While on a brief trip up north recently (Moose Lake Agate Days, naturally enough!), I spent an afternoon around Sandstone and Banning State Park. The name of the town is a good clue as to what I saw there.
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| Why, yes, sandstone! |
The newest titanosaur is the second from Uruguay and the Guichón Formation. Here's the breakdown:
Genus and Species: Mesetasaurus protector. The genus name is a reference to the locality, Meseta de Artigas. The species name is a reference to the Uruguayan national hero José Artigas, (Soto Núñez et al. 2026). His name is also part of the locality, so we end up with a multilayered reference that boils down to "José Artigas's lizard".
Citation: Soto Núñez, M., F. Montenegro, and D. Perea. 2026. A new aeolosaurini (Sauropoda, Titanosauria) from the Upper Cretaceous of Uruguay. Ameghiniana (advance online publication). doi: https://doi.org/10.5710/AMGH.19.06.2026.3689
Geography and Stratigraphy: The type and only known specimen is from Meseta de Artigas, in the northern part of Paysandú Department, Uruguay. We are in the Guichón Formation, previously noted as the source of the titanosaur Udelartitan celeste (Soto et al. 2024). At that time, an age in the first half of the Late Cretaceous was suggested. This time a somewhat younger age is proposed, "perhaps late Santonian-early Campanian" (Soto Núñez et al. 2026).
Holotype: FC-DPV 3740 (Vertebrate Fossil Collection, Facultad de Ciencias, Universidad de la República), two anterior caudals (Soto Núñez et al. 2026). The authors refer to the more anterior caudal as 3740A and the more posterior as 3740B, and suggest A is the third caudal and B is the sixth.
The two caudals are well-preserved, although unfortunately this is not the same as completeness (the processes are truncated). They are distinct from those of U. celeste, being decidedly aeolosaurine/id/inid in anatomy. They are also somewhat smaller, not that U. celeste was an especially titanic titanosaur in the first place. Not surprisingly given its anatomy, M. protector plots among the aeolosaurs, making it the second record of an Uruguayan aeolosaur after the Asencio Formation caudal in Soto et al. (2022).
I don't have a whole lot to say about this species. To make this post a little less perfunctory, I'd like to go a little farther south. Barrett et al. (2026) have reported the second titanosaur from Antarctica, after the caudal reported by Cerda et al. (2011, 2012). The new material is BAS D.8621.25 (British Antarctic Survey, Cambridge, United Kingdom), a partial anterior caudal that coincidentally enough also has a bit of an aeolosaur appearance. It comes from the early Campanian-age Beta Member of the Santa Maria Formation on James Ross Island. (For those of you keeping track, Cerda et al. also described their specimen, a partial middle caudal, from the Santa Maria Formation. The particular strata have since been placed in the overlying Snow Hill Formation as the Gamma Member.) It is not well preserved and is rather small, at only 59 mm (2.3 in) long, 89 mm (3.5 in) if you include the substantial posterior condyle. This puts it in the company of Magyarosaurus dacus. We can't be sure of the exact growth stage, but there's enough of the neural arch to show it wasn't a very young juvenile. Interestingly, although not described until now, BAS D.8621.25 is actually the first classic dinosaur fossil collected from Antarctica, way back in December 1985 (Barrett et al. 2026).
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| Several views of BAS D.8621.25. Figure 3 in Barrett et al. (2026). CC-BY-4.0. |
References
Barrett, P. M., P. D. Mannion, S. L. Beeston, M. C. Lamanna, B. Clark, A. Otero, J. P. O’Gorman, and M. Evans. 2026. A titanosaurian sauropod dinosaur from the Upper Cretaceous of Antarctica. Acta Palaeontologica Polonica 71(2): 349–362. doi: https://doi.org/10.4202/app.01315.2025.
Cerda, I., A. Paulina Carabajal, L. Salgado, R. Coria, and J. J. Moly. 2011.
The first record of sauropod dinosaurs from Antarctica. Journal of Vertebrate
Paleontology, Program and Abstracts, 2011:86.
Cerda, I. A., A.
Paulina Carabajal, L. Salgado, R. A. Coria, M. A. Reguero, C. P. Tambussi, and
J. J. Moly. 2012. The first record of a sauropod dinosaur from Antarctica.
Naturwissenschaften 99:83–87.
Soto, M., F. Montenegro, F. Mesa, and D. Perea. 2022. Sauropod (Dinosauria: Saurischia) remains from the Mercedes and Asencio formations (sensu Bossi, 1966), Upper Cretaceous of Uruguay. Cretaceous Research 131:105072. doi: https://doi.org/10.1016/j.cretres.2021.105072.
Soto, M., J. L. Carballido, M. C. Langer, J. C. G. Silva Junior, F. Montenegro, and D. Perea. 2024. Phylogenetic relationships of a new titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Uruguay. Cretaceous Research 105894. doi: https://doi.org/10.1016/j.cretres.2024.105894.
Soto Núñez, M., F. Montenegro, and D. Perea. 2026. A new aeolosaurini (Sauropoda, Titanosauria) from the Upper Cretaceous of Uruguay. Ameghiniana (advance online publication). doi: https://doi.org/10.5710/AMGH.19.06.2026.3689.
This year it works out that I'm a little early for the annual "Fossil [Group] of the National Park Service", and we continue our tour of mammal groups with a bit of a grab-bag. As with the marine reptiles, there is not one single group of marine mammals. Rather, several different groups became adapted to the oceans. There are the cetaceans, including whales, dolphins, and porpoises (and yes, dolphins and porpoises are small whales, but you know what I mean). There are the pinnipeds, including animals such as seals, sea lions, and walruses (and yes, there are two distinct groups of pinnipeds called seals, true seals without ears and eared seals that are closer to sea lions; it wouldn't be an Equatorial Minnesota post without mentioning at least a couple of caveats and technicalities, after all). Finally, we also have sirenians (dugongs and manatees) and desmostylians (extinct, kind of hippo-like things), two herbivorous groups that may or may not have been related.
