Saturday, December 10, 2016

Hopeful Dinosaurs

Silesaurus opolensis, by Scott Hartman--used with permission.
We’re going to start this discussion with an assumption so that I don’t have to go back a hundred years and talk about the history of dinosaur phylogeny: Dinosauria is a monophyletic group, and it consists of the Saurischia (Theropoda + Sauropodomorpha) and Ornithischia (everyone else). I’ll also invoke Robert Bakker’s bizarre suggestion that the Theropoda was separate from a (Sauropodomorpha + Ornithischia) group he called the “Phytodinosauria” because they all ate plants, had similar teeth and, back in 1986, Segnosaurus was thought to be maybe a missing link between prosauropods and early ornithischians (details in The Dinosaur Heresies). You'll see why soon.

Early ornithischian Heterodontosaurus, with the predentary in green. Image from Wikipedia.
Now then, ornithischians all share a unique bone called the predentary—basically a separate ossification (“beak bone”) that’s in front of the dentary and opposes the premaxillary bones. It acts a bit like the lower incisors of modern ungulates—useful for cropping vegetation. A recent paper by Nabavizadeh & Weishampel (2016) has shown that the predentary can act as a brace while the mandibles move or rotate during feeding. Fascinating stuff if you have access.

Anyway, the predentary bone is so characteristic—even moreso than the “bird-like” pelvis which would not be present in the earliest ornithischians—that O. C. Marsh tried to rename the group the “Predentata” in 1894, years after Harry Seeley formalized the Saurischia/Ornithischia divide. But it didn’t stick, despite being arguably more accurate.

So you can imagine everyone’s surprise when, in 2003, Jerzy Dzik described Silesaurus opolensis, a small, Polish herbivorous non-dinosaur with what appeared to be a predentary. Prior to this discover, the earliest relatives of dinosaurs were guys like Marasuchus, Lagerpeton, and Dromomeron: small insectivorous bipeds that, while well on the way to dinosaurhood, lacked many of the pelvic, hindlimb, and veterbral specializations that defined Dinosauria.

The skull of Silesaurus (from Dzik, 2003). Although Piechowski, Niedzwiedzki & Talanda (2015) have revised it.

Despite the differences between ornithischians, sauropodomorphs, and theropods, all three groups are united by a number of unambiguous skeletal characters—twelve, according to Nesbitt’s recent (2011) mega-analysis. As phylogenetics roughly follows the axiom “keep it simple, stupid,” it’s more parsimonious to say their common ancestor had all twelve features than to suggest that all three independently evolved those features.

Jaw/dental elements of Silesaurus
Dzik did not run Silesaurus through a phylogenetic analysis but summarized its featured confounding mixture of basal and advanced characters that would make an exact placement difficult. Commenting on the dention, for example, Dzik writes:

“…in their weak serrations, with denticles oriented slightly laterally, they are not much more morphologically derived than teeth of the associated aetosaur Stagonolepis.”
However, just in front of those teeth is (emphasis mine):

“The feature of possibly the most far-reaching phylogenetic consequences is the presence of a horny beak on the lower jaw of the Krasiejow animal.”

Dzik is unsure of what to make of all the homoplasty, and decides that either (1) Silesaurus is an early ornithischian; (2) Holy crap, Phytodinosauria is real; or (3) Silesaurus developed its ornithischian features independently of actual dinosaurs.

You know, what we really need are more “silesaurids.” Thankfully, they were just on the horizon.

Eucoelophysis - yeah, there's not much there.
In 2006, Martin Ezcurra took a look at a Triassic reptile named in 1999 by Sullivan & Lucas: Eocoelophysis baldwini from the Petrified Forest Formation of New Mexico. As you might guess from the name, Sullivan & Lucas thought it was a coelophysoid, but Ezcurra discovered that the taxon—known only from hindlimb elements—could not be referred to Dinosauria at all. Instead, it was very similar to Dzik’s recently-named Silesaurus. So here we find a silesaurid in New Mexico.

