Hyoliths IV: The Quest For Hyoliths

For being a pretty obscure group of long-extinct shelled invertebrates, hyoliths get a fair amount of study. What's been going on since we checked in a little over a year ago?

Back in the Hyolithening we looked at soft-tissue discoveries that indicated the enigmatic hyoliths were actually closely related to brachiopods. Later, in the Season of the Hyolith, came evidence of pedicles in early hyoliths, further linking hyoliths to brachiopods. Settled stuff, right? Hey, this is paleontology; we can always complicate things. There's also the hard parts of hyoliths to consider. It's been known for a while that hyolith shells and mollusk shells have certain structural similarities which must be due to either common ancestry or convergent evolution. Li et al. (2019) opted to go down to the early days of Hyolitha, publishing on the microstructure of Cambrian hyolith shells. The microstructural fabrics of the shells are more similar to what is seen in Cambrian mollusks than Cambrian brachiopods, being lamellar (fine layers of alternating materials) and composed of tiny "blade or lath-like" aragonite or calcite crystals. Li et al. (2019) interpreted the shell similarities as evidence of homology, not convergence. Would this make hyoliths mollusks? Well, no, not necessarily; it just means the mollusk line and the hyolith line diverged after evolution of the common shell structure. Li et al. proposed that the hyoliths were intermediates between mollusks and lophophorates (brachiopods, bryozoans, and horseshoe worms), with the common ancestor of the hyoliths and lophophorates evolving the basic lophophore feeding anatomy, but the lophophorates dropping the mollusk-like shell and evolving different shell structures. Under this hypothesis, you might think of a hyolith as something like sticking a brachiopod-like animal in a mollusk-like shell.

As we saw back in the Hyolithening, hyoliths were equipped with a sort of mustache of tiny tentacles for feeding. This is not the end of the digestive story, though. Berg-Madsen et al. (2018) recently described the fossilized digestive tract of the orthothecid hyolith Circotheca johnstrupi from the early Cambrian of the Læså Formation, Denmark. The guts of this hyolith formed a series of 20+ loops in a chevron pattern going off to the small pointy end of the shell, turning into a simple tube leading back to the wide end for the return chute. The chevron loops are interpreted as ventrally positioned (which would be along the flat side of the shell) and the anal tube as dorsally positioned. Based on the small number of preserved hyolith digestive tracts, the orthothecids had much more complex tracts than the hyolithids, which had a simple "U", suggesting an ecological difference. Interestingly, juvenile orthothecids also had simple "U"-shaped tracts, which may indicate the ecological factor appeared later in life (and that juvenile orthothecids were doing something different than adults) (Devaere et al. 2014).

An odd aspect of hyoliths is that they seem to have been very handy as hard substrates for epibionts. For example, Zicha et al. (in press) found that in the Middle Ordovician Šárka Formation, 60% of the colonized shells were hyolith conchs, with edrioasteroid echinoderms being particularly selective for them. Wen et al. (2019) also commented on the edrioasteroid fondness for hyoliths. They described the earliest known such colonizer, Totiglobis spencensis from the Cambrian Spence Shale (roughly what we used to call the Middle Cambrian). It was attached to a specimen of the hyolith Haplophrentis; based on the hyolith's operculum being adjacent, it seems that the hyolith acquired the epibiont during life and the two grew and prospered together.

References

Berg-Madsen, V., M. Valent, and J. O. R. Ebbestad. 2018. An orthothecid hyolith with a digestive tract from the early Cambrian of Bornholm, Denmark. GFF 140(1):25–37. doi:10.1080/11035897.2018.1432680.

Devaere, L., S. Clausen, J. J. Alvaro, J. S. Peel, and D. Vachard. 2014. Terreneuvian orthothecid (Hyolitha) digestive tracts from northern Montagne Noire, France: Taphonomic, ontogenetic and phylogenetic implications. PLoS One 9(2):e88583. doi:10.1371/journal.pone0088583.

Li,  L., X. Zhang, C. B. Skovsted, H. Yun, B. Pan, and G. Li. 2019. Homologous shell microstructures in Cambrian hyoliths and molluscs. Palaeontology 62(4):515–532. doi:10.1111/pala.12406.

Wen, R., L. E. Babcock, J. Peng, and R. A. Robison. 2019. New edrioasteroid (Echinodermata) from the Spence Shale (Cambrian), Idaho, USA: further evidence of attachment in the early evolutionary history of edrioasteroids. Bulletin of Geosciences 94(1):115–124.

Zicha, O., J. Bruthansová, and P. Kraft. In press. Epibionts on shells in the Šárka Formation: a sparsely occupied niche in the lower to middle Darriwilian (Oretanian, Ordovician) in the Prague Basin (Czech Republic). Palaeogeography, Palaeoclimatology, Palaeoecology. doi:10.1016/j.palaeo.2019.109401.

Foerstediscus splendens, a Minnesotan in Washington

Because I was in northern Virginia, I thought I'd visit the National Museum of Natural History. While nosing around the Ocean Hall, I ran into another Minnesotan: the holotype specimen of the edrioasteroid Foerstediscus splendens (Bassler 1936). I couldn't get right up to due to the glass, so the photos aren't the clearest and there's no scale, but it's still a fine specimen, and somewhat larger than I expected following my experience with the minuscule Pyrgocystis (for reference, it's described as 3 cm across, a little more than an inch).

