The Continuing Story of Nanosaurus agilis

Recently Barrett and Maidment (2025) published a paper on the state of Morrison hypsilophodont-things, which is of great interest here because after two long posts on Nanosaurus agilis, we're solidly invested in its fate. How did it fare? Short answer: not very well. But, on the other hand, neither did anybody else. Well, Drinker was shown more appreciation than probably anyone has given it since 1990, but that's not saying much.

So, what to call these happy fellows at the Denver Museum of Nature and Science?

Barrett and Maidment (2025) went over the type specimens of N. agilis, N. rex (Othnielia), Laosaurus celer, L. consors (Othnielosaurus), L. gracilis, and, via illustrations, Drinker nisti. (There is a slight advance on Carpenter and Galton 2018, in that we now get the implication that Bob Bakker has D. nisti as opposed to the whereabouts being unknown.) They find none of the type specimens to be diagnostic. The one that comes off the worst is L. consors. Material cataloged as the type is an assemblage rather than an individual. To be fair, Marsh knew he had multiple individuals at the time, but then he should have been more careful about specifying a type. The parts that had been on display as a panel mount at Yale may be one associated individual, consisting of most of the cervical and dorsal series (just centra), possibly six sacrals, parts of the left shoulder girdle and the pelvis, a partial left femur and complete left foot, and parts of the right femur, tibia, and astragalus. This is a lot of parts/partials plus a lot of plaster, which is not encouraging. The rest of the material is a chimeric mix including at least juvenile dryosaur and hypsil material. The only one of the six that ends up being interesting is D. nisti, which has some dental and jugal features reminiscent of pachycephalosaurs (but is still not diagnostic, although it would be nice to have the type material in hand to be sure).

Where does this leave the Morrison hypsil(s), which Carpenter and Galton (2018) had declared N. agilis? Anonymous, for the time being. Carpenter and Galton (2018) looked upon the pile of Morrison hypsil bits and proposed it was "all" Nanosaurus agilis. Barrett and Maidment (2025) looked upon the same pile and clutch of names and regarded it as a taxonomic dead end, to be set aside to allow a fresh start for more complete and better preserved specimens (with quarry maps and documented associations and such).

At heart, we're seeing two different approaches to taxonomy, and which one you choose depends on how pragmatic you are and how bound you feel by existing names. If you want a species with a holotype featuring robust apomorphies, N. agilis is not for you. We saw that in the comments section of the last post: most of the characters cited by Carpenter and Galton (2018) are widely distributed among hypsil-things, with just a couple that might have some particular use. However: Is there a hypsil-thing in the Morrison that is anatomically consistent across specimens, whether or not said specimens are diagnostic across Ornithischia? If so, is it reasonable to call this hypsil *something*, knowing that it may be revised later? If so, the oldest existing name is Nanosaurus agilis. If you go that route, I'd recommend looking into a neotype, though. ("All would be well, if, if, if, if, if...")

References

Barrett, P. M., and S. C. R. Maidment. 2025. A review of Nanosaurus agilis Marsh and other small-bodied Morrison Formation “ornithopods". Bulletin of the Peabody Museum of Natural History 66(1): 25–50. doi: https://doi.org/10.3374/014.066.0102.

Carpenter, K., and P. M. Galton. 2018. A photo documentation of bipedal ornithischian dinosaurs from the Upper Jurassic Morrison Formation, USA. Geology of the Intermountain West 5:167–207. doi:

The downside of reef building?

I was reviewing text for a website a few weeks back dealing with aspects of the history of life, and a couple of things struck me about biological reefs. First, a quick look at reef-builders through time (a useful overview can be found here if you'd prefer more flesh on the bones, or corallites or shells or whatever may be more appropriate):

The first multicellular reef-builders were the archaeocyathan sponges, who flourished briefly in the Cambrian but did not even make it to the end of the period. Corals, in the form of rugose and tabulate corals, spread in the Ordovician but took a while to make reefs. They were joined by stromatoporoid sponges (layered like stromatolites, spelled like stromatolites, but not stromatolites) and various microbes, with the Devonian being an apex of reef-building. The classic stromatoporoid-tabulate reefs of the Devonian went kaput in the End-Devonian extinction. Permian reefs were a conglomeration of just about everything that couldn't get out of the way: various algae, sponges, bryozoans, and other less obvious things. This assortment bought it at the end of the Permian. False starts with scleractinian corals in the first part of the Mesozoic gave way to the rudist bivalves in the Cretaceous. The rudist reefs went out with non-avian dinosaurs, marine reptiles, pterosaurs, ammonites, and so forth at the end-Cretaceous extinction. Finally we get to the big scleractinian coral reefs in the Cenozoic, with some sponge and oyster reefs and such for variety.

So far, a typical pattern: group of organisms branches into reef building, reefs spread and become ecologically complex, reefs flourish for a while, mass extinction wipes out reefs. Then after a hiatus reefs become fashionable again, with some other group laying the foundation for a new iteration, and the cycle continues. A couple of observations come to mind. First, reefs seem to be an obvious evolutionary path for immobile marine invertebrates. It may take some time, but some group always takes up the baton after another falters.

Then, the other shoe. What happens to the previous reef-builders? Seen any archaeocyaths lately? Any vacation packages advertising stromatoporoid reef visits for their island getaways? Run across any rudists while snorkeling? Could it be that once a group goes all-in on the reef habit, it's stuck with it?

Furthermore, reefs have a habit of getting smacked in mass extinction events. Does a reef inherit a narrowing range of environmental restrictions from its components as its complexity increases? Does it become vulnerable to unpredictable instability, such as some minor constituent going through a bad patch leading to collapse via a Rube Goldbergian-cascade of events? More broadly, does reef building amount to an evolutionary Faustian bargain, in which a group becomes dominant for a while by locking itself into a doomed arrangement? (Granted, we're all doomed in the final analysis, but some of us are more obviously doomed than others.) Or am I just playing the gloomy Minnesotan?