Slightly More Hopeful Dinosaurs

Lewisuchus--basalmost ornithischian? (from Ezcurra et al., 2019)

Dear me, it’s been awhile, hasn’t it? My tardiness in keeping this blog going has not been entirely intentional; 2020’s been a year for the records? I only hope it ends on December 31st and doesn’t continue on in some space-time warping extension into December 32nd. Nonetheless, I am motivated today to inform you all about a paper that brings together several topics I’ve written about in years past: silesaurids, Pisanosaurus, and the Triassic Ornithischian Crisis. Below, I will offer the briefest of recaps, but hit those links if you want a more complete picture.

Silesaurids are a group of non-dinosaur dinosauriformes, often found to be the immediate sister group to the Dinosauria. When the first silesaurids were being described, it appeared that they had a toothless predentary bone—an extra bone at the tip of the dentary that is unique to all ornithischian dinosaurs. Thus, these early silesaurid descriptions included speculation that silesaurids might represent the earliest ornithischian dinosaurs.

More silesaurid taxa turned up over the next seventeen years and it turns out that they are a diverse, widespread group of animals that may or may not form a monophyletic group (depending on the phylogeny). Additionally, their “predentary” bone is not a true predentary: The anterior tip of their dentary bones are toothless and slightly upturned, yes, but are not an entirely separate structure. The silesaurid “predentary” was more readily explained as a similar adaptation for a group of herbivorous non-dinosaurian dinosauriformes. It remains, however, a curious feature.

It’s also possible that silesaurids, as traditionally envisioned, do not represent a monophyletic “Silesauridae,” but instead a stepwise series of taxa between lagerpetids and true dinosaurs. Langer & Ferigolo (2013) take things a step farther, suggesting that while some silesaurids are outside the Dinosauria proper, others really do represent the oldest true ornithischian dinosaurs—including Pisanosaurus. Most analyses, however, have continued to find that silesaurids—monophyletic group or otherwise—represent non-dinosaurian dinosauromorphs.

Then, in 2017, Pisanosaurus was reinterpreted as a silesaurid (Agnolin & Rozadilla, 2017). If true, this would mean that there are no unambiguous ornithischian taxa known from the entirety of the Triassic period. This is surprising, but may be explained in part by the Ornithoscelida hypothesis, which posits that some branch of the Theropoda and the entirety of Ornithischia are sister groups to the exclusion of Sauropodomorpha (and herrerasaurids). Baron (2017) suggested a few different scenarios to explain what was looking like an Early Jurassic origin for ornithischians, including a position close to Chilesaurus, but also touches on the suggestions of Ferigolo & Langer (2006) and Langer & Ferigolo (2013) that at least some silesaurids represent basal ornithischians.

So, there are no Triassic ornithischians, there are no cats in America, Pisanosaurus is apparently a silesaurid, and silesaurids themselves are a group or grade of non-dinosaurian dinosauriformes. Seems clear enough!

Muller & Garcia's (2020) Ornithischia, now featuring silesaurids

Just a few weeks ago, Muller & Garcia (2020) provided a full-throated endorsement of the “silesaurids as ornithischians” position, bringing some phylogenetic support to the hypothesis. In their scenario, silesaurids represent a paraphyletic series of stepwise taxa from the base of Dinosauria towards the earliest genasaurs, represented by Scuttelosaurus. The authors show that silesaurid dentition also seems to show a stepwise sequence from blade-shaped, insectivorous/carnivorous teeth to leaf-shaped, herbivorous teeth--similar to that of sauropodomorphs.

This hypothesis has two satisfying implications: it “solves” the Triassic Ornithischian Crisis, as the oldest silesaurid is actually from the Middle Triassic (Asilisaurus) and it allows Pisanosaurus to be a silesaurid and an ornithischian simultaneously. Indeed, in Muller & Garcia’s tree, Pisanosaurus is both the most derived “silesaurid” and basalmost “traditional” ornithischian.

However, this idea presents new issues. For one, if Asilisaurus is an ancient ornithischian dinosaur, that would pull the origin of the Dinosauria back to the Ansian. If Nyasaurus (Nesbitt et al., 2013) is a true dinosaur (and not an ornithischian), that would help close the gap, as it is coeval with Asilisaurus (and Lutungutali). If Nyasaurus isn't a dinosaur, however, there’s still a considerable gap in time between Asilisaurus and the next-oldest non-ornithischian dinosaurs, which start showing up in the mid-Carnian.

The other issue—which might not be an actual issue—is that silesaurids were long thought to be outside of Dinosauria because they lacked certain features which define a monophyletic Dinosauria. If silesaurids are true dinosaurs, it means that some of these defining features arose independently in Ornithischia and Saurischia. However, given the amount of homoplasy in dinosauromorphs, this might not be surprising or even problematic. Muller & Garcia (2020) write that: 

...the branch support and bootstrap values of the present topology are generally low...However, it is not surprising. Low values occur in other topologies (traditional Ornithischia/Saurischia split and Ornithoscelida hypothesis) obtained by distinct datasets. This condition is tentatively explained by high rates of homoplasy, as the earliest members of the major subgroups were very similar in body size and morphology.

The authors use a recently-ereted clade name, "Sulcimentisauria" (Martz & Small, 2019) for a more exclusive clade of ornithischians--basically everyone except Lewisuchus, Soumyasaurus, and Asilisaurus--that have two dental characters and two femur characters, the former related to the onset of herbivory. The name was originally applied to a clade of all silesaurids apart from those three, but silesaurids themselves were, in that paper, still monophyletic. Muller & Garcia (2020) essentially used that definition here, but with traditional Ornithischia included. 

This topology also, as the authors note, results in herbivory arising just twice, in early ornithischians and sauropodomorphs, rather than thrice, when you also include silesaurids as a non-dinosaurian group of dinosauriformes.

I like this idea--it solves a lot of problems--but will require further testing and more fossils (as always). And where are all the Ansian saurischians?