Where were we? Yes, the majestic trilobite, the three-lobed former denizen of the deep, the oval with antennae, bearer of the first compound eye, shameless vandal of pristine Paleozoic sediment. Trilobites are one of the most famous types of extinct organism, perhaps not on par with certain vertebrates, but certainly the most renowned fossil invertebrates. (In second place, with plenty of daylight intervening, are ammonites. In last place, maybe edrioblastoids? Some kind of echinoderm, probably.) You can find online information on practically any aspect of trilobites you may want to know about, often lovingly illustrated with photos. You'll probably end up at Trilobite.info sooner rather than later. The American Museum of Natural History also has a good guide, with photos of several of the species mentioned below. Wikipedia and the Kansas Geological Survey have detailed entries, and more photo guides particularly relevant to our slice of time can be found at the Atlas of Ordovician Life, the Dry Dredgers, fossilid.info, and University of Georgia pages on the Cincinnatian and Nashville.
Trilobites composed a large and diverse group of extinct arthropods, the group which also includes insects, spiders, crustaceans, centipedes, and millipedes. It is unclear what their closest relatives were. At various times people have proposed that they were crustaceans or related to the chelicerates (arachnids, horseshoe crabs, and the ever-popular eurypterids, the extinct "sea scorpions"). Interestingly, there is another extinct group of arthropods, known as the aglaspidids, which are restored as something of a cross between horseshoe crabs and trilobites. People sometimes compare isopods such as woodlice (aka pillbugs) to trilobites. The major points of comparison are the flexible segmented body and the many limbs.
Trilobites showed up in the Early Cambrian and were the hot thing for most of the period before getting slugged by an extinction event. In fact, they were pounded down by several mass-extinction events, but always got back up...until, of course, they didn't, at the Permian–Triassic boundary at about 252 million years ago. They'd managed almost 300 million years, though, so there was nothing to be ashamed of. The name "trilobite" means "three lobes", and refers to the midline ridge, or axial lobe, and its flanking pleural lobes. Coincidentally, trilobites also have three divisions anterior to posterior, but these divisions are not the source of the name. The divisions are the cephalon, for the head region; the thorax, which is segmented; and the pygidium, for the tail region. Beneath the cephalon was a sort of mouthpart or mouthguard called the hypostome, which is also frequently found as a fossil. Trilobites also had a variety of more fragile pieces that didn't fossilize as well, including antennae and numerous limbs. You can see the presence of two rows of legs in trace fossils: trilobites made many of the double-trench trace fossils Cruziana (for moving animals; think cruising) and Rusophycus (for stopped animals; think R for resting, although they did more than rest when stopped—in some cases, you can see a worm burrow ending beneath a Rusophycus, which shows the end of the worm).
Trilobites are thought of as having generally been detritivores/scavengers or predators on small animals, walking at the seafloor surface. There are also examples of swimmers and floaters. As dioramas and illustrations always remind us, trilobites were in constant danger of being snapped up by passing nautiloids. Trilobites don't look very palatable, but then again practically everything in the early oceans was crunchy, miniscule, or both once hard parts caught on, so I suppose the nautiloids had to take their chances. To fend off the cephalopod menace, trilobites had their durable segmented exterior, and could roll themselves to protect their undersides. It is not uncommon for trilobite fossils to be found enrolled, perhaps because the trilobite was caught by a sudden submarine flow and went into defensive mode. Some trilobite species evolved spines. Even if not intended for defense, one does have to admit that having pointy bits does make it more inconvenient to consume the bearer. In addition, many trilobites possessed compound eyes. I have a pair of photos of a trilobite eye below. (Some trilobites, on the other hand, dispensed with eyes altogether, and probably lived too deep for light to penetrate.) But who am I to hassle you like this? You can look up this stuff in a million other places. Let's look at some practical considerations and photos.
|The eye of the trilobite. This is a tiny specimen; the eye (the crescent-shaped tower just right of center) is about 2 mm across. I collected it from the Brickyard back in 2006.|
|A little prep, and the face appears. You can see the individual lenses in the eye. Sceptaspis lincolnensis? [Edit, 2016/04/16: no, probably a Calyptaulax.]|
Trilobites grew by molting, so most trilobite fossils are not dead trilobites, but shed carapace pieces. In the Decorah, it is common to find pygidia of small trilobites, which look like segmented pie wedges in dorsal view or curly brackets in cross-section. If you haven't tried to identify your own trilobite fossils before and catch "trilobite fever", don't be surprised if you realize at some point that you've been identifying chunks of brachiopod shells as trilobites. On the other hand, you may wake up one day and realize you've got a slab crowded with pygidia. A key feature to look for on a suspect piece is the axial lobe. If one is present, it's a trilobite. Chunks of larger carapaces are also present. They often appears as relatively large (cm-scale) thin dark objects without the pores of bryozoans. If you've seen lobster or crab fossils, the surface texture and appearance may seem similar. Less common are cephalons. For small trilobites in the Twin Cities rocks, these may have a three-leafed clover shape. Once you become trained to see trilobite fossils, they start popping out. See for example the trilobite cephalon near the center of the photo below; click to enlarge as necessary. If you're looking for comparative images, you can try the links above for their photos, or Sloan (2005). Clark (1897) is another useful summary, but you'll need to translate the taxonomy.
