Three weeks of ankylosaurs and pterosaurs

March has been busy over at The Compact Thescelosaurus, with ten new entries adding three ankylosaurs, four pterosaurs, and three theropods. There's still a week to go, too. Sometimes people talk about following taxonomy as another kind of stamp collecting. I have a more artistic point of view, with each specimen as more along the lines of tiles in an incomplete mosaic or notes in an unfinished song, each contributing to a greater whole. It just so happens that the currency of the realm is in species and genera. (I also keep track because you can't tell the players without a program!) As usual, I'll leave the theropods to others, except to say that it's nice to see Arkansaurus finally graduate from informal name to the big leagues, thanks to Hunt and Quinn (2018) (the name has figured in print since 1983, after all). A few comments on the ankylosaurs and pterosaurs:

Ankylosaurs

It's interesting to contrast the decomposition of Euoplocephalus with the decomposition of Iguanodon. If you weren't there, the splitting of Iguanodon was a confusing mess that dragged on for years. I think the problem, besides Iguanodon having a more significant historical and cultural profile than Euoplocephalus, was that dividing Iguanodon had become very obvious to a lot of people, making it impossible for there to be a "controlled demolition". Instead, several authors with very different styles all went to work at the same time, clouding the whole thing with the lingering whiff of personal conflict and claim rushing. (Another problem is the system in use provides no middle ground between an interesting specimen buried under Iguanodon sp. and a fully formed italicized Linnean binomial intended to stand for the rest of time.)

The breakup of Euoplocephalus has been much quieter, perhaps in part because it had centered on disbursing previously named genera, and because people were comfortable with the biostratigraphic arguments. This month, though, we've gone beyond the liberation of Anodontosaurus, Dyoplosaurus, and Scolosaurus with Penkalski (2018), which names two new species to existing genera (Anodontosaurus inceptus and Scolosaurus thronus) and one new genus and species (Platypelta coombsi). All three taxa come from the Dinosaur Park Formation of Alberta, albeit different horizons. This would put us to eight named species of ankylosaurs over about 6 million years of time in Alberta and Montana, with the possibility of more. I don't think it's *not* possible, but I do think that our knowledge of ankylosaur variation is starting to butt up pretty hard against our current supply of specimens.

The three stars of Penkalski (2018), from images in Arbour and Currie (2013) assembled into subfigures for Wikipedia (here, here, and here), and further assembled by me.

On the other hand, it may be simpler to think of the whole in terms of five lineages, based on assignments of species to genera: the Dyoplosaurus and Platypelta lineages, which so far are fairly inconsequential unless you happen to be a Dyoplosaurus or Platypelta because they appear to have conked out low in the Dinosaur Park Formation; the Euoplocephalus lineage, which hangs around farther through Dinosaur Park Formation; the more complicated Anodontosaurus lineage, which includes A. inceptus in the middle Dinosaur Park and A. lambei in the Horseshoe Canyon Formation; and the most complicated lineage, the one with Scolosaurus, Oohkotokia, and Zuul, which is the oldest, so far, but also outlasts all of the others except for Anodontosaurus. Therefore, if this makes you feel any better, we're effectively dealing with three lineages once Dyoplosaurus and Platypelta disappear, and some of the species could conceivably just be chronospecies (i.e., Scolosaurus thronus is just S. cutleri with a new hat).

Is that's what's going on? Nuts if I know. One of the figures is a diagram of 16 "Euoplocephalus" snouts, with colored shading highlighting different groups of scales. I looked at the diagram for quite a while until it occurred to me that I could see differences just fine, but not the similarities; i.e., I couldn't tell why this species would be in this genus as opposed to that other genus. This may just be me; I don't see mythical figures in constellations or hear words in bird calls, either. As it is, I'm wary of making taxonomic distinctions and assignments based on stratigraphy; either specimens can be diagnosed based on the preserved material, or they can't. We no longer decide stratigraphy based on fossil content, after all, which is a good thing if you've ever tried to determine Croixan trilobite species from natural molds and casts in sandstone.

