Thursday, June 7, 2018

Faux Theropods

The skeleton of Effigia
Bit of a shorter post this month, as I'm prepping a photo-heavy review of the Creative Beast raptor toys that I just received. This month I’ve decided to tackle one of my absolute favorite groups of Triassic weirdos: shuvosaurids! They are a fairly obscure clade outside of paleo circles, consisting of only three (used to be four) named genera but shuvosaurids should be poster children for the concept of convergent evolution. These things are pseudosuchian theropod mimics, and not just theropods but ornithomimosaurs. Ostrich dinosaurs in the Triassic...but suchians!

The largely complete skull of Shuvosaurus
Shuvosaurus inexpectatus was the original shuvosaurid (as you might expect), a Late Triassic taxon described in by Chatterjee in 1993. While the Texas holotype (TTU-P-9280) consists of most of the skull, paratypes (TTU-P-9281 & 9282) filled in some details. He referred some associated fragmentary bits—a vertebra, fragment of scapula, and tibia—to Shuvosaurus as well, but it’s not clear whether those bones belong with the skull, and have been ignored by subsequent authors. The skull is small, probably from a juvenile, and toothless. Rather incredibly, Chatterjee referred it to the Ornithomimosauria in a new basal family, the Shuvosauridae. This doesn’t just pull ornithomimosaurs back to the Late Triassic but every other theropod lineage that branched off prior to ostrich dinosaurs (coelurosaurs, basically). Of course, this referral was met with some degree of disbelief from other workers.

The holotype of Chatterjeea
The first blow came in 1995, when Long & Murry described both Chatterjeea elegans and its new family, the Chatterjeea (sigh). The holotype, from the same area as Shuvosaurus, is an incomplete but very informative postcranial skeleton. Additional fragmentary material from Arizona and New Mexico was referred to this new genus. In short, there’s apparently a lot of Chatterjeea material. Despite being bipedal and gracile, the authors refer their new taxon to the Poposauridae. They also comment on Chatterjee’s Shuvosaurus, expressing doubt that Shuvosaurus is a theropod at all, much less an ostrich dinosaur, instead of a “highly derived pre-dinosaurian archosaur.” Further, they suspect that the holotype skeleton of Chatterjeea may belong with the holotype skull of Shuvosaurus. Although they admit that the two cannot be synonymized with certainty pending the discovery of overlapping material, that possibility can’t be ignored. If they are from the same species, then Chatterjeea becomes a junior synonym of Shuvosaurus.

Despite Chatterjeea’s poposaurid affinities, Long & Murry admit that “If it were not for a number of similarities in common with the popposaurids and the presence of a crurotarsal [ankle] joint, the chatterjeeids might be confused as a sister group to the Dinosauriformes.” Foreshadowing!
Almost on cue, Rauhut (1995), in his redescription of the skull of Shuvosaurus, refers the animal to Theropoda, but does not endorse an ornithomimosaur affiliation. After going over why Shuvosaurus can’t be an ornithomimosaur, coelurosaur, or tetanurine, he settles on a “basal theropod” position, noting that “more information is needed to assess its phylogenetic position.” Rauhut does not comment on the possibility that Shuvosaurus may be synonymous with Chatterjeea and probably considers them to be very different animals.

Onward and upward: brief mentions of Shuvosaurus in Murry & Long (1997) and Osmolska (1997) further question its theropod affinity. Hunt et al. (1998) supported the synonymy of Shuvosaurus and Chatterjeea.

The skull of Effigia okeeffeae
Then, in 2006, Effigia okeeffeae happened when a nearly complete skull and skeleton was described by Nesbitt & Norell (2006). Here was a relatively small-bodied, bipedal, toothless, puny-armed pseudosuchian complete with crurotarsal ankle that also, finally, illuminated the murky identity of Shuvosaurus. Nesbitt & Norell further endorse the idea that the Shuvosaurus skull belongs to the Chatterjeea skeleton, and so sink former as a junior synonym of the latter. The authors pull a third fragmentary taxon, Sillosuchus longicervix from Argentina (Alcober & Parrish, 1997), into the Shuvosauridae as well, demonstrating that these animals had a much wider distribution than just the southwestern United States.

The very next year, Nesbitt (2007) published a complete description of Effigia. This impressive monograph details its osteology, tests its growth (via histology), tests its relationships with other pseudosuchians, and goes over its impressive list of convergences with dinosaurs. Surprisingly, shuvosaurids form a sister group relationship with the larger, sail-backed Lotosaurus (who is also toothless) and Arizonasaurus.* Finally, Nesbitt et al. (2007), in their comprehensive review of North American Triassic dinosaurs, concluded that the vertebrae of alleged Triassic theropod Gojirasaurus quayi (Carpenter, 1997) cannot be differentiated from Shuvosaurus while its pubis and tibia cannot be differentiated from Coelophysis.

