Sunday, November 3, 2019

The Ruins of the Fabrosaur Empire

Pisanosaurus mertii depicted as a silesaurid. By Nobu Tamura, retrieved via Wikimedia Commons (CC BY-SA 4.0).
Me, a regular and consistent blogger? Surely none of you were ever fooled by this notion. The mood finally strikes again, so why don't I spend a moment catching up on one of the topics I've blogged about before? How are our fabrosaurs faring?

Unfortunately, it seems as if the situation has only grown more dire since we last checked in on the subject of Triassic ornithischians in August 2016. In that article, I walked over the history of Fabrosauridae and a number of supposed Triassic ornithischians. It seems the Revueltosaurus situation has played out again, because the most recent major paper on the subject, Agnolín & Rozadilla (2017), reinterpreted Pisanosaurus as a non-dinosaurian dinosauriform, a silesaurid, based on both dentary and skeletal characteristics. This seems pretty well supported, and in fact had already been mentioned in the literature previously (in 2015), but this wasn't known to me at the time of the article. Separately, McPhee et al (2017) positioned the South African ornithischian Eocursor as an Early Jurassic taxon instead of its previous interpretation as a Triassic animal. (A finding which is not completely resolved, but for now it holds true.)

This has the salutary effect of extirpating the ornithischians from the Triassic period altogether, which raises some serious questions about the origin and evolution of this clade of dinosaurs. Such an extensive ghost lineage has led authors to question just why ornithischian fossils are absent (or nearly so) prior to the beginning of the Jurassic. Matthew G. Baron (whose Ornithoscelida paper with other authors [2017] has been blogged to death during my long absence) has mooted a novel concept to explain away this problem, wherein Ornithischia would be nested within Theropoda, using the unusual Late Jurassic dinosaur Chilesaurus as the linchpin. Described initially as a basal tetanuran by Novas et al (2015) when it was described, its taxonomy has remained problematic since its discovery, because of a few features in its skeleton (namely the presence of a toothless premaxilla and some characteristics of the ilium) which closely resemble those of ornithischian dinosaurs. In this paper, Baron, having previously mooted Chilesaurus as the basalmost ornithischian, suggested that Ornithischia might be within Neotheropoda, positing a few different hypotheses at different positions, but generally making the case that such a relationship would both clear up Chilesaurus' taxonomic quandary, and shorten the ornithischian ghost lineage implied by the traditional phylogeny.

Of course, this does pose the issue of an equally long (or longer) ghost lineage, if we are to consider Chilesaurus the basalmost ornithischian, as it would be far removed from the current earliest known ornithischians from the first stages of the Jurassic. A Late Jurassic taxon of probable Tithonian age, Chilesaurus is separated from its purported relatives, the earliest known ornithischians, by around fifty million years, so if it is indeed the basalmost of that group, then the ghost lineage problem seems to remain anyway. Müller et al (2017) were also quick to jump onto the issue, and while one of their analyses (when recalibrated) did find Chilesaurus to be within Ornithischia, they still hedged their bets and concluded that the current data simply is not strong enough to make a sure statement one way or the other.

Where exactly can we place the enigmatic Chilesaurus among the other dinosaurs? Can the ornithischians really be nested among the theropods to explain their absence in the Triassic fossil record? Or are there, in fact, still Triassic ornithischians waiting to be discovered? All of these questions seem to be tied up in each other as of the most recent studies. Supposedly there's a lot more (a lot more) on the Ornithoscelida issue pending publication so, as always... keep an eye on the literature, guys.


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