One of the earliest posts here, way back in March 2014, was about an assemblage of high-Arctic Late Cretaceous coprolites that had been part of my graduate work. We did quite a bit with them (Chin et al. 2008), but there's always more that can be discovered. One of the things Chin et al. (2008) noticed was the rarity of body fossils for large potential coprolite producers. They broached the idea that the producers were only around part of the time, living elsewhere during the polar winter and only turning up to feed during the long sunny summer days. Duffy et al. (2026) takes this idea and runs with it.
You may recall that back in 2014 we discussed two major categories of coprolites, those with a dominantly greensand composition and those with a dominantly phosphatic composition. The greensand coprolites had various kinds of inclusions, including crustacean carapaces, bivalves, and squid hard bits* and such (interestingly without evidence of processing by teeth), suggesting consumers that were bottom-feeders, whereas the phosphatic coprolites were loaded with planktonic microfossils, suggesting filter feeding (or feeding on soft-bodied filter feeders) (Chin et al. 2008). The division is nuanced a bit more this time around, with an intermediate group with greensand embedded in phosphate. At the time, we could easily see that the phosphatic coprolites had spiral structures, as in shark excrement. As it turns out, at least some greensand coprolites also have internal tubular structures, although usually not as easy to spot (Duffy et al. 2026). What that means is that most of the coprolite producers therefore had spiral intestinal valves like sharks.
*I had the hardest time figuring out those squid-pen bits when I was working with the fossils. I thought they might be horseshoe crab tails.
There is one group that fits quite well for producing the whole range of coprolites while also not producing abundant body fossils: sturgeons. Sturgeons have spiral valves. The adults don't have teeth to shed, so they aren't leaving hundreds of potential fossils, and they aren't crushing or cutting prey. Rather, they are bottom-feeders that suck in prey items (and sediment). They are also big enough to produce the size range of greensand coprolites. Meanwhile, young sturgeons have small teeth (but they lose them), and they feed on zooplankton. Finally, there are sturgeons today that migrate from marine water to estuaries or shallow marine settings to feed on seasonal food blooms before spawning in freshwater (Duffy et al. 2026). They're about as perfect of "poopetrators"** as we could ask for.
**I still regret nothing!
It's the migration part that particularly interested Duffy et al. Most behaviors are pretty darn difficult to fossilize clearly; or, if you want to get philosophical about it, all behaviors are reflected somewhere in anatomy, but most of them produce effects that are too subtle to pick out or are swamped by the effects of other behaviors. We can look for evidence for migration in certain large land animals such as mammoths by studying stable isotopes in bones. These animals were big enough to travel long distances (and thus drink water in places with different isotopic signatures, for example) and had nice big bones that allow sampling for time sequences. The Devon Island situation is not quite so convenient, but a few lines of evidence are suggestive (Duffy et al. 2026):
All in all, I like it, and I think it's great to learn new tricks from old turds. You never know what will turn up once you start looking at these humble fossils!
References
Chin, K., J. Bloch, A. Sweet, J.Tweet, J. Eberle, S. Cumbaa, J. Witkowski, and D. Harwood. 2008. Life in a temperate Polar sea: a unique taphonomic window on the structure of a Late Cretaceous Arctic marine ecosystem. Proceedings of the Royal Society B 275(1652): 2675–2685. doi: 10.1098/rspb.2008.0801.
Duffy, F., K. Chin, S. Cumbaa, and L. Wilson. 2026. Coprolite evidence for marine vertebrate migration in the warm Cretaceous Arctic. Historical Biology. doi: 10.1080/08912963.2026.2670771.
If you frequent The Compact Thescelosaurus, you may have noticed that several sauropods formerly placed in Titanosauria have been reclassified. (Apropos of nothing, I often wonder what the people who are browsing the sheets think when they see me active. Do they get excited to see what I'm working on? Or is it an inconvenience to whatever searching or sorting they're doing? Sometimes more appear while I'm working. I know it's just a coincidence, but it amuses me to think there's some kind of alert I don't know about that is issued when I show up.) This is not the first time this has happened. Back in 2019 Mannion et al. (2019) led me to move Baotianmansaurus henanensis and Dongyangosaurus sinensis to Titanosauria? (the question mark, the second-to-last refuge of a coward) and Jiangshanosaurus henanensis and Yongjinglong datangi out of Titanosauria altogether. After Beeston et al. (2024), the diamantinasaurs were also put at Titanosauria?. There has now been another purge of Early Cretaceous forms following Mannion and de Souza Carvalho (2026).
(Wait a second... Mannion et al. 2019, Mannion and de Souza Carvalho 2026, Mannion as third author on Beeston et al. 2024... Philip Mannion, stop taking my titanosaurs!)
In this case, the affected species were Hamititan xinjiangensis, Ninjatitan zapatai, and Volgatitan simbirskiensis. Although from different continents and formations, all three share one key characteristic: Supposed Early Titanosaur. SET is almost a curse. As soon as someone starts thinking a particular sauropod represents an Early Titanosaur, it is liable to transform, as if by perverse magic, into something else, and I don't recall that any have actually gotten back to being classified as titanosaurs. The main culprit seems to be that we just don't really have a good grasp on what somphospondyls were up to in their early years. (Well, that and the inevitability that the closer you get to the base of any lineage, the more generalized the taxa. And, perhaps, sometimes people might get too enthusiastic hoping for an Early Titanosaur and read a bit more into specimens than is warranted.) Somewhere in there is the lineage that led to titanosaurs, but until they established their monopoly, it's difficult to distinguish that thread from various also-rans, plus other sauropod groups that may be confused with them when you only have a couple of bones.
Backing up for a moment, Mannion and de Souza Carvalho (2026) is not primarily about reclassifying three disparate Early Cretaceous titanosaur-like sauropods. It's actually a redescription of Triunfosaurus leonardii, another victim of SET. In a minor upset for how these descriptions usually go, the type material doesn't turn out to be chimeric (the "middle-posterior caudals" are more likely anterior, but that's about as close as it gets to major anatomical reinterpretation). This then turned into an opportunity to look at the relationships of five SETs: T. leonardii, the three mentioned above, and Tengrisaurus starkovi. Running equal weighting (EQW) and extended implied weighting (EIW) against their data, they found the following placements:
Of these five, the two with the best cases to be recognized as true Early Titanosaurs are Tengrisaurus, which always ended up within it, and Triunfosaurus, on the doorstep. Although it's tempting to take one and plant a flag for the origin of the group, the situation is too messy for anything that neat, as noted by Mannion and de Souza Carvalho (2026). There's almost no record of somphospondyls in the Late Jurassic and earliest Cretaceous, when by definition they must have been around (because their sister group Brachiosauridae was around), and when we do start seeing them, they're all over the place (Mannion and de Souza Carvalho 2026).