The authorities agree that the most diverse and abundant orders in the local Ordovician are the Asaphida, Corynexochida, and Phacopida, but after that it gets a bit more difficult. Here is what Stauffer and Thiel (1941) reported for the Glenwood, Platteville, Carimona, and Decorah, with a few (but not exhaustive) alterations to the taxonomy, and bonus trilobite orders.
Amphilichas sp. (Pl) [Lichida]
Basilicus barrandi (Pl) [Asaphida]
Bathyurus extans (Pl, Ca) [Proetida]
Bathyurus schucherti (Pl) [Proetida]
Bathyurus sp. (Pl) [Proetida]
Bumastus? trentonensis (Pl, Ca) [Corynexochida] = Bumastoides milleri, B. porrectus
Calliops plattevillensis (Pl, Ca) [Phacopida]
Calliops schmidti (De) [Phacopida]
Calliops sp. (Gl, Pl, Ca, De) [Phacopida]
Calymene senaria (De) [Phacopida]
Calymene sp. (De) [Phacopida]
Ceraurinus scofieldi (Pl) [Phacopida]
Ceraurus pleurexanthemus (Pl, Ca, De) [Phacopida]
Corydocephalus wesenbergensis paulianus (Pl, De) [Lichida]
Encrinurus vannulus (Pl) [Phacopida] = Encrinuroides rarus
Eomonorachus intermedius (Pl, Ca, De) [Phacopida]
Goldius lunatus (Pl) [Corynexochida]
Illaenus americanus (Pl?) [Corynexochida]
Isotelus gigas (Pl, Ca, De) [Asaphida]
Isotelus maximus (De) [Asaphida]
Isotelus sp. (Pl) [Asaphida]
?Nileus vigilans (Pl, Ca, De) [Asaphida]
?Onchometopus simplex (Pl) [Asaphida]
Thaleops ovata (Pl) [Corynexochida]
Now, here's a more recent version for essentially the same formations, by Rice and Hedblom (1987). Aside from 46 years of taxonomy intervening, their results are based on a consultation of Frederick Sardeson's vast collection of fossils from the area. Sardeson's collections are based on his stratigraphic scheme, which I should really get around to detailing one of these days, but suffice it to say I've pulled out the chunk that describes the Platteville, Carimona, Decorah, and a zone where the Decorah and Cummingsville (Cu) overlap. The Glenwood is technically included, but they reported finding nothing of interest, and the Carimona interval had something like three trilobite specimens. Asterisks denote species that are particularly common in those formations:
Acanthoparypha evetti (De) [Phacopida]
Amphilichas sp. (Pl) [Lichida]
Basiliella barrandi (Pl) [Asaphida]
Bumastoides milleri (*Pl) [Corynexochida]
Bumastoides porrectus (*De) [Corynexochida]
Ceraurinella scofieldi (Pl) [Phacopida]
Ceraurinella templetoni (Pl) [Phacopida]
Ceraurus pleurexanthemus (De and/or Cu) [Phacopida]
?Encrinuroides vigilans (De and/or Cu) [Phacopida]
Eomonorachus intermedius (Ca, *De) [Phacopida]
Flexicalymene senaria (De and/or Cu) [Phacopida]
Gabriceraurus mifflinensis (Pl) [Phacopida]
Illaenus sp. (*Pl, De and/or Cu) [Corynexochida]
Isotelus gigas (Ca [in figure, not mentioned in text], *De) [Asaphida]
?Isotelus simplex (*Pl) [Asaphida]
Raymondites longispinus (Pl) [Proetida]
Sceptaspis lincolnensis (Pl, De, ?Cu) [Phacopida]
Thaleops ovata (Pl) [Corynexochida]
Thaleops sp. (De) [Corynexochida]
My experience is that even though guides to Minnesota fossils call trilobites rare, pieces of trilobites are actually fairly common. It's just you'll need to calibrate your expectations away from beautiful complete specimens to a ton of Eomonarchus intermedius pygidia, indeterminate fragments of larger trilobites, and the occasional cephalon. E. intermedius is represented frequently in the photos below, at least the parts you generally get to see. At the genus level, Bumastoides and Isotelus are the other genera you are most likely to see in either the Platteville or Decorah. Both are larger than Eomonorachus and have pygidia without segmenting. From above, Bumastoides resembles a kind of pill with the two round ends separated by an accordion, while Isotelus is more distinctly angular at both ends, and has prominent eyes (in the "face", it looks a bit like a frog to me). Also, it can reach lengths greater than a couple of feet. Eomonorachus also has a bit of a pointed frog-face.
|Spot the trilobites: there's a small cephalon near the upper center margin and a couple of large fragments.|
For an additional bonus, here are the Oneota Dolomite and Jordan Sandstone trilobite faunas listed by Stauffer and Thiel (1941). As a Cambrian formation, the Jordan is more trilo-centric than the younger Ordovician formations. It is also part of a biostratigraphic system involving several other Cambrian formations we have not yet met.
Bowmania n. sp.
Dikelocephalus cf. halli
Plethometopus? n. sp.
Prozacompsus n. sp.
Clarke, J. M. 1897. The Lower Silurian trilobites of Minnesota. Pages 694–759 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.
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.
Sloan, R. E. 2005. Minnesota fossils and fossiliferous rocks. Privately published, Winona, Minnesota. Available from the Minnesota Geological Survey.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.