The Upper Ordovician of the Twin Cities metro includes, in ascending order, the St. Peter Sandstone, the Glenwood Formation, the Platteville Formation, the Decorah Shale, and a tiny remnant of the Cummingsville Formation (and yes, they each deserve an explanatory post). There are a few other formations above the Cummingsville Formation in southeastern Minnesota, but that, as they say, is for another time. In terms of fossils, we're mostly looking at the Platteville and Decorah. Calling the St. Peter Sandstone "sparsely fossiliferous" is an understatement, the Cummingsville Formation of the metro is more or less limited to a few lucky blocks near where St. Paul, West St. Paul, Mendota Heights, and Lilydale meet, and although the Glenwood Formation has a decent microfossil assemblage (if you need a hand lens or microscope to see what it is, it's a microfossil), it's been stingy with most other fossils. What we do have from these three formations is not radically different from the Platteville and Decorah, at least not in terms of phyla and classes and so forth.
The Platteville and Decorah are both shallow marine formations, albeit with somewhat different depositional conditions. They include most of the same groups, although the proportions are different. The Platteville was great for brachiopods, snails, and bivalves, while Decorah rocks are full of bryozoans, brachiopods, and crinoids (the "BBC"). The Platteville has more diverse echinoderms (sea stars, sea cucumbers, sea lilies, sea urchins... give it five-fold symmetry and slap "sea" in front of the name, and you're good to go), although this could just be due in part to a lucky site, the old Johnson Street Quarry. It also has graptolites (see below), which don't appear to have been reported from the Decorah in Minnesota. The Decorah, for my money, has more diverse fossils overall. Something shared by both formations is that most of the different types of animals were adapted to sit somewhere and filter stuff out of the water, and a substantial portion of the rest crawled or burrowed along the ground. Furthermore, aside from a few nautiloids and trilobites, these weren't particularly large animals, with lengths on the order of inches to centimeters to millimeters.
Below are the different broad groups known from the Platteville Formation, the Decorah Shale, or both, with descriptions of their characteristics and habits. Keen-eyed observers will note that the assemblage is broadly similar to the well-known fossils of Cincinnati; the Minnesota fossils are slightly older. Keen-eyed observers will also note a certain monotony to the ecology of the groups: most of them were sessile (attached to the sea floor, or to something else attached to the sea floor), filter feeders, or both. My major sources are Stauffer and Thiel (1941), Sloan (2005), and Mossler (2008). Stauffer and Thiel (1941) is a bit difficult to use because the names for some of the formations are outdated, and they didn't break out all of the groups of animals we would break out today. (eventually, I need to put up some kind of glossary, and, of course, each group deserves an explanatory post; this will hopefully serve to introduce the actors)
- Chitinozoans: flask-shaped organic microfossils that probably represent some kind of microbial resting cyst or invertebrate egg.
- Chlorophytes (green algae): I'm cheating a bit and lumping the problematic extinct receptaculitids ("sunflower corals", for their distinctive structure) with well-regarded algae. They would have been sessile and photosynthetic.
- Sponges: still around today, of course; they were sessile filter feeders.
- Corals: represented by two extinct groups, the tabulate corals and rugose corals. Tabulates were sessile colonial filter-feeders, and the local forms seem to have been fond of encrusting other things. Rugose corals included colonial forms and solitary forms; solitary rugose corals are known as horn corals, for their shape. Both groups were sessile and technically predatory, but they were feeding on tiny organisms and so can be grouped with suspension feeders.
- Conulatans/conulariids: extinct animals interpreted as jellyfish relatives. They are known from skeletons of small rods that often look like triangles with a herringbone pattern. In life, the body structure was a sort of cross between an ice cream cone and a pyramid: imagine a tall, skinny pyramid with a jellyfish instead of ice cream. They were sessile and are assumed to have been predatory on plankton, like corals.
- Bryozoans: also known informally as "moss animals", bryozoans are still alive today. The Ordovician forms were colonial, building mineralized skeletons to house the tiny individual animals. Bryozoans lived up to their common name by encrusting anything that wasn't moving too fast. They were sessile filter feeders.
- Brachiopods: also known as "lamp shells", brachiopods are also still alive today. Brachiopods have two shells covering the upper and lower parts of the animal, and the shells are typically symmetric down the center, which helps to distinguish them from clams and other bivalves. Ordovician brachiopods were sessile filter feeders.
