Poposaurus, Postosuchus, and the Dinosaur Mimic Croc Walk

Poposaurus walked like a dinosaur. That’s weird, because the Triassic critter wasn’t a dinosaur at all, but a peculiar archosaur more closely related to crocodiles. Even though paleontologists traditionally thought that dinosaurs had an immediate edge over all over forms of Triassic life thanks to muscular legs held close together right beneath their bodies, rather than sprawled out to the sides, there were also croc copycats that cramped dinosaurian style.

Two years ago, I wrote a short ScienceNOW piece about Poposaurus and a monograph describing the creature’s postcranial skeleton. Even though the 225 million year old archosaur was named almost a century ago, it has only been recently that more complete individuals have shown how strange Poposaurus truly was.

In the big picture of archosaur evolution, Poposaurus was a pseudosuchian – a group solely represented by alligators, crocodiles, and gharials today, but encompassed a surprising and frightening array of related lineages in the deep past. Roughly, the pseudosuchians are “croc-line archosaurs.” The group not only includes scattered modern species, but totally extinct groups such as the “armadillodile” aetosaurs, terrifying rauisuchians, and the strange lineage of animals that Poposaurus belonged to, called poposauroids, which included bipedal herbivores and sail-backed carnivores such as Arizonasaurus in their ranks.

Poposaurus wasn’t the only one of its kind to strut upright. The first poposauroids discovered to walk like dinosaurs were toothless, beaked herbivores. Effigia, named by paleontologists Sterling Nesbitt and Mark Norell in 2006, was a fortuitous find that outlined the shape of bipedal crocodile cousins. After that, other strange pseudosuchian fossils started to make sense. Shuvosaurus, originally proposed to be a Triassic dinosaur similar to Ornithomimus, turned out to be yet another beak-bearing pseudosuchian with a posture just like that of Effigia.

Evolution spawned anatomical convergence between dinosaurs and pseudosuchians, and, for reasons still unknown, dinosaurs succeeded while the bipedal pseudosuchians died out. The bipedal croc-line archosaurs have made one thing clear, though. Contrary to previous proposals, dinosaurs did not persist and proliferate by virtue of their posture alone.

Effigia and Shuvosaurus were beaked herbivores. What Poposaurus ate, however, has been harder to outline. What little we know of the pseudosuchian’s skull has traditionally come from hard-to-identify fragments. Based upon skull fragments found with some postcranial bones in Arizona’s Petrified Forest National Park, however, paleontologists William Parker and Sterling Nesbitt have been able to confirm that Poposaurus was a roughly 13-foot-long carnivore that retained the sharp, recurved teeth of its ancestors.

After studying a portion of the left upper jaw and a pair of pieces from the lower jaws, the paleontologists found that Poposaurus was a hypercarnivore. Therefore, Parker and Nesbitt note, herbivory and bipedalism did not evolve at the same time among the weird dinosaur mimics. If anything, the various poposauroids were Triassic oddballs who embodied a mix of postures, diets, and even ornamentation during their evolution.

But there was another bipedal pseudosuchian that was even scarier than Poposaurus. Named in 1985, and initially confused for a Tyrannosaurus ancestor (which tells you something about the animal’s fearsome appearance), Postosuchus has since been regarded as an awful apex predator of Late Triassic North America. About as long as Poposaurus, the bulkier Postosuchus had a deeper skull set with teeth that the carnivore likely put to work on aetosaurs and other armored prey of its era. Most restorations envision the hunter clambering through Triassic forests on all fours, but a rexamination of the pseudosuchian’s skeleton conducted by Jonathan Weinbaum hints that Postosuchus might have been a principally bipedal animal.

Postosuchus on all fours has always looked a bit odd to me.  Despite being cast as a quadrupedal carnivore, the creature’s forelimbs are relatively small and, as Weinbaum describes them, “slender.” The hindlimbs of Postosuchus, by comparison, were more stoutly constructed, with a large, broad foot.  The body of Postosuchus looks as if the animal could have easily balanced on two legs, and, as Weinbaum suggests, the carnivore may very well have been a habitual biped. Weinbaum acknowledges that the hypothesis needs additional testing, but there’s no reason to take the traditional, four-on-the-floor reconstruction of Postosuchus as a given.

Despite being a predator of similar size and posture, though, Postosuchus wasn’t a close relative of Poposaurus. As Weinbaum points out, Postosuchus belonged to a tangle of pseudosuchians currently categorized as “rauisuchids” – principally predatory croc cousins that were the major carnivores of their time. So if Postosuchus really did walk on two legs, then bipedal crocodile cousins evolved more than once during the Triassic. Think about that for a moment. What if, by some quirk of history, the relatively marginal dinosaurs of the Late Triassic were all wiped out, and the pseudosuchians survived. What kinds of unseen, dinosaur-like forms would have evolved? Or would the pseudosuchians have kicked off an entirely different, even stranger profusion of Mesozoic life? We’ll never know. Poposaurus, Postosuchus, and their kind offer a tantalizing, yet frustrating, glimpse at evolutionary threads that were cut short at the end of the Triassic.

References:

Parker, W., Nesbitt, S. 2013. Cranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolutionCranial remains of Poposaurus gracilis (Pseudosuchia: Poposauroidea) from the Upper Triassic, the distribution of the taxon, and its implications for poposauroid evolutionCranial 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. v. 379. http://dx.doi.org/10.1144/SP379.3

Weinbaum, J. 2013. Postcranial skeleton of Postosuchus kirkpatricki (Archosauria: Paracrocodylomorpha), from the upper Triassic of the United StatesPostcranial skeleton of Postosuchus kirkpatricki (Archosauria: Paracrocodylomorpha), from the upper Triassic of the United StatesPostcranial skeleton of Postosuchus kirkpatricki (Archosauria: Paracrocodylomorpha), from the upper Triassic of the United States. Geological Society, London, Special Publications. v. 379. doi 10.1144/SP379.7