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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.

Photo by Dallas Krentzel, image from Wikipedia.

A reconstruction of Postosuchus. Photo by Dallas Krentzel, image from Wikipedia.

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.

Image courtesy Jonathan Weinbaum.

Postosuchus as a biped. Image courtesy Jonathan Weinbaum.

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 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 States. Geological Society, London, Special Publications. v. 379. doi 10.1144/SP379.7

There are 16 Comments. Add Yours.

  1. Zach Miller
    February 21, 2013

    I dunno, man. I think Postosuchus as a biped looks just as strange–if not stranger. It looks a bit like short-legged Majungasaurus, with too much weight in front of the hips and not enough behind. The neural spines must have impeded upward mobility of the spine to “push back” the center of gravity. Damn weird if true, though.

  2. Jerrold Alpern
    February 21, 2013

    What was the relationship of Prestosuchus to Postosuchus and Poposaurus?

  3. Brian Switek
    February 21, 2013

    Zach: Hopefully we’ll see some biomechanics tests for Postosuchus in the future. And even for Poposaurus – even though some pseudosuchians mimicked dinosaurs in a rough sense, they clearly were different in the details.

    Jerrold: Prestosuchus is another “rauisuchid”, closer to Postosuchus than Poposaurus. It’ll be interesting to see where all the various “rauisuchids” eventually fall out (and I’ll be happy to stop using quotes around their name, which makes me sound like I’m being sarcastic).

  4. Alexander Kleine
    February 21, 2013

    Isn’t that a coincidence? Me and my academic advisor Jonathan Weinbaum were just talking the Rauisuchia paper that came out and we even sent me the online version. For me and Weinbaum, the bipedal posture sure is accurate enough for us.

  5. BJ Nicholls
    February 21, 2013

    Rany Irmis’ recent UFOP talk had a great takeaway lesson. Modern crocodilians are not representative of their Triassic forebears, and there are recent fossils that show that crocodylomorphs were even more diverse earlier than previously understood.

  6. 220mya
    February 21, 2013

    Brian – there’s no need to use Rauisuchidae or rauisuchid in quotes – that is a monophyletic group. There terms that require quotes are “Rauisuchia” and “rauisuchian”.

  7. Brian Switek
    February 21, 2013

    Thanks, Randy. Although I’ve seen rauisuchid in quotes, as in Weinbaum’s paper. The varying usage is sometimes a bit confusing.

  8. 220mya
    February 22, 2013

    Although in the past “Rauisuchia” and Rauisuchidae have sometimes been used synonymously, Nesbitt (2011) provided a formal phylogenetic definition for Rauisuchidae: The most inclusive clade containing Rauisuchus tiradentes (Huene, 1942) but not Aetosaurus ferratus Fraas, 1877, Prestosuchus chiniquensis Huene, 1942, Poposaurus gracilis Mehl, 1915, or Crocodylus niloticus Laurenti, 1768 (sensu Sereno, 2005).

    Thus, by definition Rauisuchidae is monophyletic, and recent phylogenetic analyses suggest it includes Rauisuchus, Polonosuchus, and Postosuchus spp.

  9. Brian Switek
    February 22, 2013

    Got it! Thanks, 220!

  10. DWalker
    February 22, 2013

    Imagine the first prehistoric findings…they all seemed weird and several nary seemed possible. Impressive data shows that biped creatures were not only a possibility but a reality. Weinbaum is brilliant.

  11. jenny
    February 25, 2013

    oh my god they are graet and cool

  12. Fuzzy Logic
    February 25, 2013

    I believe that even though life on earth in this day and age is very refined, back in the prehistoric times is where i’d like to be. The lush trees, the inviting forest. Creatures of exceptional ability such as this. It makes you wonder what could have been, dosen’t it?

  13. Cai Yuan
    February 28, 2013

    “Rauisuchidae” in scare-quotes is justified by its probable inclusion of Teratosaurus, however, giving Teratosauridae Cope 1871 priority.

  14. David Peters
    March 1, 2013

    What is it about Poposaurus and poposaurids that make paleontologists think they were croc relatives? The calcaneal tuber? The large reptile tree at reptile evolution.com recovers poposaurids as phytodinosaurs with a convergent development of a calcaneal tuber, the only dinosaurs to do so. This is convergent with crocs themselves, which also developed a calcaneal tuber after basal crocs, like Gracilisuchus and Scleromochlus, did not have one. One trait, even an ankle trait, does not trump the suite. It’s time to add more pertinent taxa and rerun the analysis.

  15. mo
    July 28, 2013
  16. David Peters
    July 28, 2013

    ^ In this case all those traits are all in the literature.

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