References
Beeston, S. L., S. F. Poropat, P. D. Mannion, A. H. Pentland, M. J. Enchelmaier, T. Sloan, and D. A. Elliott. 2024. Reappraisal of sauropod dinosaur diversity in the Upper Cretaceous Winton Formation of Queensland, Australia, through 3D digitisation and description of new specimens. PeerJ 12:e17180. doi: 10.7717/peerj.17180.
Mannion, P. D., and I. de Souza Carvalho. 2026. Re-evaluation of the Early Cretaceous titanosauriform sauropod dinosaur Triunfosaurus leonardii from the Triunfo Basin, Brazil: implications for the initial radiations of Somphospondyli and Titanosauria. Zoological Journal of the Linnean Society 207(1): zlag073. doi: 10.1093/zoolinnean/zlag073.
Mannion, P. D., P. Upchurch, X. Jin, and W. Zheng. 2019. New information on the Cretaceous sauropods of Zhejiang Province, China: impact on Laurasian titanosauriform phylogeny and biogeography. Royal Society Open Science 6(8):191057. doi: 10.1098/rsos.191057.
There are so many outstanding fossil sites and productive formations (this link is just a sample) that it's not really feasible to be conversant with all of them and still have time for normal human interactions and responsibilities. I'd love to have that knowledge, but realistically I'd be doing pretty darn good if I only knew North America. A few of them on the linked list are not all that far from the Late Ordovician of the Twin Cities in time and space. We saw the Winneshiek Shale briefly when looking at the Decorah impact crater. Another example is the Waukesha Biota in southeastern Wisconsin, dating to the early Silurian. This assemblage came to mind because of the publication this month of Waukartus muscularis, a cousin to modern millipedes (Briggs et al. 2026).
The Waukesha Biota is found in basal dark shale of the Brandon Bridge Formation, otherwise composed of reddish dolomite. The productive beds are quite limited in distribution, described as extending about 350 m (about 1,150 ft, or not much more than a fifth of a mile) (Briggs et al. 2026). The strata were deposited at the toe of an erosional scarp at the beginning of a marine transgression (Briggs et al. 2026); think of them as akin to sedimentary filler. One of the things that's easy to forget when dealing with Paleozoic marine assemblages that are packed with shells and other hard parts, like our old friend the Decorah Shale, is that there were also a lot of things that just didn't fossilize well, particularly "worms" and arthropods that did not have the convenient durable exoskeletons of trilobites. You can find evidence of them through burrows and microfossils, but it's just not the same thing (it's hard to establish the taxonomic diversity, for one thing!). The Waukesha Biota is a Konservat-Lagerstätte, meaning the preservation is exceptional, and so we get to see those soft-bodied organisms. In fact, the Waukesha Biota is kind of Bizarro World as far as the Paleozoic is concerned, with uncommon brachiopods, crinoids, and mollusks, but abundant and diverse arthropods and "worms" (Wendruff et al. 2020). Preservation seems to have been greatly enhanced by microbial mats (Wendruff et al. 2020).
One of these otherwise unlikely fossils is the present subject, Waukartus muscularis. The genus name refers to Waukesha and limbs, which are an important part of the story, and the species name refers to the preservation of musculature (Briggs et al. 2026). This animal is not actually something that was just found; reports of this fossil animal go back to the 1980s. It was mentioned in the earliest papers on the Waukesha Biota (Mikulic et al. 1985a, 1985b) as a "myriapod-like animal". (Myriapoda is the group including centipedes and millipedes.) Specimens representing parts and sometimes counterparts of nearly three dozen individuals have been found. They top out at a little less than 3 cm (1.2 inches) long and perhaps 10% of that wide, and would have looked rather like chunky basic millipedes from a human's-eye-view. The body features a head, as many as 11 trunk segments (each segment looking deceptively like they were actually two parts), and a terminal segment. Despite the fairly large sample size, there isn't an especially well-preserved head, but there appears to have been four appendages on the head and eyes, likely on stalks. The terminal section is also poorly preserved but had a pair of blade-like projections on the underside (Briggs et al. 2026).
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| Some of the 35 individuals of Waukartus muscularis, including the holotype (A–E, part and counterpart) (Figure 1 in Briggs et al. 2026, which see for full caption; 5 mm scale in A–D and F, 2 mm for E, H, and I, 1 mm for G and J; ). CC-BY-4.0. |
The limbs are the feature that has drawn the most comment. There is one pair of (rather stocky) limbs per trunk segment, unlike true millipedes, which have two. (Hence the scientific name for the group, "Diplopoda", meaning "double feet".) They are uniramous rather than biramous, the ancestral arthropod condition. A uniramous limb has "one branch", whereas a biramous limb forks into two branches. Many aquatic arthropods have biramous appendages and use one branch for locomotion and the other for respiration (think trilobites). Terrestrial insects, arachnids, and myriapods have uniramous limbs, and this has long been thought to be a specific adaptation to living on land (having feathery gill-like things on your limbs like trilobites did isn't quite as useful in the open air). Waukartus, though, was found in marine shales with nothing thought to be definitively terrestrial, and so is thought to have been marine as well. Therefore, uniramous limbs may not have been a terrestrial adaptation, at least in myriapods, but something that came in handy when the move occurred (exaptation, or pre-adaptation if you're older than you'd like to admit) (Briggs et al. 2026). It may have been respiring through its cuticle (Briggs et al. 2026), which is a neat trick you can get away with when you're a little less than 3 cm long.