Effigia okeefeae by Carl Buell
Shuvosaurids present an incredible case of convergent evolution with theropods. Nesbitt (2007) presents an impressive list, but the ones I found most interesting were:
  • No osteoderms, and this also appears to be the case in the shuvosaurids’ sail-backed sister group including Arizonasaurus and Lotosaurus.
  • Strongly reduced forelimb—what could it do with those tiny hands?
  • Four sacral vertebrae. It’s apparently rare to find a croc-line archosaur with more than two. Furthermore, in Effigia and Shuvosaurus, the vertebrae are fused into a rigid rod of bone.
  • An expanded pubic boot, which is very common among theropods (think Allosaurus).
  • Toothless jaws, which must be exceptionally rare in pseudosuchians.
Effigia was a dog-sized animal, but Shuvosaurus may have grown much, much larger based on the vertebrae originally assigned to “Gojirasaurus.” When it was originally described, “Gojirasaurus” was thought to be as large as Dilophosaurus!

The Hayden Quarry dinosauromorphs by Donna Braginetz
Shuvosaurids were living alongside lagerpetids, silesaurids, and basal theropods (like Tawa, Chindesaurus, and Coelophysis) during what I assume to be their entire history. And that’s weird because all four of these groups had similar bauplans and were not too disparate in size (see image at left which does not include a shuvosaurid). However, they likely avoided direct competition by going after different food: shuvosaurids were probably herbivores and/or insectivores, silesaurids were definitely herbivores, and basal theropods were predators of small vertebrate game. Can’t really be sure what Dromomeron was eating because there’s no skull material associated with any specimens (that I know of), but as a lagerpetid, I have to assume it was predating insects and tiny vertebrates.

Effigia's "crocodile-normal" ankle
I wonder if the shuvosaurid ankle structure, which Nesbitt (2007) compares to Alligator, differentiated it from contemporary ornithodirans. Ornithodirans have limited motion in the ankle joint, being only able to bend the foot in a fore-and-aft arc. Crocodilians, however, have some rotation in the foot relative to the lower leg. Unfortunately, I don’t think anybody’s done any speculation on the functional morphology of these suchian dinosaur-mimics. Paleoartist Gabriel Ugueto has wondered if shuvosaurids were plantigrade or not given their ankle structure--I tend to think walking would've been difficult if they were flat-footed but it's an interesting question. Effigia remains the youngest shuvosaurid known, living alongside Coelophysis towards the end of the Late Triassic. The group does not appear to have survived the End-Triassic Extinction. Perhaps their niche was taken over by basal ornithischians.

Anyway, there you have it. Shuvosaurids remain my favorite example of convergent evolution, even moreso than fully-shelled placodonts or dolphin-shaped ichthyosaurs. These are animals that, if you never found their ankle bones, might today be classified as aberrant theropods.

*This group (sail-backed poposaurs) fascinates me and I don’t understand why nobody’s gotten around to describing Lotosaurus, an especially interesting taxon whose skeleton and skull are apparently completely known based on dozens of individuals who all died together. It was “announced” in a one-paragraph paper from 1975 (Zhang) and since then there’s only been a publication on the taphonomy of a giant Lotosaurus bonebed (Hagen et al. 2018) but no real osteological description has been put forward.

1 comment:

  1. So if Ornithomimus is convergent with an ostrich is Shuvosaurus=ostrich?
    I'm joking I'd think it unlikely croc-line archosaurs would have feathers. Not impossible though.
    I'm not always a big fan of convergent evolution because it confuses non-sciency people. They think it's true that there's a master plan for evolution. That it's inevitable that for every era there has to be an elephant type, a cow type, a whale type, ostrich type, when these animals (sauropods, elephants, bison, ceratopsians, rhinos, brontotheres, baleen whales, pilosaurs, etc.) are very different and the convergence is superficial.
    Inside the animal's skeleton, metabolism, respiration, aren't similar at all. The convergence might be because they're utilizing a similar food source, or method of locomotion, or they're just big. Or it could be random.
    I think part of the lesson with Shuvosaurus is the danger of extrapolating entire animals from a few convergent pieces. Parts of the skeleton of Shuvosaurus look like the perfect basal ornithomimid but with more pieces it's clearly a pseudosuchian. The arms are especially un-ornithomimid.
    I get confused when I see a picture of an animal, see certain features on it. But when I look up the known skeletal structure it's fragmentary. This is fine, it works much of the time but when you're jumping 50 million years it's a bad idea. Especially when it's applied to a basal ancestor type.