Hyoliths and Scenella

One of the aspects of Paleozoic fossils you learn to appreciate is just how many groups had a brief moment in the sun and then disappeared. I'm not just talking about famous things, like trilobites or ammonites (which saved their best for the Mesozoic, of course). You couldn't wade around in an Ordovician sea without crushing or disturbing something that has no living relatives. In fact, given the diminutive size of most of these things, you would probably endanger an entire fauna with each step, so by all means be careful next time you happen to swing by the Ordovician. Today's spotlight shines on two groups that hang around the fringes of Mollusca, and which show up in small numbers in the Ordovician of the Twin Cities: the hyoliths and scenellids.

Hyoliths

Let's start with the group we actually know a few things about. Hyoliths are primarily known from small triangular shells. No, they aren't the same as conulariids; despite both groups having struck on the eternal appeal of the triangle, hyoliths opted for a simpler structure with an oval or triangular cross-section, and did not go in for tiny mineralized rods in a herringbone pattern. However, they can be confused if all you've got to go on is a natural mold. Hyoliths appeared during the Cambrian and hung around into the Permian, but you'll mostly hear about them in connection to the Cambrian, when they were all the rage along with trilobites, brachiopods, eccentric soft-bodied critters, and various shelly things of shady affiliation. In addition to the handsome triangular shell, your standard hyolith came equipped with an operculum, a little door or cover with which it could shut out the outside world, and a pair of rod-like things known as helens. Hyoliths have not bequeathed a great body of literature unto posterity, so what exactly they did with their helens is unclear. There's a charming reconstruction of a hyolith here that shows the helens, sticking out like a handlebar mustache that's losing its curl. The University of Kansas has a number of photos of specimens with all of the parts in place here.

What exactly a hyolith was, and what it did, are also unclear. To briefly summarize, after Malinky et al. (2004): in days of old, they tended to get shuffled off with "pteropods" (free-swimming marine snails). This has hung over a bit, as seen in their common inclusion with the mollusks. Over the years, they have been postulated as free-swimming, as deposit-feeders, and as filter-feeders. The UGA Stratigraphy Lab has them tentatively as mollusks, while the Baltoscandia fossil site (which also has many fine photos) includes them without comment as mollusks. In the Twin Cities rocks, we apparently get to make do with one named species and material that is indeterminate at the species level. Courtesy of Stauffer and Thiel (1941):

Hyolithes baconi (originally listed under Gastropoda) (Platteville, Decorah)
Hyolithes sp. (originally listed under Gastropoda) (Decorah)

Hyoliths were also noted in the introduction to the second volume of the great monograph on Minnesota paleontology (as gastropods, naturally), but the chapter dealing with gastropods (Ulrich and Scofield 1897) did not include them; Conularia suffered a similar fate. Perhaps they got misplaced? At any rate, I have included below a photo of Platteville hyoliths as natural casts.

Hyolithes baconi from the collections of the University of Minnesota. GB17238, Platteville Formation, Minneapolis. Note the triangularity, as well as the sugary sparkly medium gray Platteville matrix (that appearance is how you know it's been recrystallized into dolomite from limestone).

Scenellids

Now we're into a real realm of mystery. Scenellids, represented by Scenella, are known from little cap-like shells, with a peak and often a combination of radial and concentric striations. That's it. "Are there no artful reconstructions?", you ask. Don't worry—you can do it yourself! Get a conical sun hat or a pileus-like hat. Submerge it in water. You now have a reasonable, albeit vastly magnified reconstruction of a Scenella in situ according to current knowledge, or a drowned hat in your bathtub, depending on your powers of imagination. "But what's under the hat?" Nuts if I know. While I suppose there is a remote possibility that during the Cambrian and Ordovician, there existed a species of extremely tiny fashion-conscious gnomes that sunburned easily and took care to avoid being buried at sea, it is rather more likely that scenellid fossils were the hard parts of a group of mollusks. The basic structure is limpet-like, so not unreasonably Scenella has historically been considered a snail. With the march of science, its postulated affiliation is now often with our old friends the monoplacophorans, or sometimes a group of quasi-nexus mollusks known as helcionelloids. A more radical interpretation considers the Scenella fossil as not a shell, but a float for a jellyfish-like hydrozoan.

Quite a few species have been reported for Scenella in the metro area. Stauffer and Thiel (1941) listed the following:

Scenella affinis (Decorah, Cummingsville+Prosser)
Scenella affinis obsoleta (Cummingsville+Prosser)
Scenella beloitensis (Platteville)
Scenella compressa (Platteville)
Scenella magnifica (Platteville)
Scenella montrealensis (Platteville)
Scenella obtusa (Carimona)
Scenella radialis (Cummingsville+Prosser)
Scenella superba (Platteville)

Ulrich and Scofield (1897), describing gastropods and various things that later turned out to only resemble gastropods, were reasonably happy to put any limpet-like shell with a central apex and radiating striae in Scenella. The shells Ulrich and Scofield described ranged from a centimeter in greatest dimension to several cm (or a couple of inches) across, and from low umbrella or sun hat shapes to pretty steep cones, not quite bullets but not too dissimilar.

We saw this one a while back; it's Plate LXI from Ulrich and Scofield 1897. Figures 31 to 41 represent five species of Scenella, varying from a relatively low apex to a fairly steep cone. For the record, 31 and 32 are S. radialis from St. Paul, 33 and 34 are S. beloitensis from Beloit, 35 is S. superba from Cannon Falls, 36 and 37 are S. affinis from Goodhue County, and 38 to 41 are S. compressa from somewhere on Planet Earth. You'll find the descriptions, and some other species, from pages 837 to 842.

References:

Malinky, J. M., M. A. Wilson, L. E. Holmer, and H. Lardeus. 2004. Tube-shaped incertae sedis. Pages 214–222 in B. D. Webby, F. Paris, M. Droser, and I. G. Percival, editors. The great Ordovician biodiversification event. Columbia University Press, New York, New York.

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., and W. H. Scofield. 1897. The Lower Silurian Gastropoda of Minnesota. Pages 813–1081 in Ulrich, E., 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.