Sacisaurus agudoensis - assuming all those bones are from Sacisaurus.
Also in 2006, Ferigolo & Langer describe Sacisaurus agudoensis, a Brazilian animal known from remains that are unfortunately mixed in with that of an unambiguous (and larger) saurischian dinosaur. The authors spent a lot of time discussing the predentary, which is different in Sacisaurus—unfused, it is a paired structure that meets at the midline and seems to be a part of the dentary rather than a separate element, which may be important later. Ferigolo & Langer entertain the idea that Sacisaurus and Silesaurus are, in fact, basal ornithischians but come up against the same wall that Dzik did in 2003: the combination of advanced and basal characters in other parts of the (known) skeleton are confusing.

Jaw/dental elements referred to Sacisaurus.
But Dzik and Ferigolo & Langer are both assuming that the thing at the front of the dentary is a predentary. What if it’s…not?

A few years later, in 2010, Nesbitt et al. described yet another silesaurid, this time from Tanzania: Asilisaurus kongwe. Known from remarkably complete remains, Asilisaurus also extends the Silesauridae back into the Anisian part of the Middle Triassic (and, by implication, Dinosauria). The authors never refer to the animal’s “predentary” as such, instead saying that the “anterior portion of the dentary tapers to a point like that of both Silesaurus and Sacisaurus…” Like Sacisaurus, the “predentary” of Asilisaurus is not clearly a separate element.

Asilisaurus kongwe - the gray bones are unknown (so that's a lot of known bones).
Now this is important: in all ornithischians, even old adults, the predentary is very clearly separate from the dentary. According to Nabavizadeh & Weishampel (2016), that was the whole point of the predentary: it provided an anchor for both sides of the jaw have some flexibility.*

What if the “predentary” of silesaurids is just a toothless beak-shaped extension of the dentary bones?

Two additional silesaurids have been named since 2010: Morocco’s Diodorus scytobrachion (Kammerer, Nesbitt & Shubin 2012) and Zambia’s Lutungutali sitwensis (Peecook, et al. 2013). Neither of them preserves the anterior dentary, so they are ambiguous as to “predentary” anatomy. Rather, they speak to the rapid diversification and widespread nature of the group.

Langer et al. (2010) briefly discuss silesaurids and noted that phylogenetic analyses which included Silesaurus and Sacisaurus did not recover an ornithischian position for either animal. If silesaurids had predentary bones, they were probably independently derived—strange though that may be.
All this was pretty much confirmed when I read Nesbitt’s enormous 2011 archosaur phylogeny monograph. At almost 300 pages long, this is an astounding piece of work that everybody interested in archosaurs should, at the very least, skim. But the interesting bit we’ll be discussing is about (surprise) silesaurids.

As noted, no previous phylogenetic analyses had found support for Silesaurus as a true, blue dinosaur. Rather, it was consistently found to be just outside Dinosauria. The differences were whether Silesaurus, Eucoelophysis, and Pseudolagosuchus were monophyletic or formed a stepwise series of taxa towards Dinosauria.

In Nesbitt’s analysis—which includes Silesaurus, Sacisaurus, Eucoelophysis, Asilisaurus, Pseudolagosuchus and Lewisuchus—silesaurids are found to be a monophyletic sister group (Silesauridae) to Dinosauria. The most interesting thing (to me) is that Pseudolagosuchus and Lewisuchus (which might be synonymous) are found to be the most basal members of this family, meaning that many of the allegedly dinosaurian features of Silesaurus are not in the earliest silesaurids. Thus, over the course of their evolution, silesaurids converged to an incredible degree with early dinosaurs.

For example, the “predentary” that we’ve been talking about is not present in Lewisuchus—which, in fact, has carnivorous dentition (but keep reading). Thus, silesaurids evolved towards herbivory in much the same way that ornithischians must have--a stunning case of parallelism. And that predentary is really just a “tapering, toothless anterior end of the dentary,” not a separate bone.

