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The Poposaurus, The Strange Dinosaur-like Creature of The Triassic

Mamerto Adan is a feature writer back in college for a school paper. Science is one of his many interests, and his favorite topic.

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The notion that dinosaurs are reptiles is an outdated idea. Tagging them as giant lizards weren’t the fault of early scientists though. Their knowledge was limited by the available information and fossils at that time. Dinosaur remains though were well known even before science recognized these giant beasts, and in fact the Chinese thought they were dragon bones. And when the tooth of Iguanodon was discovered by Mary Ann Mantell, its resemblance to the choppers of modern iguana led to the early conclusion that it’s basically an oversized lizard. Lumbering lizards in fact. Creatures with barrel-like body as what’s presented in the Crystal Palace models. Fast forward today and the knowledge about dinosaurs greatly improved, and we now know that they are not giant reptiles, but sleek monstrous bird relatives.

But back in the Triassic age, there are strange bipedal creatures that looked like dinosaurs, but not exactly dinosaurs. In fact, they are reptiles and they are relatives of modern crocodilians. Unlike modern crocodiles sprawled in the swamp, they walked on their hind legs, like what theropods did.

The name Poposaurus might sound cartoonish, but they are one of those bizarre Triassic reptiles that mimic a dinosaur’s body plan. And people who are turned off by feathered raptors might find them amusing.

Discovery

Poposaurus ilium.

Poposaurus ilium.

It was way back in 1904 when fragments of Poposaurus remains was found. Paleontologist J.H Lees described this fossil (part of the hip or the ilium) to be a body part of Paleohirnus bransoni, an unrelated extinct Triassic reptile. And by the way, it was found in the Popo Agie Formation. As more complete fossils became available, paleontologist M. G. Mehl realized that the ilium described by Lees belonged to a different species. With differently shaped hips, limb bone and vertebrae, he named this new species Poposaurus, from the place it was discovered.

But back then, the distinction with reptiles and dinosaurs was not well established. In 1921, Franz Nopcsa, a Hungarian paleontologist classified it as a dinosaur (ornithischian to be exact). According to him, it bears resemblance to dinosaurs like iguanodons.

It stayed in the dinosaur family for years, and Edwin Harris Colbert even classified it as a theropod (in 1961). But in 1977, Peter Galton reclassified Poposaurus as a pseudosuchian, a genus of reptiles that predates the dinosaur. We will learn more about this family of Triassic crocodile thingy later, but as more fossils of Poposaurus was uncovered, it became clear that the creature might be a hyper-carnivore rather than a plant eater.

The Pseudosuchians

A collection of Pseudosuchian species.

A collection of Pseudosuchian species.

The animal was a part of strange division of reptiles back in the Triassic known as Pseudosuchus. They are under the Archosauria, which also includes dinosaurs and modern birds. Basically, it meant “false crocodile,” and yes, it is more crocodile than birds. This made the name Pseudosuchus a misnomer, because they are true crocodilians and not mimic creatures. One might think that being a croc relative means they look like modern crocodiles. Pseudosuchians have a vast diversity of alien looking reptiles with different forms of lifestyles. Simply they came in many styles and shapes. They could be scavengers like Ornithosuchids, a few were herbivores complete with armor plating, and some are enormous predators lime the Saurosuchus.

The heyday of these crocodile thingies was in the late Triassic. This was where our Poposaurus appeared. It seems that they occupied the niche that dinosaurs will dominate in later age, seeing on their body plans. People could only guess what forms of dinosaur-like creatures would have evolved, if the Pseudosuchians continue to exist. Unfortunately, much to the frustrations of some, that never happened. The Triassic period ended with mass extinction, and some blamed it on climate change probably due to volcanic eruptions. Other theories include meteor impact. Nevertheless, the extinction of these “false crocodiles” gave way to the dinosaur take-over. At the start of the Jurassic age, it will be the dinosaur’s turn to succeed them as the dominant terrestrial carnivores and herbivores.

What it Looked Like

Poposaurus size compared to a human. Restoration by Jeff Martz.

Poposaurus size compared to a human. Restoration by Jeff Martz.

Back to our Poposaurus, you might be wondering what the creature looked like if it was alive. I mean it supposed to look a dinosaur.

And it is.

Seeing the reconstruction of the skeleton, the Poposaurus seems to resemble a rudimentary therapod. A raptor to be exact. And being a reptile, it never had feathers hence fans of the featherless raptor will be delighted.

Starting with the skull, it was never flat like a crocodile, but deep like a theropod and I don’t blame early paleontologists mistaking it for a dinosaur. But it was the legs that I found to be intriguing. M.G. Mehl described the Poposaurus as well muscled, light in weight and possibly bipedal. He arrived to this by basing the long limb bone and deep hip sockets. And do note that bipedal dinosaurs also have the same features, hence this indicates that we are dealing here with a large bipedal crocodile. This was confirmed in 2011, when a nearly complete specimen was discovered. This newly discovered remains also have front and hind limbs, and it had shorter arms but longer legs.

The approach to being upright differs among Poposaurus and other Pseudosuchians. In the case of Poposaurus, it had a “pillar erect stance.” Simply the hip sockets face downwards over the head of the femur. Dinosaurs on the other hand had hip sockets facing laterally and the head of the femur is bent to fit in. Lastly, the muscles of the legs resembled that of crocodiles rather than birds.

Overall, a Poposaurus had the appearance of a scaly raptor, only with less upright stance and shorter legs (the creature is 13 feet long).

Other Bipedal Triassic Croc

Postosuchus size compared to a human. Image from Wikipedia.

Postosuchus size compared to a human. Image from Wikipedia.

As what’s mentioned earlier, Pseudosuchians come in all shapes and sizes, from armored to bipedal. With that said, the already terrifying Poposaurus was not the only bipedal croc to roam the Triassic.

Again, the name Postosuchus (meaning “crocodile from Post”) shared a not so terrifying name with Poposaurus. But one will be thankful that no one is around in the woods to terrorize campers. The skeleton of the Postosuchus is large and robust, and with small forelimbs, it is also capable of bipedal locomotion. At 13 feet long it’s a size larger than Poposaurus and preyed on other Pseudosuchians.

The variety of these Triassic crocodile things will require more than two articles to discuss. Nevertheless, knowing these early forms of monstrosity just added color to the underrated lost world of the Triassic.

References

1. Bryan Switek (May 11, 2011) "Triassic Crocodile Cousin Walked Like a Dinosaur." Science Magazine.

2. Mehl, M.G. (1915). "Poposaurus gracilis, a new reptile from the Triassic of Wyoming". The Journal of Geology

3. Weinbaum, J.C.; Hungerbühler, A. (2007). "A revision of Poposaurus gracilis (Archosauria: Suchia) based on two new specimens from the Late Triassic of the southwestern U.S.A." Paläontologische Zeitschrift.

4. Gauthier, J. (1986). "Saurischian monophyly and the origin of birds". In Padian, K. (ed.). The Origin of Birds and the Evolution of Flight. Memoirs California Academy of Sciences.

5. eyer, Karin; Carter, Joseph G.; Sues, Hans-Dieter; Novak, Stephanie E.; Olsen, Paul E. (2008). "A new Suchian Archosaur from the Upper Triassic of North Carolina". Journal of Vertebrate Paleontology.

Comments

Mamerto Adan (author) from Cabuyao on May 23, 2019:

Thanks eric for tip!

Eric Evinczik on May 23, 2019:

It looks like 13 feet not 13 meters... Must be a typo....