cheeks (and perhaps more, but there's a bryozoan in the way; Figure 2A), plus an overturned glabella from what appears to be the same taxon (Figure 2B). The arrangement of the structures near the eye doesn't seem quite right for Eomonorachus, so I'm tentatively calling it Calyptaulax, which is still in the same subfamily as Eomonorachus (Eomonorachinae) and is relevant to the stratigraphy per DeMott (1987) and Midwest Paleo. Another eomonorachine is also present in the Decorah, Sceptaspis lincolnensis, and it looks very similar to the other two; one upshot of this is maybe technically the common pygidium of the Decorah ought to be identified only to subfamily, instead of to genus and species (at least if the identifier is not putting in the heavy lifting; there are subtle differences in the pygidia, as described in DeMott 1987).
|Figure 2. Two possible Calyptaulax from the Brickyard, with the whole face visible in A and an overturned glabella in B.|
Cheiruridae: CheirurinaeI've also collected or seen a few cheirurine heads (and one forked pygidium; Figure 8B): a partial cranidium (mostly glabella) that may be Gabriceraurus (Figure 3A); a cranidium seen in situ in the Platteville in Minneapolis that I am more comfortable attributing to Gabriceraurus (Figure 3B); a partial cephalon with a narrower glabella than 3A, possibly Ceraurus (Figure 3C); and an upturned glabella of a cheirurine (Figure 3D). The glabellae are more squarish or rectangular than the eomonorachine glabellae, with a more D-shaped or rectangular anterior portion and similarly-sized rounded lobes behind. They also reach larger sizes. Cheirurines could be very ornate, with long cheek and pygidial spines, and we have the pygidium photo to prove it.
Other headsThere are two other specimens I'm reasonably confident in identifying. One is a very nice Flexicalymene cranidium (family Calymenidae) found by my mother in the uppermost Decorah (or basalmost Cummingsville) of southeastern Minnesota (Figure 4A). The other is a specimen unfortunately represented only by a couple of mediocre to poor photos, because the specimen was on a piece we gave away at the Minnesota Earth Science Teachers Association (MESTA) meeting back in February (Figure 4b); this is also the case for Figure 5B. If you're one of the people who picked up Decorah slabs at MESTA, you can check for them. (This also included Eomonorachus in Figure 1D and the hypostome in this post.) Anyway, this rounded and furrowed glabella appears to pertain to the cheirurid Acanthoparypha.
|Figure 5. Two difficult specimens. A) An overturned partial cephalon, with one cheek spine showing and someone else's pygidium. B) A pustular glabella with equant lobes, looking vaguely cheirurid (and blurry).|
Illaenids: at the other endAs pointed out in Figures 1D and 3D, there are some odd thin rounded fragments that may represent parts of illaenid trilobites. Our leading Decorah candidates are Bumastoides, Illaenus, and Thaleops. There are a couple of pieces that look a bit like deflated footballs, and which seem reasonably like Bumastoides pygidia (Figure 6). There's also one very strange object that could be part of the front end or back end of Thaleops (more so the back end), but it is poorly preserved and could be other things instead (Figure 7). The illaenids were often rounded fore-and-aft, as these fragments suggest; Bumastoides didn't even have cheek spines.
|Figure 6. Possible Bumastoides pygidia. A) A partial piece, outlined for clarity. B) A whole piece, no outline necessary.|
|Figure 7. A mystery piece, somewhat resembling a Thaleops pygidium if I'm feeling generous.|
PygidiaWith the pygidia, Eomonorachus intermedius (or Eomonorachinae indet.) is the champ (Figures 8A and 8E), but I've found a few others, including a cheirurine forked pygidium (Figure 8B), part of an Isotelus pygidium sticking out of the side of a slab (well, it might be the front end, because the tip of the cephalon could be very similar to the tip of the pygidium) (Figure 8C), and a small, narrow pygidium that doesn't seem to match the others (Figure 8D).
Other piecesChunks of Decorah frequently have various pieces of trilobites, including spines, cheeks, hypostomes, and pieces of the thorax (Figure 9). Very dark fragments are frequently trilobite pieces.
|Figure 9. Trilobite bits. A) A group of spines and pygidia. B) The dark object from the top of Figure 1C, most likely a chunk of an Isotelus judging from the size (one of the glabella from Figure 1C is also visible, along the left).|
DeMott, L. L. (posthumous, edited by R. E. Sloan, F. C. Shaw, and R. P. Tripp). 1987. Platteville and Decorah trilobites from Illinois and Wisconsin. Pages 63–98 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.
Rice, W. F., and E. P. Hedblom. 1987. Brachiopods and trilobites of the Sardeson Beds in the Twin Cities. Pages 131–135 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.