Hesperosuchus: Beast of the Week

This week we will be checking out a relative of modern crocodilians that has tricked scientists multiple times!  Enter Hesperosuchus agilis!

Hesperosuchus was a pseudosuchian (related to crocodilians) that lived in what is now Arizona and New Mexico, USA, during the late Triassic period, about 210 million years ago.  From snout to tail the most complete skeleton measures about four feet (1.2m) but other fossils suggest it could have grown larger, up to about six feet (1.8m).  The genus name translates to "Western Crocodile" due to where it was fond and its relation to modern crocodilians, and the species name to "agile" in reference to how it was likely a swift and agile runner in life.

Hesperosuchus stealing an egg from the dinosaur, Coelophysis. Watercolor reconstruction by Christopher DiPiazza.

When Hesperosuchus was first discovered it was initially thought to be a kind of meat-eating dinosaur.  This is due to the fact that it had hollow bones, a trait that at that time was not known to be in pseudosuchians.  Hesperosuchus is just one of many examples of how the group that includes modern crocodilians was extremely diverse and evolved into many different forms during the Mesozoic, especially the Triassic.  Not only did Hesperosuchus have hollow bones, but it also possessed fully erect upright posture with long slender limbs.  It may have even been able to run on its hind legs.  

Hesperosuchus also possessed a laterally compressed skull (again like most theropod dinosaurs) which is in contrast to modern crocodilians, all of which have skulls that are more flattened top to bottom, an adaptation for spending much of their time in the water.  Aquatic crocodilians had not yet evolved in the Triassic and all of their closest relatives from then, like Hesperosuchus, were actually land animals.  Hesperosuchus possessed pointed teeth that were serrated on both the front and rear edges.  Like modern crocodiles it had a slight notch in its snout where its bottom teeth interlocked with its top jaws.

Hesperosuchus skull image from the 2012 paper by Nesbitt et al., referenced below.

 Like most pseudosuchians, Hesperosuchus also had osteoderms, bony armor that exists in the skin, in the form of two rows running down its back.  This was likely for defense against predators, of which there were many Hesperosuchus had to be weary of.  That being said it appears this armor may not have protected Hesperosuchuchus every time.  There is a famous skeleton of the dinosaur, Coelophysis, has what appears to be bones from Hesperosuchus inside its stomach cavity. 

When alive, Hesperosuchus would have lived alongside other pseudosuchians, like the herbivorous aetosaurs and shuvosaurs, and the large  carnivore, Postosuchus.  It also would have shared its environment with crocodile-resembling (but not) phytosaurs, as well as the carnivorous dinosaur, Coelophysis.  

References

Clark, James M.; Sues, Hans-Dieter; Berman, David S. (19 January 2001). "A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs". Journal of Vertebrate Paleontology. 20 (4): 683–704.

Colbert, E. H. 1952. A pseudosuchian reptile from Arizona. Bulletin of the American Museum of Natural History 99:561–592.

Nesbitt, Sterling J.; Turner, Alan H.; Erickson, Gregory M.; Norell, Mark A. (22 December 2006). "Prey choice and cannibalistic behaviour in the theropod Coelophysis". Proceedings of the Royal Society B. 2 (4): 611–4.

Nesbitt, Sterling J.; Turner, Alan H.; Weinbaum, Jonathan C. (September 2012). "A survey of skeletal elements in the orbit of Pseudosuchia and the origin of the crocodylian palpebral". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 103 (3–4): 365–381.