With a new skull reconstruction, this post was modified Nov 2, 2013.
Previously Poposaurus (Fig. 1) was only known from post-cranial remains. A complete Poposaurus, lacking a skull and a few cervicals, was described by Gauthier 2012 (Fig. 1).
Recently Parker and Nesbitt 2013, reported on a partial maxilla, dentary and prearticular, along with associated pubis and ischium (PEFO 34865). They matched their new find to prior Poposaurus specimens (Fig. 2), such as YPM 057100. In consideration of the PEFO tooth they wrote, “We fully describe the cranial elements and demonstrate that P. gracilis was a toothed hypercarnivore.”
There’s more than one way to rebuild that skull.
The pelvis – is this a match?
The new pelvis, PEFO 34865, (Fig. 3) is indeed most similar to that of the YPM Poposaurus pelvis (among the pelves I’ve been able to see), but, they’re not the same. Several traits unite the two pelves including the kink in the anterior ilium, the widely diverging ventral elements and the semi-open acetabulum (a dinosaur trait). The curved pubis and pubic medial flange with obturator foramen separates them. Rauisuchid taxa with a curved booted pubis include Arizonasaurus, which Parker and Nesbitt (2013) consider to be poposauroid. However, the large reptile tree separates Arizonasaurus from poposaurids.
With regard to the Parker and Nesbitt statement about hypercarnivory, let’s take another look at the post-cranial skeleton of Poposaurus (Fig. 1, YPM specimen). Are those tiny harmless hands those of a hypercarnivore? By comparison, the hands of Postosuchus (Fig, 3) look equally harmless, except for that can opener claw on digit 1. The elongated metatarsals of Poposaurus are also traits shared with dinosaurs, especially phytodinosaurs that depend on fleeing to avoid predation. Postosuchus (Fig. 3), by contrast, has small feet and short metatarsals relative to the tibia. Take poposaurs out of the Rauisuchia (because they don’t belong) and there are no rauisuchians with long metatarsals.
Parker and Nesbitt (2012) report, “The remains of poposauroids have long been confused with those of early dinosaurs because of the striking convergences (Nesbitt & Norell 2006; Nesbitt 2007; Nesbitt et al. 2007; Gauthier et al. 2011). The extraordinary disparity of poposauroid body plans, locomotor styles and dentition is unique within Pseudosuchia; however, the evolutionary sequence of acquisition of these features as well as the loss of others within the clade is incompletely understood.”
About that ventrally concave (bent) maxilla…
Fellow poposaur, Silesaurus has a bent maxilla and a straight premaxilla ventral rim. Lotosaurus has a straight maxilla and a bent premaxilla. All rauisuchians have a convex ventral maxilla with longer teeth.
About that tooth…
Sure that tooth has that carnivore look, but more than half is buried in the maxilla. That likely preserved the tooth with the bone. Ancestral taxa, such as Pampadromaeus, have similar sharp, even recurved teeth. More derived poposaurids do not. The teeth of the PEFO specimen trend toward less recurved and more symmetrical. Daemonosaurus, nested with basal ornithischians, but has hyper-carnivorous teeth, so there is precedent and analogy.
About that antorbital fenestra…
Other poposaurids have a tall open antorbital fenestra. the PEFO specimen does not. It more closely resembles that of rauisuchians. However, a small, but not elongated, antorbital fenestra is found in Daemonosaurus and other ornithischians.
About that robust dentary…
Other poposaurids don’t have a robust anterior dentary, but Pisanosaurus does. Rauisuchids have a robust anterior dentary.
About that robust skull…
The robust skull of the PEFO specimen stands out from the more gracile skulls found in other poposaurids (Fig. 1), but this is not unheard of in other phytodinosaurs. The armored ornithischians, like Scelidosaurus, also reduce the antorbital fenestra. Perhaps the post-crainia of the PEFO specimen is likewise armored and perhaps secondarily quadrupedal.
The poposaur list – different here
Among their poposaurids, Parker and Nesbitt (2013) include the long-necked Qianosuchus. In the large reptile tree that taxon nests with Ticinosuchus and aetosaurs. Parker and Nesbitt include the sailbacks Arizonasaurus and Xilosuchus among the herbivorous poposaurids. The large reptile tree nests those with the similarly carnivorous rauisuchians.
Poposaurus should have a phytodinosaur (herbivore) skull
All known poposaurs recovered in the large reptile tree are either toothless or, like Pisanosaurus and Silesaurus, have a plant-eater morphology and teeth. However, the sisters of the ancestors of poposaurs (like Pampadromaeus) had teeth typical of carnivores. Among the ornithischia, Heterodontosaurus retained fangs. We should also expect the skull of Poposaurus to be relatively short, like that in Daemonosaurus, Heterodontosaurus and Lotosaurus, with a large orbit and either a large antorbital fenestra.
At the base of the Phytodinosauria the teeth do not yet reflect an herbivorous diet, whether in the sauropodomorpha, ornithischia or poposauridae. Dinosaurs were diverging rapidly. Less perfect designs became extinct more quickly than the better morphologies. The PEFO specimen is among these. It’s type did not last long.
So is PEFO 34865 a Poposaurus?
The pelvis of the PEFO specimen is very close to Poposaurus, but it is also distinct. What remains of the skull does not closely resemble any other taxon in the archosaur clade of the large reptile tree. The skull is robust, which is expected in consideration of the robust cervicals. The anteriormost dentary is missing. The question is, were paired predentaries present? Or just a continued uplift of the mandible tip?
Incomplete remains are always a challenge. Parker and Nesbitt (2013) write, “Poposauroids display a complicated pattern of unusual character suites unlike any other group of pseudosuchians or any other archosaur group in the Triassic.” Let’s keep this discussion going.
Final passing thoughts
Parker and Nesbitt (2013) report, “Interestingly, the morphology of the neural spines that make up the sail in Lotosaurus adentus differs significantly from those of Arizonasaurus and Ctenosauriscus, supporting the idea that it was independently derived (Butler et al. 2011; Nesbitt 2011).” That was promoted long ago by the results of the large reptile tree. I’m glad to see others are finally starting to catch up to this. Now let’s get poposaurids back into the Dinosauria, where they belong1
Gauthier JA, Nesbitt SJ, Schachner ER, Bever GS and Joyce WG 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Mehl MG 1915. Poposaurus gracilis, a new reptile from the Triassic of Wyoming. Journal of Geology 23:516–522.
Parker WG and Nesbitt 2013. Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolution. Geological Society, London, Special Publications 379: 22 pp.