Postosuchus: Beast of the Week

This week we will be checking out a beast that shows us that evolution is capable of truly amazing things and how deceiving looks can really be.  This creature also helps teach us it's okay to change our perceptions as we learn new things, especially in science.  Enter Postosuchus kirkpatricki!

Postosuchus was a pseudosuchian archosaur, more closely related to crocodilians than to dinosaurs, that lived in what is now the Southern United States, specifically New Mexico, Arizona, Texas, and as far east as North Carolina, during the Triassic Period, between 221 and 203 million years ago.  It was one of the largest animals in its environment, with adults measuring between thirteen and fifteen feet long from snout to tail, and ate meat, probably hunting many of the other Triassic animals from the area, including fellow pseudosuchians, Shuvosaurus and Desmatosuchus, and the dinosaur, Coelophysis.  The genus name, Postosuchus, translates to "Post Crocodile", in reference to Post Town, Texas, where the first Postosuchus bones were unearthed.  The species name is in honor of the Kirkpatrrick family, who helped in Postosuchus' excavation.

Postosuchus life reconstruction in watercolors by Christopher DiPiazza.

 
Postosuchus has gone through a few makeovers over the years since it was discovered.  Because the first group of specimens' bones were not always found articulated, and often times among those of other species, early Postosuchus skeletal reconstructions accidentally were partially made of bones from other kinds of animals.  An example of this would be a the pelvis, hand, and toe bones, which would later be found to have really belonged to Shuvosaurus.  Because of this, Postosuchus was often depicted as being able to walk on four or two legs when alive.  However, we now know that it was much more likely to have been an obligate biped, only able to walk on its hind legs, like most theropod dinosaurs. This is thanks to more complete remains of Postosuchus found more recently, proving that the arms and hands were actually much smaller than previously thought, and were incapable of supporting its weight even if they could reach the ground.  The pelvis was much larger than originally thought, too, meaning the animal's center of gravity was over the hips.

Postosuchus brass skeletal mount on display at the Mesalands Dinosaur Museum in Tucumcari, New Mexico.  This mount is actually outdated.  Parts of the pelvis actually belong to Shuvosaurus, and the four-legged stance is improbable.  The vertebrae, especially behind the skull, would have much shallower neural arches in reality.

Postosuchus' skull was tall overall, but narrow laterally in the front, gradually getting wider towards the back.  It had prominent ridges over the eyes, forming an overhanging shelf, perhaps to help block out sun glare in life.  The eye sockets were relatively large, and partially faced forwards, suggesting Postoscuchus may have relied on its sense of site to hunt.  Due to large hollow areas connected to the nostrils, it likely also had a good sense of smell in life.  Postosuchus' jaws were lined with relatively long, pointed teeth that were serrated, for better cutting meat.  Some of the teeth were much longer than the others in certain parts of the jaws, like the tip of the lower jaw, and the mid-section of the upper jaw.  This would have made Postosuchus' mouth act like a deadly trap if it were to bite into a still-living/struggling animal.

Brass cast of Postosuchus skull at the Mesalands Dinosaur Museum.  Note how the longest teeth are in the middle of the upper jaw, and the front of the lower jaw.

Osteoderms have also been discovered with Postosuchus.  Osteoderms are pieces of bone that grow under the skin of an animal.  Modern crocodilians are most famous for having osteoderms, which they use as armor, as well as for built-in solar panels, to better absorb heat from the sun.  The osteodersm associated with Postosuchus appear to have been growing in two rows down the back.  It is still uncertain as to what their purpose was.

Photographs of Postosuchus foot bones from Karin Peyer's 2008 paper, describing the specimen found in North Carolina.  Note how Postosuchus would have walked on four toes on the ground.  This is different from what you'd see in MOST theropods.  (Therizinosaurs walk on four toes.)

Within pseudosuchia, Postosuchus belongs to the family called rauisuchidae.  Rauisuchians all exhibited fully erect posture, and typically had large heads with sharp teeth.  Not surprisingly, they are often mistaken for meat-eating dinosaurs.  They even possessed hollow bones, a trait normally associated with dinosaurs.  However, Postosuchus, like all pseudosuchians, shows some key differences from dinosaurs that give it away as more closely related to crocodilians.  It's ankles are the biggest giveaway because they could rotate, like those of modern crocodilians, rather than the more rigid, one-way bending ankles all dinosaurs have.  It also would have walked with it's first four digits touching the ground and its outermost fifth digit, which was much shorter, out to the side.  Postosuchus was another amazing example of convergent evolution.  In this case, it evolved the same body design you would see in large meat-eating dinosaurs like Megalosaurus or Allosaurus...before large, meat-eating dinosaurs even evolved!

That is all for this week!  As always feel free to comment below or on our facebook page.

References

Chatterjee, Sankar (1985). "Postosuchus, a new Thecodontian reptile from the Triassic of Texas and the origin of Tyrannosaurs". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 309 (1139): 395–460.

Long, Robert A.; Murry, Phillip. A. (1995). "Late Triassic (Carnian and Norian) tetrapods from the southwestern United States". New Mexico Museum of Natural History and Science, Bulletin. 4: 1–254.

Nesbitt, Sterling J.; Norrell, Mark A. (2006). "Extreme convergence in the body plans of an early Suchian (Archosauria) and Ornithomimid Dinosaurs (Theropoda)". Proceedings of the Royal Society B. 273 (1590): 1045–1048.

Novak, Stephanie E. (2004). A new specimen of Postosuchus from the Late Triassic Coelophysis Quarry, siltstone member, Chinle Formation, Ghost Ranch, New Mexico (M.Sc. thesis). University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.

Peyer, 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. 28 (2): 363–381.