Points to you if you too noted "Middle Ordovician"; with redefinition and redating of the Ordovician, "Late Ordovician" would be more accurate.

Here's a closer view of the specimen. "S. 4079" is the Smithsonian's number; it was originally a University of Minnesota specimen, UMPC 4742a, but it was donated to the Smithsonian (Rice 1990). From my own experience with the UMPC fossils, I think all of the edrioasteroids they had on hand by approximately the mid-1930s went to the Smithsonian at Bassler's request, with the specimens I photographed coming with Sardeson's collections a few years later.

"Edrioasteroid" means "seated star", which is reasonably self-explanatory.

Edrioasteroids are among the rarest fossils from the Decorah and Platteville formations of Minnesota, with their other competition in that category being other echinoderms such as sea stars and cystoids. Seeing as how these fossils are very rare here except in certain places such as the former Johnson Street Quarry, the obvious scenario is that non-crinoid echinoderms only thrived under certain conditions that are poorly represented in the known outcrops of the Platteville and Decorah. (Alternatively, they *were* more common than we know, but they decomposed so completely to their constituent plates that they've been unrecognizable; be that as it may, though, the Platteville/Decorah echinoderm diversity doesn't seem to be as high as in other formations of similar age in the central US.)

Foerstediscus splendens was collected from the Ford Plant (Bassler 1936). Work on the Ford Plant, Ford Bridge, and Ford Dam (Lock and Dam No. 1) in the 1920s made that area a great place for collecting, as discussed in this post. Clinton R. Stauffer named 56 species of microfossils from collections at the bridge (Stauffer 1933, 1935), and August Foerste named the nautiloid Metaspyroceras perlineatum (Foerste 1932) from the vicinity of the plant and dam. F. splendens was found by Irving G. Reimana from the top of the Rhinidictya bed of the Decorah Shale (Bassler 1936), otherwise known as Bed 4 (Sloan 1987), or, a little more informally, the lower third of the Decorah Shale above the Carimona Member. (One of these days I ought to outline the bed divisions.) In more practical terms, Bed 4 is the Decorah you see at Shadow Falls, so the top of Bed 4 would be somewhere at or a little higher than the highest part of the ravine. Strangely, Stauffer and Thiel (1941) attributed Foerstediscus to the underlying Carimona Member, or Bed 3, despite Stauffer presumably being the one who gave Bassler the stratigraphic information in the first place.

Looking east from Minnehaha Park in June 2013, toward the defunct Ford Plant (across Lock and Dam No. 1, aka the Ford Dam).

The closure and dismantling of the Ford Plant over the past few years provide an unusual opportunity to look at current events in a geological frame of mind. The Ford Plant was a fixture of southwestern St. Paul for decades (and it happened to be there because there was a young river gorge carved by a post-glacial waterfall that could be harnessed for hydroelectric power, and because there was once a pure sandy beach that could be turned into auto glass). Its day is now over, though, and soon its former grounds will be rebuilt into something entirely different. Ten thousand years ago, mammoths and giant beavers roamed its grounds. Seventy million years ago it was probably a coastal plain near a sea filled with ammonites and marine reptiles. 454 million years ago edrioasteroids, nautiloids, and other marine invertebrates lived and died in a shallow tropical sea that had submerged the pure sandy beach. Underneath all of it are volcanic rocks more than a billion years old that fill a scar from Michigan to Kansas. Every part of history is made up of what came before it, and shapes what comes after it.

References

Bassler, R. S. 1936. New species of American Edrioasteroidea. Smithsonian Miscellaneous Collections 95(6).

Foerste, A. F. 1932. Black River and other cephalopods from Minnesota, Wisconsin, Michigan and Ontario (Part 1). Denison University Scientific Laboratories Journal 27:47–137.

Rice, W. F. 1990. Catalog of paleontological type specimens in the Geological Museum, University of Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Information Circular 33.

Sloan, R. E. 1987. History of study of the Middle and Late Ordovician rocks of the Upper Mississippi Valley. Pages 3–6 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. 1933. Middle Ordovician Polychaeta from Minnesota. Geological Society of America Bulletin 44(6):1173–1218.

Stauffer, C. R. 1935. The conodont fauna of the Decorah Shale (Ordovician). Journal of Paleontology 9(7):596–620.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

All of the other echinoderms

If you thought that echinoderms have a variety of seemingly unrelated body plans today (sea stars, crinoids, sea urchins, sea cucumbers...), you should have seen them during the Paleozoic, when several now-extinct classes populated the oceans. The group with probably the most recognition are the blastoids, or sea buds, which had stalks like crinoids but with a nut-like structure instead of a cup-like structure as the business end. The rocks of the Twin Cities area are not known to have produced blastoids, but they have produced rare examples of four other groups not including the crinoids we saw before. They are: asteroids (sea stars), rhombiferan cystoids, edrioasteroids, and stylophorans (a.k.a. carpoids, a.k.a. homalozoans). Three of these groups are extinct, and it doesn't take much to guess which. This diversity of echinoderms is not particularly unusual; Cincinnatian rocks have all the same classes as well as brittle stars and cyclocystoids (extinct and known mostly from their ring-like outer structures), and the Bromide Formation of Oklahoma, of comparable age, outdoes the Minnesota rocks handily. But for the luck of the cosmic draw, any of these extinct groups might be around today.