- Cornulites: extinct "tube worms" that were probably either relatives of bryozoans and brachiopods, or mollusks. They were sessile and often attached themselves to other animals, and are thought to have been filter feeders.
- Hyoliths: enigmatic extinct animals known from small conical shells; they may have been mollusks. It's not clear what they were doing and how they went about it, but presumably they knew.
- Scenellids: enigmatic extinct animals known from disc-shaped convex fossils. If they were mollusks, they probably lived like snails and monoplacophorans on the sea floor. It has also been suggested that the fossils were floatation structures of jellyfish-like animals, in which case they were free-floating predators.
- Monoplacophorans: rare today, and represented only by animals with limpet-like shells; Ordovician monoplacophorans also included some coiled examples. As one might imagine, they lived on the sea floor and did not move particularly swiftly. The UCMP describes them as eating detritus, although I suppose some Ordovician forms may have been more ambitious.
- Rostroconchs: extinct mollusks known from shells that look something like clams and something like tacos; they were single-shelled mollusks, but the shell was folded over on itself. Like clams, they were probably sessile filter feeders.
- Bivalves: clams, oysters, scallops, and their friends. Most bivalves are sessile filter feeders, and the Ordovician forms were probably similar. Bivalve shells are not typically symmetric, but the two shells are often symmetric along the hinge (they are mirror images).
- Nautiloids: at last we come to something moving on its own. Nautiloids include the modern Nautilus and its relatives, almost all of which are extinct. Nautiloids are swimming predators. Ordovician examples had elongate conical shells instead of the spiral shell of Nautilus. Illustrators almost always include a nautiloid seizing a trilobite in their Ordovician scenes.
- Snails (gastropods): if you don't know what a snail is... Snail shells are common Ordovician fossils. These ancient snails filled a variety of roles, from grubbing in the muck, to parasitism, to grazing on algae, to predation.
- Segmented worms: worms, probably polychaetes ("bristle worms") are represented by burrows and microscopic jaw elements (scolecodonts). Like snails, worms are quite versatile.
- Trilobites: one of the most famous groups of extinct animals; they looked vaguely like roly-poly bugs. The types we see in the metro probably mostly ate detritus from the sea floor.
- Ostracodes (seed shrimp): crustaceans protected by a pair of shells. Most modern ostracodes are mm-scale, but cm-scale examples can be found in the Platteville. Ostracodes include bottom dwellers and planktonic forms, and do a bit of everything in terms of feeding habits.
- Sea stars/starfish (asteroids): these animals are rare Twin Cities fossils, and were probably similar to modern sea stars: mobile bottom-dwelling predators.
- Brittle stars (ophiuroids): similar to sea stars, but with detritivore or scavenging habits instead of being predators.
- Crinoids (sea lilies): modern crinoids include some mobile forms, but the local Ordovician crinoids were, literally, stuck in the mud. Their bodies consisted of long stalks supporting a cup-like body with numerous feathery feeding arms. Sessile filter feeders.
- Cystoids or rhombiferans: these extinct echinoderms looked something like crinoids, but instead of the cup-like body and feathery arms, they had a more bulbous structure with projections, sometimes resembling a balloon or cartoon head. Sessile filter feeders.
- Carpoids, or stylophorans, or homalozoans...: enigmatic extinct echinoderms; the local varieties had pill- or capsule-like bodies with a short appendage (kind of like this). What exactly they did is unknown, but it must have been nefarious to warrant so many names.
- Edrioasteroids: extinct echinoderms that looked something like a bulb or disc with a starfish attached. Sessile filter feeders, often encrusting other things.
- Conodonts: extinct eel-like swimming animals, either very early vertebrates or not too distant from vertebrates. They are known from tooth-like elements, and may have been predatory, or (what else?) filter feeders.
- Graptolites: colonial animals related to (are?) pterobranch worms. Local forms include some that look like thin, double-sided saws; each "sawtooth" would have held an individual animal. They are interpreted as planktonic, either sinking slowly or suspended with floatation structures. If they were like modern pterobranchs, they would have been... filter feeders.
|The aftermath of a swingin' New Year's party in St. Paul, 454 million years ago. Things got awkward when the trilobites moulted.|
Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.
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.