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.

Your Friends The Titanosaurs, part 17: Malawisaurus, Mansourasaurus, and Maxakalisaurus

A pair of African titanosaurs (Malawisaurus dixeyi and Mansourasaurus shahinae) get the call this week, along with Maxakalisaurus topai from Brazil. It's also the second consecutive entry where there are rumblings of an imminent redescription of one of the guests: last month it was the Magyarosaurus complex, this month it's M. dixeyi. What's going on with probably the most widely known African titanosaur? Read on to find out!

Compact Thescelosaurus Year Four

It's time for the annual update to The Compact Thescelosaurus, now entering its fifth year. This time, ichthyosaurs join the group, which is really stretching it for me, but hopefully it's not entirely in vain. Ichthyosaurs fared a bit better than plesiosaurs in that *only* 91 of the 224 species (40.6%) included are of questionable validity, rather than 173 of 318 (54.4%). (Only.) My go-to work was McGowan and Motani 2003 on Ichthyopterygia (Handbook of Paleoherpetology 8), just to figure out what was there. Ichthyosaurus itself, like Plesiosaurus, accumulated an awe-inspiring retinue of species over the years, but it's not alone. A couple of areas of ichthyosaur taxonomy are pretty volatile at the moment as massively specious genera are re-evaluated, so expect revision. (Also expect the usual formatting bugs and obvious corrections that come with a new sheet.)

Sure, like you don't know what an ichthyosaur looks like. This was found at Wikimedia Commons, but originally came from a short item by William Diller Matthew in the American Museum Journal for January 1908.

What I Did While I Was Out: On the Way to the Grand Canyon

As I mentioned a few months ago, I've been spending a lot of time on the fossils and rocks of Grand Canyon National Park this year. Part of why I've been doing this culminated on Saturday, September 28: our special National Fossil Day event, held at the park as part of their centennial festivities. (You can see the whole group that worked the event here.) We've been working for months on a Grand Canyon NP paleontological inventory, and to further that work as well as help at the public event, I spent the end of September in Arizona, visiting various places.