Strolling on the Magnolia Member by Hidden Falls

If you're looking for something geological to do in the Twin Cities while we're still under our unseasonably warm and dry weather, may I suggest paying a visit to the new park area above Hidden Falls? [Update, 2022/11/01: this park is called Uŋčí Makhá Park.] As part of the conversion of the former Ford Plant environs, part of the area of the creek into Hidden Falls has been daylighted. The landscaping has produced a mini-bedrock gorge that exposes significant vertical and bedding-plane surfaces of the Magnolia Member of the Platteville and the overlying Carimona Member of the Decorah.

There's nothing quite like this kind of exposure in the Twin Cities; we don't have a lot of exposed non-vertical bedrock in the first place, and this particular stratigraphic interval tends to be out of reach. The closest might be the platform below the overlook at Shadow Falls, but that's more limited in extent and has more of a stair-step profile.

Bonus points for spotting the Deicke K-bentonite.

Many of the exposed bedding plane surfaces reveal the shell beds the Magnolia is known for. The fossils are almost entirely brachiopods (with a few snails) and are represented by dolomitized molds and casts, giving them that characteristic sugary appearance.

See the little bumps? Brachs.

Enlarge for a world of brachiopods.

Here's a closer view showing a few nice examples, representing multiple species.

Also, just for fun, some of the stones used for landscaping are loaded with burrows.

If you stop by, please don't attempt to remove the fossils; it's a park, after all, and the fossils aren't really going to come off in one piece because they're molds and casts. Just enjoy the experience of walking on the seafloor without ever getting wet!

Abbott and Costello Meet the Hyolith

I seem to have missed doing a hyolith post last year, which is really a shame and is all on me. Honestly, where else are you going to get the latest information on hyoliths? Social media? Cable news? Public radio? No, of course not! This is really a public service I'm running here, and you'll thank me for it someday. With that out of the way, what's been going on in the world of hyoliths over the past couple of years? A few thoughtfully curated highlights:

If you've ever spent time looking at the origins and relationships of gastropods, you'll be familiar with the seemingly endless debates about whether such-and-such is a gastropod or something that just happens to have a shell that looks like a snail shell (like monoplacophorans, helcionelloids, and maybe bellerophonts). We've had a bit of that with the local snail-oids, although by the Late Ordovician most of the hard cases had been cleared out. Down in the Cambrian things are more complicated. One example is Protowenella, an itty-bitty (smaller than 1 mm long) shell thing that looks kind of like a Phrygian cap. (Okay, fine, it looks like an exaggerated Smurf hat.) Is it a gastropod, a monoplacophoran, or a helcionelloid? According to Peel (2021), it is none of these. Instead, it is... a hyolith.

(Admittedly, the surprise you are feeling is probably tempered by the fact that this is a post about hyoliths, so it wouldn't have made sense for it to be a chiton or graptolite or something.)

Peel based his conclusion on the presence of a bilaterally symmetric operculum (a mineralized cap that covers the shell aperture) with features consistent with a hyolith origin. Gastropods frequently have opercula, but they aren't symmetric, and hyoliths are the only thing known to have had opercula in the Cambrian. If this referral is accurate, it would be something of an unexpected expansion of hyolith morphological talents. Hyoliths, of course, are famous for having long triangular shells with triangular cross-sections, whereas Protowenella as mentioned looks like a curled-over pointed hat.

Now let's turn from something that doesn't look like a typical hyolith but has something that *is* typical, to something that looks like a typical hyolith but is missing something expected. Hyoliths are generally divided into two groups, the orthothecids and hyolithids. Orthothecids showed up first and have a flat, retracting operculum (rather than the more complex operculum of hyolithids) and no helens (the paired spiny appendages that make hyolithids look kind of like they have wide spindly mustaches). It turns out that there are some hyoliths with hyolithid anatomy except for no helens. Liu et al. (2022) examined one such example, "Ambrolinevitus" ventricosus, an early Cambrian form from China (which they moved to Paramicrocornus, also known to lack helens). The implication is that the hyolithid body shape evolved before helens. Therefore, whatever ecological specialization was held by hyolithids over their earlier cousins, it was underway before helens appeared.

Finally, I leave you with Sun et al. (2021), a description of some beautifully preserved Cambrian hyoliths from China. Assigned to the new species Novakotheca weifangensis, they're small (less than 2 cm at most, although some have even smaller brachiopods attached) and pretty typical in shape. The neat thing about them is the preservation of mineralized soft parts of the digestive tract, interpreted as including a pharynx, esophagus, stomach, U-shaped intestine, and possible digestive gland. The anatomy is more complex than previously thought, and suggests that at least some hyoliths were not simply filter feeders.

References

Liu, F., C. B. Skovested, T. P. Topper, and Z. Zhang. 2022. Hyolithid-like hyoliths without helens from the early Cambrian of South China, and their implications for the evolution of hyoliths. BMC Ecology and Evolution 22: article 64. doi:10.1186/s12862-022-02022-9.

Peel, J. S. 2021. In-place operculum demonstrates that the Middle Cambrian Protowenella is a hyolith and not a mollusc. Alcheringa: An Australasian Journal of Palaeontology 45(4):385–394. doi:10.1080/03115518.2021.2004225.

Sun H., Sun Z., and Zhao F. 2021. Exceptionally preserved hyolithids from the middle Cambrian of north China. Geological Magazine 158(11):1951–1959. doi:10.1017/S0016756821000510.

Compact Thescelosaurus Year Seven

Here we are at the second weekend of October, which means it's time for three things: National Fossil Day; a new sheet for The Compact Thescelosaurus; and our annual roundup of what's been added to the spreadsheet. National Fossil Day falls on Wednesday, October 12 this year, although events occur throughout the month (especially the weekends before and after), so check your nearest museum or National Park System unit for events! Our fall Park Paleontology newsletter is also up for viewing (including more fun with packrats).