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| Waukartus muscularis out for a stroll (head lower center, terminal segment upper left) (Figure 5 in Briggs et al. 2026, restoration by Leia Francis). CC-BY-4.0. |
References
Briggs, D. E. G., J. C. Lamsdell, J. Kluessendorf, and D. G. Mikulic. 2026. A marine stem-myriapod from the Silurian Waukesha Lagerstätte, Wisconsin, USA: terrestrial traits pre-date the transition to land. Proceedings of the Royal Society B: Biological Sciences 293(2070). doi: 10.1098/rspb.2026.0131.
Mikulic, D. G., D. E. G. Briggs, and J. Kluessendorf. 1985a. A Silurian soft-bodied biota. Science 228: 715–717.
Mikulic, D. G., D. E. G. Briggs, and J. Kluessendorf. 1985b A new exceptionally preserved biota from the Lower Silurian of Wisconsin, USA. Philosophical Transactions of the Royal Society of London. B, Biological Sciences 311: 75–85.
Wendruff, A. J., L. E. Babcock, J. Kluessendorf, and D. G. Mikulic. 2020. Paleobiology and taphonomy of exceptionally preserved organisms from the Waukesha Biota (Silurian), Wisconsin, USA. Palaeogeography, Palaeoclimatology, Palaeoecology 546(109631). doi: 10.1016/j.palaeo.2020.109631.
The latest friendly titanosaur to come along hails from Morocco, making it the first named Moroccan titanosaur but not the first record. Other occurrences are mentioned here. For some reason it seems like the new names are always specimens that weren't included. Still lots of titanosaurs out there!
You never know where interesting geology will turn up. I've been poking around the Twin Cities for years and I'm still coming across places I'd never dreamed were there. Case in point: today's pictorial topic, Fish Creek Canyon. I know the name sounds like a place you might find in Montana or Idaho, where the water is cold, the fish are biting, and the bears are waiting for you to wander away from your cooler, but this particular Fish Creek Canyon is just about where St. Paul, Maplewood, and Woodbury meet. It's not a huge canyon, and I don't imagine the fish are very large these days, but it's the kind of little hidden backyard gem that makes exploring worthwhile. I visited it with a friend back in late November, not long before our on-off winter hit the "on" switch for the first time.
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| Down in the valley on a great fall day, with Fish Creek in view. |
Fish Creek is spread across two city jurisdictions. Maplewood has Fish Creek Natural Area, which is mostly the heights above 494 on the south and east and Highway 61 on the west. If you scout around, you'll find that this is one of a handful of small bluff-top parks between Battle Creek Regional Park and 494 overlooking 61. Adjacent on the north is the actual canyon of Fish Creek, which is within St. Paul and owned by Ramsey County Parks but apparently not organized at the moment, per se. The area is also largely within Mississippi National River and Recreation Area (so I'm also *officially* curious). The bluff-top lands are well worth walking around in their own right. There are no official access points to the canyon from the bluff-top, but it is possible to reach the canyon via social trails, leading to a steep descent into the narrow valley.
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| The view from the bluff, with the November afternoon sun on a clear day. |
The canyon itself is not vast or deep. The water power that carved the original ravine has long since dwindled to a hoppable creek. And yet, what the creek has cut for itself is such a perfect little feature, complete with a miniature waterfall.
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| You're walking along, and then the creek disappears. |
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| A miniature waterfall. |
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| There ought to be gnomes. |
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| It widens a bit going down (note that this feature was put in by people, presumably for water management, although it's not bad for aesthetics, either). |
The rock that has been cut through here is the St. Peter Sandstone in its case-hardened form, producing the steep-sided, narrow slot. If you've driven Highway 61 near here, you may have noticed how the St. Peter sinks out of view for most of the stretch between Battle Creek and Camels Hump in Cottage Grove. It's still there, it just only shows its face sporadically, and this is the most picturesque place to find it. Interestingly, the location of Frederick Sardeson's "Highwood" collecting locality for St. Peter Sandstone fossils was supposedly a little north of here, about a mile and a half south of Battle Creek Park on 61. This works out to about the area where the ravines now occupied by Highwood Avenue and Springside Drive empty out, so our old friend was successfully trying his luck out here back in the 1890s.
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| Sometimes the St. Peter feels like crumbling, and sometimes it feels like holding a wall. |
There are two great lost fossil sites in the Twin Cities area. (The Brickyards don't count; they're not lost, they just aren't open to collection.) One is the Johnson Street Quarry, where workers cut into a bed in the Hidden Falls Member of the Platteville Formation that had unusually abundant echinoderms. As described in Sloan et al. 1987: 200, "Sardeson mined out a spot in this unit in the old Johnson Street Quarry in Minneapolis (now filled with garbage, and covered with Interstate 35) that produced about 20 specimens of the starfish Protopalaeaster narrawayi, several specimens of the crinoid Cremacrinus arctus (Fig. 16.2), edrioasteroids, cystoids, brachiopods, bryozoans, molluscs, and graptolites." This is slightly out of date; instead of a dump, there's now a Quarry Shopping Center with a Cub Foods, Home Depot, and Target, although even with all those options you can't get an edrioasteroid there anymore. Regardless of the exact character of the overburden, it seems unlikely that anyone will be doing any paleontological follow-up there anytime soon. The other locality is the Afton graptolite locality in the St. Lawrence Formation. We already had a post on why this locality was important; what I'm curious about is where exactly it was. A locality, even if "lost", had to have been *somewhere*, and apart from the scientific and historic interest, there very well could be similar fossils in rocks nearby. Indeed, Hughes and Hesselbo (1997) reported graptolites in the lowest strata of the St. Lawrence Formation in their Afton section, where collection may have postdated the road work that destroyed the classic location. For some reason, despite its “classic” nature, nobody ever saw fit to just put a pin on the map. What clues do we have?
There's an electronic stack of topics I've considered for blog posts, and within that a subset earmarked "do before hanging up the keyboard". One of these, almost from the very beginning, was a bibliography of Quaternary vertebrate locality inventories like the kind Oliver Perry Hay pulled together. Being a detail hoarder, I've always loved having this kind of information. Maybe I'm the only one, but it's my blog and if I want to put together a page of old locality lists, I'll do it. One might think "If I have to find this information, all I need is the Paleobiology Database" and call it a day, to which I just smile and nod politely. Anyway, you can find the page here.
