AsteroidsOrdovician sea stars are not terribly unlike modern sea stars, so we can assume they probably lived similar lives, moving across the sea floor at a leisurely pace in search of food. Stauffer and Thiel (1941) identified three taxa in the Platteville and Decorah of Minnesota:
Palaeaster sp. (De) [genus now Promopalaeaster]
Protopalaeaster narrawayi (Ca) [genus Hudsonaster, and found in the Platteville as well]
Urasterella ulrichi (Pl)
Promopalaeaster looks a lot like your basic sea star. Hudsonaster, should you ever run into it, is a small (on the order of an inch) stocky sea star. The Cincinnati Museum Center has a photograph of an example from Minneapolis in their collections: search either for "Hudsonaster" as "Taxon" and "Minnesota" as "Sites", or "IP36520" as "Catalogue #". Urasterella, at the other end of proportions, has very long arms. I have seen the latter two in collections, but not Promopalaeaster.
Rhombiferan cystoidsTime was, you just called these cystoids and left it at that, but of course cystoids ungratefully turned out to not be a natural group. Given that "rhombiferan" sounds geometric and fairly non-threatening, and "cystoid" sounds like something that might not be covered by your medical insurance, perhaps it was for the best.
The basic cystoid body plan features a crinoid-like stalk with a sort of sack-like structure covered by polygonal plates instead of the crinoid "cap and branching arms". Atop the body were long slender appendages called brachioles, used for the business of bringing food down to the body. Rhombiferans get their name from rhomb-shaped structures on the body that look like vents, which were used for respiration. The rhombs also have the side effect of looking like mechanical eyes, so the whole thing may look like an alien head with a long snake-like neck and tentacles for hair. Some were more specialized: Pleurocystites, Amecystis, and their ilk, known from the Decorah, had flattened bodies with distinct upper surfaces (with the respiratory rhombs in Pleurocystites and large plates) and lower surfaces (featuring the inconspicuous anus and a pavement of smaller plates surrounded by large marginal plates), plus tapering stalks that did not form obvious holdfasts or other attachments, and a pair of brachioles. They don't look like crinoid stems with a bag- or sack-like thing on top. They look more like weird armored fish with paired antennae, and in fact have sometimes been interpreted as mobile, with the flexible stalk as a means of propulsion. There is a toy of Pleurocystites, and it is somewhat more disturbing than the fossilized faux-fish below (identified as Pleurocystites, but more likely Amecystis). Go ahead, imagine these things creeping along the seafloor by either using the stalk as sort of a flagellum or as a hook or prop to drag itself along. Other rhombiferans reported from the Platteville or Decorah, like Glyptocystites, were of a more typical form, with numerous small brachioles atop the body.
|This Decorah Shale specimen in the collections of the University of Minnesota is identified as Pleurocystites, but the absence of visible rhombs suggests that it is the similar genus Amecystis. This is the upper or "antanal" surface.|
|These raspberry-like structures from the Decorah (University of Minnesota again) are identified as possible cystoids, but I haven't been able to match them to the usual suspects to my satisfaction. (note, 2016/02/18: I suspect that they may be paracrinoids, which would be interesting if accurate because paracrinoids have not been reported from the Decorah of Minnesota. Some Ordovician examples are included in photos here. There are other echinoderms that can have similar surfaces, though.) (further note, 2017/06/27: I have also seen apparent inarticulate brachiopods with similar surfaces.)|
Stauffer and Thiel (1941) reported a handful of taxa from the Platteville, Decorah, and overlying Galena Group units. Apparently at that time they did not know of pleurocystitids from the Decorah:
Cheirocrinus sp. (Pl)
Eurycystites granosus (Ca, De) (this name shows up nowhere but in lists derived from Winchell and Ulrich 1897, so I suspect it is something that Ulrich meant to name but never got around to, or published under a different name. Wonder where it is?)
Glyptocystites sp. (Ca, De, Gp)
Pleurocystites squamosus (Gp)
Pleurocystites sp. (Gp)
Kolata et al. (1987) published a stratigraphic table of echinoderms from the Ordovician rocks of the upper Mississippi Valley, but unfortunately did not break their lists down by location. According to their tables, rhombiferans which would be appropriate for the local Platteville and Decorah include:
EdrioasteroidsEdrioasteroids were Paleozoic echinoderms that looked something like a sea star glued to a disc- or bulb-like object, in turn attached to something else, be it the seafloor or a brachiopod shell. The five "arms" (ambulacra) contained the food grooves, protected beneath cover plates. As encrusting animals, they did not get around much once they had settled down, although some forms may have been less firmly cemented than others. Others appear to have been capable of vertical extension and retraction, perhaps to expand their filter-feeding range while also being able to pull everything back into protection when needed. A handful of edrioasteroid genera and species have been reported from the Carimona and upper Decorah of Minnesota. Stauffer and Thiel (1941) came up with the following (all included under "Cystoidea"):
Agelacrinites n. sp. (Ca, De)
Foerstediscus splendens (Ca) (although the original description has it as plain old Decorah)
Hemicystites paulianus (De) (a synonym of Isorophusella incondita?)
Pyrgocystis sardesoni (Ca, De)
Kolata et al. (1987) throw in a couple more as potentially present:
Edrioaster bigsbyi (which is known to be present in the Prosser)
Hemicystites curtus (Carneyella pilea)
The type specimens of F. splendens and H. paulianus are in the National Museum of Natural History, and photos are online. From the collections database, just search using the scientific name of either, or the specimen number (4079 for F. splendens and 42114 for H. paulianus). P. sardesoni may be the most common in the metro area; at least multiple specimens have been found.
|Pyrgocystis sardesoni from Bather (1915)|
|And in person (University of Minnesota). (In case you were wondering, they weren't all that big.)|
Stylophorans ("carpoids")Stylophorans are one of the most enigmatic fossil groups, not helped by their habit of traveling under a variety of aliases (carpoids being the traditional name). They are usually described as echinoderms, although have occasionally been described as related to the ancestry of chordates (not widely supported). The stylophorans of interest for us have a sort of capsule-shaped body with a short stocky midline appendage at one end and a pair of thinner appendages at the corners of the other end. I do have some concern that this basic shape, when poorly preserved, can be confused with poorly preserved pleurocystitids. Fossils identified as "carpoids" from the Twin Cities are in collections but aren't published. Two species have been named from equivalent rocks in the upper Mississippi Valley, and could conceivably be represented here. They are Ateleocystites guttenbergensis and Willmanocystis denticulatus (Kolata and Jollie 1982).
References citedBassler, R. S. 1936. New species of American Edrioasteroidea. Smithsonian Miscellaneous Collections 95(6).
Bather, F. A. 1915. Studies in Edrioasteroidea. VI. Pyrgocystis N. G. Geological Magazine 6(1):5–12.
Brower, J. C. A new pleurocystitid rhombiferan echinoderm from the Middle Ordovician Galena Group of northern Iowa and southern Minnesota. Journal of Paleontology 73(1):129–153.
Kolata, D. R., and M. Jollie. 1982. Anomalocystitid mitrates (Stylophora–Echinodermata) from the Champlainian (Middle Ordovician) Guttenberg Formation of the upper Mississippi Valley region. Journal of Paleontology 56(3):631–653.
Kolata, D. R., J. C. Brower, and T. J. Frest. 1987. Upper Mississippi valley Champlainian and Cincinnatian echinoderms. Pages 179–181 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.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.
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.