|Here it is, with scale artfully copied from elsewhere in the cropped photo and placed in a more convenient location (mm divisions, one full cm included). A crinoid columnal is visible in the upper left corner.|
This was not a very large animal, only about an inch across or so. It's not from the former Johnson Street Quarry, the fabled lost storehouse of Platteville echinoderms in the Twin Cities. Actually, none of the three specimens in this post are from that site. It's possible they're from the Hidden Falls Member, as the Johnson Street specimens were, but I don't have stratigraphic information lower than formation for these specimens, two of which were found on loose slabs.
|Another view, taken at closer range and thus a bit sharper, but with no scale.|
Hudsonaster was not the only sea star in the Platteville. Urasterella was a larger, more lanky sea star, although since we're talking about the Ordovician of Minnesota, "larger" still isn't enormous by our standards. (It's still pretty big for a non-cephalopod of the Platteville, though.) The specimen is not as complete as the Hudsonaster, but there is one nearly complete arm and most of two others, with the missing sections partially recorded by natural molds.
|And I've rather brilliantly arranged to have my reversible transparent ruler wrong-side-out.|
From a quick glance of the exteriors, neither Hudsonaster or Urasterella are all that different from sea stars you might see today. Going from the familiar to "what-is-that-and-what-happened-to-it", we have a fossil identified as the rhombiferan cystoid Pleurocystites. Because only one side is visible, I'm not entirely sure if it is Pleurocystites or a close relative (the very similar genus Amecystis is also known from this interval in the Upper Midwest per Kolata et al. 1987), but it's a reasonable identification.
|With quarter for scale; the block is also peppered with partial crinoids.|
It somewhat resembles a deflated balloon on a string, or some kind of odd fish, and again is not very large. Rhombiferan cystoids were stalked ancestrally, but these pleurocystitids seem to have been using theirs for something else, perhaps on the order of a flagellum. It's not apparent in this specimen, but at the opposite end from the "stalk" were two long appendages that looked like antennae but were actually part of the feeding apparatus (only the bases are visible here). A few formations up, in the lower Prosser Formation, a mass-death assemblage in southern Minnesota yielded dozens of Pleurocystites (Sloan and DesAutels 1987), so whatever they were doing, they were at least briefly successful at it.
Strangely enough, members of a completely different group of echinoderms, the solutan "carpoids", hit upon an astonishingly similar body plan at almost the same time (Kolata et al. 1977). The Prosser bed also includes specimens of these solutan doppelgängers (Sloan and DesAutels 1987). A quick way to tell the two types apart is the "stalk": pleurocystitid "cystoids" have a simple "stalk" of stacked pieces, but the lookalike solutans have complex, "braided" "stalks". (Also, pleurocystitids have two appendages opposite the "stalk" and the lookalikes have one, but those aren't always as easy to see.) The Science Museum has one of these solutan lookalikes, Dendrocystis, on display. The fad for echinoderms that looked like deflated balloons with antennae did not last for either version, with both lineages going extinct long before the end of the Paleozoic.
Kolata, D. R., H. L. Strimple, and C. O. Levorson. 1977. Revision of the Ordovician capoid family Iowacystidae. Palaeontology 20(3): 529–557.
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.
Sloan, R. E., and D. A. DesAutels. 1987. The Wagner Quarry cystoid bed: a study in Prosser (Sherwood) paleoecology. Pages 60–62 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.