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| Pop quiz! Mammoth or mastodon? |
A couple of thematic inventories have just been published with the involvement of the NPS Paleontology Program. One of these is the first part of a paleobotany series, eventually to be three parts covering the Cenozoic, Mesozoic, and Paleozoic. The first part, Matel et al. (2026), covers the Cenozoic and is available for download through February 24. This is one of the projects from the National Park Service-Paleontological Society Paleontology in the Park Fellowship Program. In it, we've worked to provide information on Cenozoic paleobotany for every park unit where it's known. (It's easier to do concisely when the record is one piece of petrified wood than when the park is, say, Florissant Fossil Beds National Monument, so necessarily the scale of detail varies.) For practical reasons the focus is on macrofossils, with pollen and such as an adjunct.
The other new inventory, Santucci et al. 2025, covers NPS cave paleontology from 2002 to 2023. This is presented as an update of a previous report (Santucci et al. 2001). I'm taking the opportunity here to add a postscript because the past couple of years have been very productive. First of all, we came upon a new park record: Kalaupapa National Historical Park, on the north side of Molokaʻi, Hawaii. Although main focus here is historical, this park has a very interesting geological story too, involving the collapse of a volcano and the subsequent growth of another smaller volcano. The smaller volcano is now extinct, leaving behind Kauhakō Crater. Inside the crater people have found bird bones in a cave. One is a partial ulna of the extinct flightless ibis Apteribis glenos, the other is a coracoid of Pterodroma hypoleuca, the modern Bonin petrel, which does not live on the island today (Olson and James 1982, 1991).
Also, a few things in press or under study have progressed in the past couple of years. Avid readers of this blog will know that the "musk ox" of Muskox Cave at Carlsbad Caverns National Park is now named Speleotherium logani (White et al. 2025). The Cumberland Bone Cave monograph has also been published (Eshelman et al. 2025). Most notable, though, has been the publication of work on Mammoth Cave National Park. The park inventory (Toomey et al. 2025) was mentioned here a few months back. There has also been a good short summary of the Mississippian vertebrates (Hodnett et al. 2024a) and no fewer than five new taxa: ctenacanthiforms Troglocladodus trimblei and Glikmanius careforum (Hodnett et al. 2023); petalodonts Clavusodens mcginnisi (Hodnett et al. 2024b) and Strigilodus tollesonae (Hodnett et al. 2024c); and holocephalan Macadens olsoni (Hodnett et al. 2025).
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| Clavusodens mcginnisi on the prowl, among the crinoids. Illustration by Benji Paysnoe for the National Park Service. |
References
Eshelman, R. E., C. J. Bell, R. W. Graham, H. A. Semken, Jr., C. B. Withnell, S. G. Scarpetta, H. F. James, S. J. Godfrey, J. I. Mead, J.-P. Hodnett, and F. V. Grady. 2025. Middle Pleistocene Cumberland Bone Cave Local Fauna, Allegany County, Maryland: a systematic revision and paleoecological interpretation of the Irvingtonian, Middle Appalachians, USA. Smithsonian Contributions to Paleobiology 108.
Hodnett, J.-P., R. Toomey, H. C. Egli, G. Ward, J. R. Wood, R. Olson, K. Tolleson, J. S. Tweet, and V. L. Santucci. 2023. New ctenacanth sharks (Chondrichthyes; Elasmobranchii; Ctenacanthiformes) from the Middle to Late Mississippian of Kentucky and Alabama. Journal of Vertebrate Paleontology 43(3): e2292599. doi: 10.1080/02724634.2023.2292599.
Hodnett, J.-P., R. Toomey, R. Olson, K. Tolleson, R. Boldon, J. Wood, J. S. Tweet, and V. L. Santucci. 2024a. Sharks in the dark: Paleontological resource inventory reveals multiple successive Mississippian Subperiod cartilaginous fish (Chondrichthyes) assemblages within Mammoth Cave National Park, Kentucky. Park Stewardship Forum 40(1): 53–67. doi: 10.5070/P540162921.
Hodnett, J.-P. M., H. C. Egli, R. Toomey, R. Olson, K. Tolleson, R. Boldon, J. S. Tweet, and V. L. Santucci. 2024b. Obruchevodid petalodonts (Chondrichthyes, Petalodontiformes, Obruchevodidae) from the Middle Mississippian (Viséan) Joppa Member of the Ste. Genevieve Formation at Mammoth Cave National Park, Kentucky, U.S.A. Journal of Paleontology 98(6): 1087–1097. doi: 10.1017/jpa.2024.40.
Hodnett, J.-P., R. Toomey, R. Olson. J. S. Tweet, and V. L. Santucci. 2024c. Janassid petalodonts (Chondrichthyes, Petalodontiformes, Janassidae) from the middle Mississippian (Viséan) Ste. Genevieve Formation, Mammoth Cave National Park, Kentucky USA. Historical Biology 36(9):1783–1792. doi: 10.1080/08912963.2023.2231955.
Hodnett, J.-P., R. Toomey, H.-D. Sues, V. Santucci, K. Tolleson, and J. Tweet. 2025. A new euchondrocephalan chondrichthyan (Chondrichthyes, Euchondrocephali) from the Middle Mississippian (Viséan) Joppa Member of the Ste. Genevieve Formation at Mammoth Cave National Park, Kentucky, USA and a reassessment of the Lower Mississippian (Tournaisian-Viséan) “Helodus” coxanus Newberry, 1897. New Mexico Museum of Natural History and Science Bulletin 100: 87–93.
Matel, T. P., I. B. Huegele, C. R. Cace, K. M. M. Bober, L. D. Boucher, V. E. McCoy, E. J. Hermsen, S. R. Manchester, C. C. Visaggi, J. S. Tweet, and V. L. Santucci. 2026. Cenozoic paleobotanical resource inventory of the National Park System. Elements of Paleontology. Cambridge: Cambridge University Press. 10.1017/9781009770477.