Material that's not Lewisuchus.
Well wouldn’t you know it, Bittencourt et al. redescribed Lewisuchus in 2014 and came to some different conclusions. They noticed things in the holotype that hadn’t been recognized before, including pterygoid teeth and a single row of presacral osteoderms. Some elements previously referred to or associated with the holotype (including, unfortunately, the lower jaw and pedal elements) “more probably correspond to proterochampsid remains.” Thus, we don't actually know what the lower jaw of Lewisuchus looked like.

Lewisuchus and Pseudolagosuchus share a tibia, but:

“There is a size difference between these (P. major is about 20% larger), but both are badly preserved at their proximal and distal portions, hampering comparisons … the available information from the material attributed to any of these taxa is not enough for a formal synonymization between them.”

Bittencourt et al. ran their revised Lewisuchus through a phylogenetic analysis and found it to be more derived than Marasuchus but just basal to (Silesauridae + Dinosauria). If Lewisuchus is not a silesaurid (if it's distinct, Pseudolagosuchus might still be), then silesaurids might not have converged as much as Nesbitt (2011) suggested.

Tangentially, Benton & Walker (2010) found old children’s book favorite Saltopus as falling between Marasuchus and Lewisuchus on Ye Archosaur Phylogeny. Remember Saltopus? Man that takes me back.

As to the predentary, it would appear that silesaurids do not have a genuine, separate, predentary bone. Rather, the anterior tip of the dentary is simply toothless and comes to a point, mimicking the appearance of a predentary (which is amazing in itself). This closeup image of the dentary of Silesaurus from Holliday & Nesbitt (2013) pretty much confirms it:

The top two are Silesaurus. Clearly not a predentary.
Thus, as currently understood, Silesauridae contains the following taxa: Asilisaurus, Eucoelophysis, Diodorus, Lutungutali, Sacisaurus, and Silesaurus.

Pseudolagosuchus might be a silesaurid, but might also be synonymous with Lewisuchus. Two silesaurids—Asilisaurus and Lutungutali—are from the Anisian while later silesaurids are from the Norian. The group lived alongside dinosaurs for millions of years but went extinct prior to or at the Triassic/Jurassic boundary (as far as we know). The fact that Silesauridae occurs in the Anisian means that Dinosauria has a ghost lineage going back there as well—currently, the earliest skeletal evidence for dinosaurs is Carnian. I should note here that trackway evidence also favors an Anisian origin for Dinosauriformes (Brusatte, Niedzwiedzki & Butler, 2010).

While it's outside the scope of this post, I will mention that Griffin & Nesbitt (2016) looked at ontogeny in Asilisaurus and Piechowski, Talanda & Dzik (2014) looked at skeletal variation and ontogeny in Silesaurus. Both very interesting papers and probably have implications for the growth strategies of basal dinosaurs as well.

When I first read about Silesaurus way back in, oh, probably 2007, I was excited by the prospect that it might represent the most basal ornithischian, and that even took me on a little bit of a “diphyletic origin for the Dinosauria” bender, but as I read more about it and its cousins I came to appreciate both its unique qualities and the rampant homoplasty that paleontologists have to deal with around the base of the Dinosauria.

But as Darren Naish has pointed out on numerous occasions when discussing bird origins, it’s not like the tree is being completely renovated every time something new is found—instead, just a few taxa are moving around, and not even a lot. For example, Lewisuchus was a silesaurid, now it’s just outside Silesauridae.  To me, this means that the tree is actually very stable—something my younger self did not appreciate.

One more thing, sports fans. In a conference presentation (I have the abstract, but it's in Spanish), F. L. Agnolin argued that Triassic ornithischian Pisanosaurus mertii might actually be a silesaurid. It's not published yet, but I hope it is soon (in English). If true, I think that would mean there are no unambiguous Triassic ornithischians, right? Crazy to think about. 

So there you go: Silesauridae. What weird, wonderful animals. I'll leave you with this wonderful illustration of Silesaurus from Matt Celeskey. Thanks to Matt and Scott for letting me use their great pictures for this post.

Silesaurus by Matt Celeskey - used with permission.

*Does the predentary fuse to the dentary in old ceratopsids? I don't think it does.

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