|Sorry, but I don't seem to have a better picture of local hyoliths, unless the things mentioned in this post or this post are hyoliths.|
This week, the big paleontological news had nothing to do with dinosaurs, or mammals, or anything with bones at all for that matter. Instead, hyoliths got to be the subject of dozens of news articles, for the happy reason that their relationships are no longer quite so enigmatic. Undergrad Joseph Moysiuk of the University of Toronto and colleagues have presented research on the little guys showing that they were equipped with a tentacle-laden feeding apparatus, making them next cousins to...
[oh, the cheap stunt of the jump break!]
...brachiopods. Specifically, they had a number of small tentacles (12 to 16 in the specimens the team examined) surrounding the mouth, kind of like a very shaggy mustache. This is comparable to the brachiopod lophophore. The elongate body of the hyolith is more comparable to another group related to brachiopods, the phoronids or "horseshoe worms", so hyoliths and brachiopods may be descended from a tubular phoronid-like animal (phoronids being one of those soft-bodied phyla with inconveniently blank fossil records). One interesting point in the scientific article, seemingly not included in the media coverage, is that the main shell and lid-like operculum of hyoliths are interpreted as homologous to the ventral and dorsal shells of brachiopods, respectively. Moysiuk et al. interpreted their hyoliths as filter feeders, using the paired curved helens as props to raise the lophophore above the substrate. If you'd like to see more, NPR has a very good article with very helpful illustrations. The New York Times and Smithsonian have good articles as well. For the actual publication by Moysiuk et al., you can see at least the abstract here; I clicked on a figure and got to read the article but not download it (although that may have only worked once), so feel free to try that for yourself. Now maybe they'll work on conulariids!
This seems as good a place as any to revisit the hyoliths of Minnesota. We didn't really get into it at the time, but hyoliths are not only found in the Platteville and Decorah. They are also present in the Cambrian rocks of the state. For reference, the major formations, in ascending order (older to younger), are the Mount Simon Sandstone, Eau Claire Formation and partially equivalent Bonneterre Formation, Wonewoc Sandstone, Tunnel City Group (ex Franconia Formation), St. Lawrence Formation, and Jordan Sandstone. Stauffer and Thiel (1941) had Hyolithes primordialis and H. sp. in the Eau Claire Formation (their "Eau Claire Member of the Dresbach Formation"), H. attenuatus and H. sp. in the siltstone facies of the St. Lawrence Formation (their Lodi Member), and H. sp. in the Jordan Sandstone (their Norwalk Member). Walcott (1914) reported Hyolithes? corrugatus from the St. Lawrence Formation of Osceola, Wisconsin, just on the other side of the St. Croix, but whether or not this is really a hyolith is up for questioning; Malinky (1988) found it very unlikely that the type material of that species belongs to a hyolith, so if the Osceola material is anything like the type, who knows. Honestly, I'm wary of giving too much weight to detailed classification of the local forms, given that a lot of the fossils are probably not that well preserved and Hyolithes hosted no fewer than 363 species and subspecies back in 1946 (Sinclair 1946). Mossler (2008) noted the presence of hyoliths (no species specified) in the Eau Claire, Bonneterre, Wonewoc, and St. Lawrence formations. (Stauffer and Thiel also have Hyolithes alatus possibly in the Devonian rocks of the state, but this species is apparently an uncoiled snail, as per La Rocque 1949.)
Another little update: the taxonomy of the Platteville and Decorah hyoliths I presented back in the original hyolith post was out of date. Malinky (1990) named a new genus and species, Solenotheca bakerae, based on a specimen from an unknown formation (presumably the Platteville or Decorah) of Minneapolis, and tentatively assigned the "default" Minnesota Ordovician hyolith Hyolithes baconi to this genus as well. The mystery triangle is not entirely unlike Malinky's figures of Solenotheca, but the surficial structures are different enough that I would not consider them the same thing at this time (Solenotheca has narrowly-spaced lines, the mystery triangle has wider-spaced lines apparently separating surficial "sheets").
La Rocque, A. 1949. New uncoiled gastropods from the Middle Devonian of Michigan and Manitoba. Contributions from the Museum of Paleontology, University of Michigan 7(7):113–122.
Malinky, J. M. 1988. Early Paleozoic Hyolitha from North America: reexamination of Walcott's and Resser's type specimens. Journal of Paleontology 62(2):218–233.
Malinky, J. M. 1990. Solenotheca, new Hyolitha (Mollusca) from the Ordovician of North America. Proceedings of the Biological Society of Washington 103(2):265–278.
Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.
Moysiuk, J., M. R. Smith, and J.-B. Caron. 2017. Hyoliths are Palaeozoic lophophorates. Nature. doi:10.1038/nature20804.
Sinclair, G. W. 1946. Notes on the nomenclature of Hyolithes. Journal of Paleontology 20(1):72–85.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.
Walcott, C. D. 1914. Cambrian geology and paleontology II. No. 13. Dikelocephalus and other genera of the Dikelocephalinae. Smithsonian Miscellaneous Collections 57(13):345-413.