Olson, S. L., and H. F. James. 1982. Prodromus of the fossil avifauna of the Hawaiian Islands. Smithsonian Contributions to Zoology 365: 1–59.
Olson, S. L. and H. F. James. 1991. Descriptions of thirty-two new species of birds from the Hawaiian Islands. Part I. Non-Passeriformes. Ornithological Monographs 45:1-88.
Santucci, V. L., J. Kenworthy, and R. Kerbo. 2001. An inventory of paleontological resources associated with National Park Service caves. National Park Service Geological Resources Division Technical Report NPS/NRGRD/GRDTR-01/02.
Santucci, V. L., J.-P. Hodnett, P. Seiser, J. S. Tweet, and J. Wood. 2025. National Park Service cave paleontology: 2002-2023. Journal of Cave and Karst Studies 87(4):108–116. doi: 10.4311/2024PA0119.
Toomey, R. S., J. S. Tweet, and V. L. Santucci , editors. 2025. Mammoth Cave National Park: Paleontological resource inventory (public version). Science Report NPS/SR—2025/243. National Park Service, Fort Collins, Colorado. https://doi.org/10.36967/2308547
White, R. S., J. I. Mead, and G. S. Morgan. 2025. Logan's austral scrubox, a new ovibovine (Mammalia: Artiodactyla: Bovidae) from Muskox Cave, Eddy County, New Mexico. New Mexico Museum of Natural History and Science Bulletin 101: 473–494.
What do you get the formation that has five established titanosaur species and a couple of ne'er-do-wells? A sixth titanosaur, of course!
All joking aside, it's misleading to think of "Titanosauria" as a relatively small-scale group like Diplodocidae or Brachiosauridae when it's really a massive, sprawling complex including multiple distinct lineages that essentially monopolized all things sauropod in the Late Cretaceous. We talk of the Morrison Formation having diplodocids and dicraeosaurids and camarasaurids and brachiosaurids and whatever else, and that may sound more impressive than the Bajo de la Carpa Formation having six different titanosaurs. "What? You need all those titanosaurs?" "All those titanosaurs" are functionally replacing each of those smaller clades of the Morrison, as well as apparently just about everything else large and herbivorous. It's just the relationships and smaller divisions are still fuzzy. Eventually, we'll have a better grasp. But enough philosophy! Let's bring on today's guest, Yeneen houssayi. (And thank you to Alberta Claw and Stephen Poropat for supplying me with the paper!)
You wait long enough, and history starts to pile up, like falling snow (only history doesn't melt away and has a way of churning back to the surface). In paleontology, we've just passed two centuries with Megalosaurus (1824) and Iguanodon (1825). I was thinking about the topic of this post and how it was a relatively recent name, even though it felt old, but then realized that 1960 is not quite as recent as it was when I first read about Silvisaurus in the mid-1980s. I'm not tracking stats to any great degree, but it's probably safe to say there aren't a lot of people reading this who were mid-career when the scientific description of Silvisaurus condrayi was published.
My first love in science, the first thing that really interested me intellectually, was astronomy. It was early 1986, around the time I turned five. (I was a weird kid.) Anyway, I was really into Star Wars, and wanted to go to space and explore different planets. I had (and still have) a book, "Astronomy Today", with lots of great illustrations and diagrams; sure, I wasn't *reading it* reading it yet, but I got a lot out of those illustrations. Then I discovered that, contrary to the movies, we couldn't exceed the speed of light, so we were basically stuck here. It's a nice enough solar system, but, well, y'know... the realization was terribly disappointing, and space went from an obsession to a pleasant side interest (I still love going to the planetary science sessions when attending GSA annual meetings).
At the time, we had just moved to Tucson. There's a lot of difference geologically between southern Arizona and south-central Minnesota; for starters, a lot more of the geology is right there at the surface in Arizona, and a lot more of it is Mesozoic. Still, Arizona is not Dinosaur Central; it's more like a nice suburb. After nearly 40 years I can't say what it was about dinosaurs that caught my eye. Maybe it was the British Museum toys at the Arizona-Sonora Desert Museum. Whatever it was, the feedback loop started: kid shows interest, people around start feeding the interest, interest intensifies...
The step that sealed the interest in paleontology came at Christmas 1986. There was one rectangular gift with unusual heft. Under the wrapping paper was the Normanpedia, "The Illustrated Encyclopedia of Dinosaurs". Could I read it? Well, okay, sure, I was a great reader for five-and-a-half, but it's safe to assume it took some time and practice before I was conversant (and the book probably accelerated the process, so on top of his scientific accolades, David Norman can take some credit for improving at least one kid's literacy). But don't forget the second word of the title! The restorations, the photos, the skeletal diagrams! I went through reams of paper tracing skeletons, then drawing outlines around them on the other side of the sheets. That way, there would be a skeleton on one side, a drawing on the other, and if I held the paper to the light, both of them would be there, skeleton revealed inside the body. I loved that book to death; it's in a box of mementos somewhere at home, dust jacket long gone, spine held together by heavy-duty tape, a slip of paper taped over the "dinosauroid" drawing because it creeped me out. I had to get a second copy in the Nineties because the first was falling apart. The second copy also started falling apart after about ten years, so now I'm on my third. Anyway, no particular moral or commentary or insight this time, just wanted to share some holiday nostalgia. See you in 2026!
Answer: They are the only three U.S. states for which I can't find a record of a mastodon or mammoth.
Hawaii is not exactly a surprise. Certainly proboscideans can swim, but if you want a large mammal that can swim halfway across the Pacific Ocean without dying, it's going to look and function suspiciously like a cetacean instead.
Delaware and Rhode Island, on the other hand, doubtless had mammoths and mastodons tromping over them at some point, because they're surrounded by states (and submerged coastal plains) that have their fossils. They're just both small states not blessed with the most helpful geology, and mantled by urbanization. Rhode Island gets an additional strike from being subjected to glaciers. Someday the fossils will turn up, though. Maybe there's already something; maybe somebody has a funny-looking doorstop they found in 1973 and never paid much attention to, or there's a notice in a local paper published in 1989 about a couple of campers who found a tooth and turned it over to a local historical society, or somebody will hop into the comments of this post and say, "Hey, ya numbskull, ya missed this!"
