Hyoliths VII: The New Blood

Have you ever been working on some mundane task when you suddenly wondered about the latest news from the world of hyoliths? Taking a walk, or merging onto a busy highway, or applying shampoo in the shower? All right, probably not, but if so, we're here for you!

Appropriately for this time of year, we have some news of hyoliths meeting or escaping grisly demises. (Or not, but that's taphonomy for you.) Kraft et al. (2023) published on an exceptionally well-preserved specimen of the Middle Ordovician central European trilobite Bohemolichas incola, including gut contents. The hyoliths are only a small part (quite literally!) of the story, which is well worth checking out if you have any interest in trilobites. The small trilobite (on the order of 35 mm or 1.4 inches long) apparently ate every darn thing it could fit in its mouth that wasn't putting up too much of a fight, including tiny hyoliths, ostracodes, stylophoran echinoderms, and chunks of shells.

The trilobite in question (Figure 1 from Kraft et al. 2023; scale bar 10 mm or 0.4 inches). Hyolith bits are in purple, including one recognizable shell under the trilobite's pygidium (tail segment). CC BY 4.0.

Paleozoic examples of the bilobed trace fossil Rusophycus are often attributed to resting trilobites, and one of the things you can do when you're not moving is pick up a snack. Lee et al. (2025), in a description of Cambrian Rusophycus from China, included an example where the trace was associated with hyolith shells. Unlike classic Rusophycus, thought to occur at the seafloor surface, this example was interpreted as a burrow. Also unlike classic trilobite predation trails, in this case the food had a hard shell. The trace-maker is thought to have been scavenging for hyoliths that had been transported from elsewhere.

Returning to the Ordovician of central Europe, we find a hyolith that was not eaten, although not from lack of trying. Fatka et al. (2023) reported a specimen of Elegantilites custos with healed damage in the form of scratches on its operculum. The culprit in this case is thought to have been an echinoderm, possibly an ophiuroid (brittle star) trying to get in.

Perhaps you'd prefer to think of your hyoliths more in terms of a grand and proud lineage, rather than delicious treats for every passing trilobite and brittle star. If so, Liu et al. (2024) have an analysis of Cambrian hyoliths for you. Using all valid Cambrian genera (N=115), they considered a set of 20 morphological characteristics over time and space. Overall hyolith taxonomic diversity peaked in Series 2 of the Cambrian (roughly speaking, the time when trilobites appeared and therefore kind of like the old "Early Cambrian"). They then keeled over sharply and were at lower levels for much of the rest of the Cambrian, locally reviving to a certain extent in the Early Ordovician. Their decline may have been due to an ocean anoxic event (the Sinsk Event) around 513 to 508 million years ago. The two major wings of hyoliths, the hyolithids (the kind with helens and complex opercula; filter feeders?) and orthothecids (the kind without helens and with simple opercula; deposit feeders?), did not follow the same curves: the orthothecids peaked sooner and felt the bite sooner, whereas the hyolithids didn't really get started until Series 2 and actually peaked just after it before suffering their drop. Morphological diversity was greatest in Series 2, although granted hyoliths had a somewhat limited repertoire.

The curve of Cambrian hyoliths. Figure 1 in Liu et al. (2024). CC BY 4.0.

References

Fatka, O., M. Valent, and P. Budil. 2023. The first healed injury in a hyolith operculum. The Science of Nature 110(50). https://doi.org/10.1007/s00114-023-01879-0.

Kraft, P., V. Vaškaninová, M. Mergl, P. Budil, O. Fatka, and P. E. Ahlberg. 2023. Uniquely preserved gut contents illuminate trilobite palaeophysiology. Nature 622: 545–551. https://doi.org/10.1038/s41586-023-06567-7.

Lee, D.-C., M.‑K. Oh, Y. Zhang, X.‑L. Zhang, J.‑H. Lee, K. Liang, and W. Li. 2025. Two new probable feeding traces of Rusophycus from the Cambrian of China: tracemaker’s behavior and formation mode. Geosciences Journal 29: 1–17. https://doi.org/10.1007/s12303-025-00007-6.

Liu, F., T. P. Topper, L. C. Strotz, Y. Liang, Y. Hu, C. B. Skovsted, and Z. Zhang. 2024. Morphological disparity and evolutionary patterns of Cambrian hyoliths. Papers in Palaeontology 10(2). https://doi.org/10.1002/spp2.1554.

Your Friends The Titanosaurs: Utetitan zellaguymondeweyae

Back almost four years ago when we first waded into Alamosaurus, I noted that "I was waiting on it because I was concerned that it might suffer a taxonomic detonation at any time, so I thought I'd hold off as long as possible." Titanosaurs work on their own schedule. Although people have danced around the issue for decades by suggesting that A. sanjuanensis is dubious or refraining from including some specimens in the species, nobody had taken the step of formally proposing another name for any of the material until Gregory S. Paul in October 2025 with Utetitan zellaguymondeweyae. This genus and species are based on the North Horn specimen that has long been the "practical purposes" type* of A. sanjuanensis.

*Seems like there ought to be a name for non-type specimens that everyone uses instead of the type. "Apatotype", for "deceptive type"? Or "pseudotype" for "false type"? Think like the "Mantel-piece" of Iguanodon mantelli. Brachiosaurus had a whole apatotype species until it was split off for Giraffatitan.

Compact Thescelosaurus Year Ten

Twelve months have rolled along since our previous check on The Compact Thescelosaurus, now clocking in at a decade of existence. With Triassic and Jurassic pseudosuchians for the previous two years, the obvious addition was Cretaceous pseudosuchians, of which there were more than the previous two additions combined. The Cretaceous section wasn't quite so dominated by one group like the Jurassic was by thalattosuchians, but it was a great time to be a notosuchian.

Like this one, Simosuchus clarki. I took this photo when the traveling exhibit visited the Science Museum 11 years ago; Wikipedia is running a photo of the same mount from another stop on the itinerary.