Crinoids

For the fossil enthusiast in Minnesota, there are few groups with a bigger gap between "what they show in the books" and "what you actually find" than crinoids. The ideal of a crinoid fossil is a long segmented stem or stalk connected to a cup-like structure crowned by a group of delicate feathery arms. The Digital Atlas of Ordovician Life and the UGA Stratigraphy Lab's pages on Cincinnatian fossils have many images of these ideal crinoid fossils, if you're feeling like living vicariously. The reality in Minnesota is a bit more like this:

Of course, they are not always found loose, but you get the idea.

Crinoids are one of the most abundantly represented fossil groups in the Ordovician rocks of Minnesota, and they are almost invariably represented by the above pieces-parts. Crinoid bodies have a distressing tendency to disintegrate to their constituent parts unless the burial conditions are right, and apparently those conditions were rare here.

I'm getting a bit ahead of things here, though. Crinoids, like brachiopods and bryozoans, are still around today but not nearly as prominent as they were at their Paleozoic heights. They are one of the great branches of the echinoderms, along with such other luminaries as sea stars (asteroids), sea urchins (echinoids), and sea cucumbers (holothuroids). Like other echinoderms, they have five-fold symmetry (which gives sea stars their five arms) and an internal calcitic skeleton. Crinoids have tube feet, like sea stars, found on the arms. The tube feet help whisk particles of food from the surrounding water into food grooves on the arms, and eventually into the body. The guts, such as they are, are concentrated in the cup-shaped structure, which is known as a calyx. (It shouldn't surprise me that crinoids have predators, but they just don't seem that appetizing. Filling, sure.) Crinoids come in both stalked and unstalked varieties. The stalked varieties are also known as "sea lilies" and are more or less stuck in one place, although some can move. The unstalked varieties are free-swimming and are known as "feather stars". In the Ordovician, there was apparently not much call for swimming crinoids or for stalked crinoids to move, so the primary mode of transport for a grown stalked crinoid was involuntary. The Minnesota contingent is almost entirely represented by similar fossils to those in the above picture, resembling gears, sprockets, rings, Cheerios, LifeSavers, etc., or short stacks of the same. These are "columnals", the pieces that made up the stalks. Some are plain round, some have nodes, some have an outer rib, some have striations, some have just one central pore, some have a flower-like five-lobed pore, some have a central pore and five little pores around it... Plates from the calyx are also sometimes found.

Although crinoids were known to the 19th-century geologists in Minnesota, they didn't get much attention aside from Ulrich (1886) naming Cremacrinus punctatus from "Finn's Glen" (Shadow Falls). They didn't even get a section in "The Geology of Minnesota", although Winchell and Ulrich (1897) did include them in their biostratigraphy, along with some names that Ulrich apparently intended to publish but never got around to. Frederick Sardeson was the major early contributor on Minnesota's crinoids. Among his innovations was a taxonomy of crinoid holdfasts (the part of the stalk used to anchor the crinoid to something) (Sardeson 1908), which didn't take but was an interesting contribution; and careful reconstruction of crinoid calyces, which showed the synonymy of several species (Sardeson 1939). Stauffer and Thiel (1941) included crinoids in their faunal lists, but their crinoid lists have been superseded by Brower and Veinus (1978), a detailed studied of Decorah and Platteville crinoids from the Twin Cities. This publication, along with Brower's short summary (Brower 1987), are about all you need to know what kind of crinoids have been found in the metro, and from which rocks. Both of them are also freely available online. There is one slight drawback: there isn't a huge amount of information on isolated columnals.

Per Brower (1987), the crinoids known from the Platteville and Decorah of the Twin Cities are as follows (the codes in parentheses: HF=Hidden Falls Member of the Platteville Formation, Ca=Carimona Member of the Decorah Shale, De4, 5, and 6=lower, middle, and upper Decorah Shale above the Carimona):

Archaeocrinus sp. (De4)
Carabocrinus dicyclicus (De4–6)
Ca. magnificus (De4–6)
Cremacrinus arctus (HF)
Cr. punctatus (De4–5)
Cupulocrinus gracilis (HF)
Cu. jewetti (Ca, De4–5)
Cu. canaliculatus (De5)
Glyptocrinus tridactylus (De6)
Grenprisia billingsi (De4–5)
Isotomocrinus tenuis (De5)
Palaeocrinus angulatus (De4)
Periglyptocrinus spinuliferus (De4)
Porocrinus pentagonius (De4–5)
Po. elegans (De5)
Pycnocrinus sardesoni (De5–6)
Py. multibrachialis (De5)
Unknown camerate crinoid (HF)
Unknown cincinnaticrinoid (HF)
Lichenocrinid holdfasts (HF, Ca, De4–5) (resemble small rough-surfaced rings attached to brachiopods)
Lobate/digitate holdfasts cemented to bryozoans (De4–5)
Massive conical attachment disk (De6)
Stem with grasping cirri on bryozoan (De5)
Tree-stump cirrus root (De5)

This group includes both classic-looking crinoids and some oddballs. Carabocrinus is restored as a large cup on a very short stalk. Cremacrinus is noted for having very thick and heavy arms, and is restored as essentially seated on the seafloor (Brower and Veinus 1978). Identifiable crinoids are rare in the Platteville, being limited to the Hidden Falls Member, and more specifically to one locality in the Hidden Falls Member, the old Johnson Street Quarry. This site was located in an area now bounded by 18th Avenue NE, New Brighton Boulevard, I-35W, and Johnson Street NE, which is now the Quarry Shopping Center. It was noted for yielding various articulated echinoderms, including sea stars, cystoids, and edrioasteroids as well as crinoids (all will be explained, eventually). Brower and Veinus (1978) suggested that the Platteville Formation was deposited under less energetic water than the Decorah, and was thus less suitable for the filter-feeding crinoids. The traditional picture of the crinoid rooted to the ground is inaccurate for these formations; instead, most of the crinoids appear to have been attached to bryozoans and brachiopods. Brower and Veinus (1978) interpreted this as evidence of unsuitable substrate.

References:

Brower, J. C. 1987. The Middle Ordovician crinoid fauna of the Twin Cities area. Pages 177–178 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.

Brower, J. C., and J. Veinus. 1978. Middle Ordovician crinoids from the Twin Cities area of Minnesota. Bulletins of American Paleontology 74(304):372–506.

Sardeson, F. W. 1908. Discoid crinoidal roots and Camarocrinus. Journal of Geology 16(3):239–254.

Sardeson, F. W. 1939. Carabocrinus and species making. Pan-American Geologist 71(1):27–38.

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. 1886. Remarks on the names Cheirocrinus and Calceocrinus, with descriptions of three new generic terms and one new species. Minnesota Geological and Natural History Survey Annual Report 14:104–113.

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.