Now, the nitty-gritty: every once in a while I'd wondered how many states have records of mammoths or mastodons, and occasionally puttered in a source or two. This time I decided to be more thorough. I checked the Hay inventories, Neotoma (which has the old FaunMap contents), and the Paleobiology Database for starters. These three together narrowed down the list of absences to Delaware, Hawaii, New Hampshire, and Rhode Island. If you're curious: Hay didn't have mammoths or mastodons in those four plus Maine and basically omitted Alaska, but had all the rest plus the District of Columbia; Neotoma was missing the four plus Alabama, Mississippi, North Dakota, Vermont, and D.C.; and the PBDB was missing the four plus Connecticut, Louisiana, Massachusetts, Maine, New Jersey, North Dakota, Vermont, and D.C. This is why it's important to check multiple sources; no one source has everything.
Putting aside Hawaii, that left Delaware, New Hampshire, and Rhode Island to check out. Now was the time to drill into the resources of various search engines and museum databases/iDigBio. For the record, I leaned on Google Scholar, Google Books, and the news tab of regular Google (not Google News), with various permutations of "mammoth", "mastodon", "Mammuthus", and "Mammut" plus a state, with additional terms as necessary to knock out particularly obnoxious false positives. This eliminated New Hampshire, which now has a record from a find on land [note, 2026/06/02: nope, see comment below] after a couple of teeth found nearby offshore. For a short time I'd thought I'd found one for Delaware, thanks to a short piece in the Winter 1994 issue of "First State Geology" (p. 3). The article mentioned that Jeremy Cloutier of Milford had donated a mastodon tooth to the Delaware Geological Survey, but the tooth derived from offshore clam dredging and therefore doesn't count for our purposes. So, for now, we're still at Delaware, Hawaii, and Rhode Island. (Still a few field days 'til Christmas, if you're feeling like pulling off a Christmas miracle!)
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| "Archie", the world's largest mounted Columbian mammoth skeleton, on display in Elephant Hall, University of Nebraska State Museum. |
Allow me to indulge in a bit of puffery and congratulations...
Let's go back a couple of years ago. We in the NPS Paleontology Program knew that the new edition of the Utah Geological Association's "Geology of Utah's Parks and Monuments" was not going to have an update of the overall park paleontology chapter. (Which is fair enough, since it was 35 pages in the previous update and was not getting shorter. The new volume is still well worth seeing, though, especially if you haven't seen the earlier editions.) We also were working with Tut Tran, then putting the finishing touches on the paleontological inventory report for Bryce Canyon National Park. These touches included some clever biostratigraphic figures of a kind we'd never used before. Thus was an idea born: Tut would prepare a standalone article featuring similar figures for the rest of Utah, with contributions from various luminaries in NPS and Utah paleontology. This article, "A Visual Paleontological Inventory of Utah’s National Park Service Areas", is now available for your edification and reading pleasure. Congrats, everyone!
Utah has 13 National Park Service units: five parks (Arches, Bryce Canyon, Canyonlands, Capitol Reef, and Zion), six monuments (Cedar Breaks, Dinosaur, Hovenweep, Natural Bridges, Rainbow Bridge, and Timpanogos Cave), one national recreation area (Glen Canyon), and one national historical park (Golden Spike). (There are a few other units in the state designated as "National", such as Grand Staircase-Escalante National Monument and Flaming Gorge National Recreation Area, but they are under different agencies.) This is omitting a few NPS trails (California, Mormon Pioneer, Old Spanish, and Pony Express) and the Virgin Wild and Scenic River, which are complicated to deal with because they don't have formal boundaries the same way the other types of units do; one day I'd like to go over rivers and trails thoroughly, but that's for another day. Anyway, the 13 units give Utah one of the best state-wide paleontological records in the NPS, up there with Alaska and California. Each one of them has something, although certainly some have more than others. The five national parks, Dinosaur NM, and Glen Canyon NRA are the most abundantly supplied and feature the longest records.
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| Figure 2, comparing the stratigraphic records of Utah's NPS units. Click to embiggen. CC BY 3.0 US. |
These seven units are fairly similar stratigraphically and in fossil content, with the exceptions of Bryce Canyon (getting started when most of the others are tapering off) and to a certain extent Dinosaur (longer record and some different formations due to distance from the other units). The classic assortment of rocks generally includes the Permian, Triassic (Moenkopi and Chinle), Jurassic (Glen Canyon Group, San Rafael Group, and Morrison Formation), and part of the Cretaceous (Cedar Mountain Formation and Naturita, the old Dakota Sandstone). Arches, Canyonlands, Capitol Reef, and Glen Canyon in particular are geological and paleontological siblings. There are some older and some younger formations, but generally the sweet spot is between about 300 and 100 million years ago.
You might think, based on Dinosaur's reputation, that the Morrison Formation is the big producer, but as it turns out that's not the case. The Lower and Middle Jurassic rocks that make up so much of the scenic vistas, although not big bone producers, are perhaps the most iconic fossil producers with their fossil tracks. (Which is fitting for a park, because just like you can't put a vista in a box and take it somewhere else, fossil tracks are best appreciated where they are found.) The Chinle and Moenkopi are also good but a bit overlooked (Utah's Chinle having the excuse of being in the shadow of Arizona and New Mexico). Another interval that is very productive is the late Quaternary: several units have cave/rock shelter assemblages, usually thanks to packrats, with the assistance of other animals. Glen Canyon in particular is noted for dung caves, including Bechan Cave ("big feces" in Navajo) and its supply of mammoth dung. The Cretaceous is coming into its own with work at Bryce Canyon, Dinosaur, and Glen Canyon; in fact, work on the Cretaceous made our article outdated almost the day it was published, thanks to Pahasapasaurus gillettei from the Tropic Shale of Glen Canyon (Schmeisser McKean 2025). We were able to get October's Athenar bermani in there in the proof stage, but the plesiosaur managed to wait just long enough. (Darn it.)