Pterosaurs

The classic story about the decline and fall of pterosaurs is that birds out-competed them, shoving them out of more and more niches until the pterosaurs disappeared up their own specializations at the end of the Cretaceous. It's one of those post-hoc morality tales people are prone to seeing: something is dead and gone, therefore it was always a failure waiting to happen, done in by its own flaws or sacrificed to progress. "Being in the wrong place at the wrong time" just doesn't seem to be satisfying enough, especially where there's a living group on hand like birds that can be held up as "superior" (ignoring that the two groups coexisted for at least 90 million years, or well over half the 150 million or so years of Pterosauria). Phooey. If you're looking for a lesson from the extinction of a lineage with 150 million years under its figurative belt, it's "deserve's got nothing to do with it."

Pterosaurs historically looked suspicious because they do seem to have been dwindling after attaining heights in diversity in the latter part of the Early Cretaceous. Then, as the story goes, there's one last hurrah with the Western Interior Seaway, and we're left with only azhdarchids to close out the Mesozoic. Let's look a little closer, though. A couple of things, to summarize and embellish on Longrich et al. (2018):

Early Cretaceous pterosaur diversity is indeed great, but it's propped up by the Jehol and Santana, and to a lesser extent the recycled pterosaurs of the Cambridge Greensand. The seemingly low end-Cretaceous diversity owes a lot to the less-than-ideal geologic conditions. Where do we find lots of pterosaurs? Fine-grained lacustrine or marine rocks. What's happening at the end of the Cretaceous? Shallow continental seas are receding. Do we generally find lots of pterosaur remains in sedimentary rocks that are not fine-grained lacustrine or marine? No, just bits. What kinds of pterosaurs are best known from the end of the Cretaceous? Robust, terrestrially adapted azhdarchids. Also, saying "only" azhdarchids is starting to look a bit like saying "only" titanosaurs, what with being found across five continents and spawning a decent number of species in their relatively limited time.

All this above is fine and dandy, but it would be nice to have some non-azhdarchids to put some teeth in it (well, maybe not "teeth", since we're looking at toothless pterosaurs, but you get the idea). This is just what Longrich et al. (2018) did, with a collection of pterosaur specimens from end-Cretaceous phosphates in Morocco. Previously limited to the azhdarchid Phosphatodraco, focused study has unearthed numerous pterosaur fossils, which the authors divided among two additional azhdarchid species (as-yet unnamed), three nyctosaurids (Alcione elainus, Barbaridactylus grandis, and Simurghia robusta), and a pteranodontid (Tethydraco regalis). They also interpreted several poorly known Late Cretaceous species as representing other non-azhdarchid lineages. Whether or not you accept all of their identifications, pterosaurs were not just impotently awaiting their doom in the Late Cretaceous.

Behold, the state of pterosaur–bird competition in the Maastrichtian, via Longrich et al. (2018: figure 20). Clearly those birds are poised to take over the azhdarchid niches. After all, we've seen how successful they are in the azhdarchid ecospace today. Pterosaurs from marine settings are in blue, pterosaurs from terrestrial/freshwater settings are orange to brown.

References

Arbour, V. M., and P. J. Currie. 2013. Euoplocephalus tutus and the diversity of ankylosaurid dinosaurs in the Late Cretaceous of Alberta, Canada, and Montana, USA. PLoS ONE 8(5):e62421.

Hunt, R. K., and J. H. Quinn. 2018. A new ornithomimosaur from the Lower Cretaceous Trinity Group of Arkansas. Journal of Vertebrate Paleontology online edition: e1421209.

Longrich, N. R., D. M. Martill, and B. Andres. 2018. Late Maastrichtian pterosaurs from North Africa and mass extinction of Pterosauria at the Cretaceous-Paleogene boundary. PLOS Biology 16(3):e2001663.

Penkalski, P. 2018. Revised systematics of the armoured dinosaur Euoplocephalus and its allies. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 287(3):261–306.