The Year of the Ceratopsian Ankylosaurs

Life restoration of an advanced stegosaur- I mean an ankylosaur (Ankylosaurus magniventris) by Emily Willoughby,  CC-BY-SA.

When digging into the history of North American fossil interpretation for the eventual next edition of my Beasts of Antiquity series, one thing that I found a bit weird was the constant reference to ankylosaurids and nodosaurids as types of stegosaurs. To a modern reader, this seems off. After all, the group of armored dinosaurs, Thyreophora ("shield bearers"), is divided into two major groups: Ankylosauria and Stegosauria, each with a few well supported subgroups. It makes sense that these close relatives might once have been classified together, and stegosaurs were discovered first, lending them priority of name. But what changed? Neither ankylosaurs nor stegosaurs are particularly large groups (especially the stegosaurs), and it seems odd that 20th century taxonomists would want to raise a group as small as the modern idea of Stegosauria to the level of "suborder".  Why were ankylosaurs eventually spun off, leaving the more primitive stegosaurs behind? I decided to do a little digging to find out.

The name Ankylosauria was coined without comment (or any indication that this was a new name at all) by then-American Museum of Natural History president Henry Fairfield Osborn in his 1923 description of Psittacosaurus. The skull of Psittacosaurus, according to Osborn, bore many similarities to ankylosaurs, and Osborn seems to have initially interpreted it as an ancestor of ankylosaurs! Specifically, Osborn cited the presence of jugal horns to unite early ceratopsians with ankylosaurs. Earlier that same year, a paper describing Protoceratops also used ankylosaurs as a favorable point of comparison. In it, Walter Granger and William Gregory regarded Protoceratops as sharing features with both ceratopsids and ankylosaurs. What's more, they considered ankylosaurs to be very different, anatomically, from the stegosaurs they had long been allied with.

What develops when looking at these two 1923 papers in an historical context is a sense that ankylosaurs were being re-interpreted not as stegosaur relatives, but as what we might now term the sister group of the ceratopsids. Granger and Gregory attributed this hypothesis to a 1919 book by Othenio Abel, in which Abel apparently classified ankylosaurs within the Ceratopsia. According to Gregory and Granger, the skull features of Protoceratops solidified this view, and they speculated that Protoceratops may have been the common ancestor of ankylosaurs and ceratopsians (they only tentatively classified Protoceratops as a ceratopsian, suggesting that it might better be placed in its own suborder, Protoceratopsia). In this new paradigm, ankylosaurs could not have been members of the Stegosauria, nor were they enough like ceratopsids to be considered true ceratopsians. The AMNH researchers therefore had no choice but to create a new "suborder" for them, the Ankylosauria.

Of course, the current scientific consensus is that ankylosaurs were indeed close relatives of stegosaurs and actually quite distant from the ceratopsids, psittacosaurids, and protoceratopsids. The opinion that they belonged to a distinct suborder because they were ceratopsian-like doesn't seem to have been actually expressed anywhere in those 1923 papers, though it's fairly clear from the context. There was never any discussion of why ankylosaurs were suddenly removed from Stegosauria and raised to a suborder. The hypothesis that ankylosaurs were ceratopsian relatives doesn't seem to have gained much traction following the publication of those two papers, and by 1948, scientists like Ned Colbert were back to considering them as separate from ceratopsians, though still retaining them in their own suborder alongside Stegosauria. Apparently, these later researchers missed the point of the original reclassification, otherwise they may as well have continued to classify them as members of a broader Stegosauria. As a result, a new name was needed to take on the role Stegosauria had previously, that of encompassing all of the armored ornithischians. The name Thyreophora, originally coined by Franz Nopcsa in 1915, was available for this purpose, though Nopcsa had by that time come to use the name to include stegosaurs, ankylosaurs, AND ceratopsians.

The last vestiges I can find of the "old" use of Stegosauria is in a paper which discusses a little more about the historical use of Thyreophora as a suborder for various armored dinosaurs, and mentions that the suborder containing Scelidosaurus is named either Thyreophora or Stegosauria, recognizing the two as historical synonyms (Casamiquela, 1967).

Another thing I found that I hadn't heard before when digging into this issue is that Thyreophora (Meigen 1803) is also the name of a genus of bone skipper flies. That's not an issue under the ICZN, but the PhyloCode may have a problem here...*

*ICPN Article 13.3 says that the first name established under the Code has priority, not the oldest name. Since the Code hasn't actually been enacted yet, it's going to be a race between dinosaur and fly researchers to see who can first claim the name for themselves!

References

• Abel, O. (1919). Die Stiamme der Wirbeltiere. Berlin: W. de Gruyter.
• Granger, W., and Gregory, W. (1923). "Protoceratops andrewsii, a pre-ceratopsian dinosaur from Mongolia." American Museum Novitates, 72.
• Osborn, H.F. (1923). "Two Lower Cretaceous dinosaurs of Mongolia." American Museum novitates, 95.
• Casamiquela, R.M. (1967). "A new Triassic ornithischian dinosaur (Pisanosaurus mertii; Ornithopoda) from the Ischigualasto Formation, Argentina." Revista de la Asociación Paleontológica Argentina, 4(2).