This article is not a rewrite of the NPS paleontology chapters in the old UGA volumes. It sets out to do different things and is much more visual in focus, rather than textual. (It even has its own nifty cover image, done by artist Benji Paysnoe in the spirit of the great "Vertebrate Paleontology of Utah" volume.) The main things you will find in this that you won't find in the older chapters are Tut's outstanding diagrams and a grand 62-page appendix listing fossil taxa from each park unit by formation, with citations. Between the nine authors, I think we were able to put together a pretty darn comprehensive appendix, although doubtless we missed something. (If you spot something, let us know, so it can be included if we get the opportunity to do an update.) Me, of course, I'm hoping for more Paleozoic invertebrates, but I'm not turning up my nose at anything!
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| Figure 39, a sample diagram (Rainbow Bridge NM). You should go to the article to see what's been found at the big parks! CC BY 3.0 US. |
We're all excited for this to be out, and we hope you find it useful as well, as a unique look at the paleontological riches of Utah's NPS units.
References
Schmeisser McKean, R. L. 2025. A new species of Pahasapasaurus (Plesiosauria: Polycotylidae) from the Upper Cretaceous Tropic Shale (lower Turonian) of southern Utah, U.S.A. Cretaceous Research 106269. doi: https://doi.org/10.1016/j.cretres.2025.106269.
Tran, T., A. R. C. Milner, J. S. Tweet, D. D. DeBlieux, R. Hunt-Foster, A. B. Shaffer, J. I. Kirkland, E. Warner-Cowgill, and V. L. Santucci. 2025. A visual paleontological inventory of Utah’s National Park Service areas. Geology of the Intermountain West 12: 221–292. doi: https://doi.org/10.31711/giw.v12.pp221-292.
It would be easy to think that we've got the large Late Pleistocene mammals of North America pretty well locked down. After all, every self-respecting animal of that stripe has a long list of synonyms and pseudonyms, sometimes going well back into the 19th century. (All right, to be fair, horses are a mess if you look closely.) Even here, though, there can be surprises. Recently we had the Pacific mastodon and the re-establishment of the dire wolf in its own genus, and now we have the recognition of an entirely new genus and species of scrubox, Speleotherium logani (White et al. 2025).
If you know a bit of Greek, you'll get the idea that the name indicates we're dealing with a beast ("therium") associated in some way with caves ("speleo"). The holotype and best specimen of Speleotherium logani did indeed come from a cave, and in fact was partially encrusted with cave deposits (which certainly give it an unusual look but haven't made it easy to prepare or interpret). A few years ago the skull was photographed for a photogrammetric model, and you can take it for a spin and see the deposits, particularly on the right side of the face. This specimen and others were discovered in 1976 in what is now called Muskox Cave in its honor, within Carlsbad Caverns National Park, New Mexico. The species name honors the discover, Lloyd Logan. Although the name literally translates to "Logan's cave beast", the authors suggest the common name "Logan's austral scrubox" instead (White et al. 2025).
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| Figure 31 from Kottkamp et al. (2020), used in several other places as well (including White et al. 2025). You're looking at the back of the skull, going off to the right. Other bones are also visible, as well as the coating of cave mineralization. |
The Muskox Cave fossils were long thought to belong to the shrubox Euceratherium (which is the name used for it at the 3D model above). That's how we labeled them in recent inventories of the park's fossils, for example (Kottkamp et al. 2020, 2022). It was not until White et al. began to prepare the specimens that it became apparent that there was something else here. Euceratherium has a narrow "forehead" and complexly twisted horn cores, whereas the Muskox Cave skull has a broad "forehead" and less curvy cores. The metapodials (metacarpals and metatarsals) are also shorter and stockier, which twigged something else; the authors knew of similarly proportioned metapodials of an animal similar to but much smaller than Euceratherium from sites in Mexico and Belize, but had never had a face to go with them. Speleotherium is therefore not a "one-off" but something that had a fairly wide range in North and Central America, and there are probably other examples that are currently listed as Euceratherium (White et al. 2025). The metapodial anatomy is rather similar to the same bones of the takin (White et al. 2025), which is a Himalayan bovid that's kind of in-between things like muskox, sheep, and goats in appearance and habits. That may give an idea of what Speleotherium was doing. The short and stocky metapodials suggest it was active in rugged terrain, but based on fossil distribution it was not limited to higher elevations (White et al. 2025).
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| The skull in the process of being captured by photogrammetry (producing the 3D model linked above), showing its best side. Figure 36 in Kottkamp et al. (2020). |
Given we're dealing with National Park Service fossils, you can imagine it's been on my radar. It showed up in the 2001 inventory of NPS cave fossils as what is now a grainy photo (Santucci et al. 2001), but we've since been able to feature the image in higher resolution glory for Carlsbad-specific inventories (Kottkamp et al. 2020, 2022). There's been a soft spot for it around here; it even got a coloring page, now part of the "Cenozoic Life in the National Parks" coloring book (just mentally substitute Speleotherium for Euceratherium as the label; the coloring page was prepared long before the bones got their new name).
References
Kottkamp, S., V. L. Santucci, J. S. Tweet, R. D. Horrocks, E. Lynch, and G. S. Morgan. 2020. Carlsbad Caverns National Park: Paleontological resource inventory (public version). Natural Resource Report NPS/CAVE/NRR—2020/2148. National Park Service, Fort Collins, Colorado.
Kottkamp, S., V. L. Santucci, J. S. Tweet, R. D. Horrocks, and G. S. Morgan. 2022. Pleistocene vertebrates from Carlsbad Caverns National Park, New Mexico. New Mexico Museum of Natural History and Science Bulletin 88:267–290.
Santucci, V. L., J. Kenworthy, and R. Kerbo. 2001. An inventory of paleontological resources associated with National Park Service caves. NPS Geologic Resources Division, Denver. Technical Report NPS/NRGRD/GRDTR-01/02.
White, R. S., J. I. Mead, and G. S. Morgan. 2025. Logan's austral scrubox, a new ovibovine (Mammalia: Artiodactyla: Bovidae) from Muskox Cave, Eddy County, New Mexico. New Mexico Museum of Natural History and Science Bulletin 101: 473–494.
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.
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| 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.
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| 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.
