Monday, August 14, 2017

And Then There Were None

Today, we add an addendum to what’s turned out to be one of my favorite blog posts: Hopeful Dinosaurs. In that essay, I introduced you all to a group of near-dinosaur dinosauriforms called silesaurids. These small, quadrupedal, herbivorous critters all hail from the Late Triassic, but were impressively cosmopolitan, being found in the United States, South America, Africa, and Poland.

Among their more interesting features is a pseudo-predentary that was initially thought to be a precursor to the ornithischian unifier. However, this pseudo-predentary is not a separate ossification, but merely a pointed, toothless anterior end of the normal dentary bones. While silesaurids are usually found to be just outside of the Dinosauria, a few authors (Dzik, 2003; Ferigolo & Langer, 2007; Langer & Ferigolo, 2013) have suggested that silesaurids, either as a monophyletic or paraphyletic group, were basal ornithischians (actual phylogenetic support for this idea, per Langer & Ferigolo, 2013, remains low).

Toward the end of that post, I briefly went over a conference abstract by Federico L. Agnolin in which he suggested that foundational Triassic ornithischian Pisanosaurus mertii was actually a silesaurid. This is surprising given that taxon’s status as the Last Surviving Triassic Ornithischian.

Oh, maybe a little background: Prior to 2005, Triassic ornithischians were downright common, but fragmentary and based on teeth. Ornithischians, indeed, have unique dentition. But in 2005, Parker et al. went and harshed that mellow by demonstrating that Late Triassic archosaur Revueltosaurus callenderi was a pseudosuchian, not an ornithischian as had been previously believed, and had independently evolved ornithischian-like dentition. Furthermore, Silesaurus was described in 2003 and combined a dinosauriform skeleton with ornithischian-like dentition: two knocks against Triassic ornithischians that were based on teeth alone. Parker et al. (2005) sank several ornithischian taxa: Galtonia, Tecovasaurus, Lucianosaurus, Pekinosaurus, and Technosaurus were no longer unambiguously referable to Ornithischia. They write that “naming isolated herbivore-like teeth can be hazardous because future work may show that separate species possess identical dental morphologies.”

Gorgeous Revueltosaurus skeletal by Jeff Martz. Obviously not a dinosaur.
After this Great Purge, the authors state that only Pisanosaurus (Casamiquela, 1967) and an unnamed Argentinian heterodontosaurid (Baez & Marsicano, 2001) remain as unquestionable Triassic ornithischians. Sadly, since the heterodontosaurid’s description, Irmis et al. (2007) wrote that its poor preservation requires further evidence to confirm a heterodontosaurid identity, Sereno (2012) wasn’t sure it could be diagnosed to Heterodontosauridae at all, and Olsen, Kent & Whiteside (2011) claimed that the age of the Laguna Colorada Formation, where it found, could be Late Triassic or Early Jurassic. Thus, a great deal of ambiguity surrounds this “Triassic heterodontosaurid.”

Since Parks et al. (2005) sank most Triassic ornithischians, only one other has been discovered: Eocursor parvus was described in 2007 and 2010. This South African ornithischian is wonderfully complete and well-preserved. However, Olsen, Kent & Whiteside (2011) wrote that the Lower Elliot Formation might also be Early Jurassic (this was reaffirmed at an SVP talk last year, apparently). Thus, Pisanosaurus is the only “surviving” unambiguous ornithischian dinosaur. Right?

Well, maybe not.

As Angnolin & Rozadilla (2017) demonstrate, there’s not much keeping Pisanosaurus in the ornithischian camp. On the contrary, it shares a lot of features with silesaurids including (but not limited to) details of tooth, sacral vertebrae, pelvis, tibia, and foot morphology. To make a long story short, they write that: “Pisanosaurus lacks any features that unambiguously position it among ornithischians and even dinosaurs, while possessing several traits that are shared with basal dinosauriforms of the clade Silesauridae.”

Pisanosaurus then (top) and now (bottom).
This implies a 30 million-year gap between Pisanosaurus and the earliest records of ornithischians in unambiguously Early Jurassic sediments, like Laquintasaura and Lesothosaurus. This could mean several things:

1. Ornithischians are still a valid “suborder,” but were ridiculously rare components of their ecosystems until after the Triassic-Jurassic extinction event, after which they diversified rapidly;
2. Ornithischians are a subgroup of Theropoda, branching off after the Triassic-Jurassic extinction event. Animals like Chilesaurus make me think this might be a serious possibility.
3. Phytodinosauria is real, but Ornithischia still didn’t diversify until the Early Jurassic. Animals like Chilesaurus (which retains some weirdly sauropodomorph features) make me think this might be a serious possibility.
4. I sort of said this already, but some or all silesaurids could actually be basal ornithischians. This would force a lot of convergence with saurischians (Sauropodomorpha + Theropoda for the sake of argument).

UPDATE: Baron gave a talk at SVP about Chilesaurus and Ornithoscelida and it sounds like things might be WAY crazier than we think based on what's currently published. But that's all I'll say.

But whatever the answer, the point here is that there are no more unambiguous Triassic ornithischian dinosaurs, which is really weird.

Oh, by the way, if Pisanosaurus is a silesaurid, shouldn’t the group be renamed Pisanosauridae? After all, Casamiquela (1976) considered Pisanosaurus unique enough from other ornithischians that he gave it its own family, the Pisanosauridae. Given the principle of priority, Pisanosauridae should have priority over Silesauridae. However, the authors argue that Pisanosauridae has not been employed for decades, whereas Silesauridae is well-known and widely accepted today.

Thus, “for the sake of clarity,” Agnolin & Rozadilla recommends retaining Silesauridae. “Further,” they write, “it is probably best not to abandon Silesauridae at least until further evidence definitively shows that Pisanosaurus belongs within this clade.” I think they made a pretty good case already, but I also appreciate the clarity argument. 

1 comment:

  1. It is a tough call... but look at the data and don't exclude Haya.