Herrerasaurus: Beast of the Week

Today we will be looking at one of the earliest dinosaurs.  Enter Herrerasaurus ischigualastensis!

Herrerasaurus lived in what is now Argentina, in South America, during the Triassic Period, roughly 231 million years ago.  The largest specimen on record would have been about twenty feet long from snout to tail, but all adults may not have reached this size, possibly averaging out at around fifteen feet.  When alive Herrerasaurus would have eaten meat.  The genus name translates to "Herrera's Reptile", in honor of the goat herder who discovered the first bones of this dinosaur, Victorino Herrera.

My Herrerasaurus ischigualastensis life reconstruction in watercolors.

Herrerasaurus had a boxy profile to its skull, which was armed with a lower jaw that was able to flex back and fourth.  This is an unusual adaptation for dinosaurs, but is common in many modern lizards, like monitors, to help manipulate large chunks of food to the back of the mouth to be swallowed.  (lizards evolved this trait independently of Herrerasaurus, however.)  Herrerasaurus was also armed with extremely long, dagger-like teeth, that curved towards the back of the mouth, implying it was a meat-eater in life.

Cast of the first discovered Herrerasaurus skull on display at the Academy of Natural Science in Pennsylvania.

Herrerasaurus had strong arms, each armed with three curved claws.  It had proportionally short thighs, and long lower legs and feet, suggesting it was a fast runner in life.  Its tail was not very flexible because of the bony structures in its vertebrae, which is also a characteristic of dinosaurs that were good runners, to aid in maneuverability.  

Herrerasaurus had a number of odd characteristics about its anatomy that have caused scientists to dispute over what kind of animal it really was.  It walked in a fully erect posture, like all dinosaurs, but the socket where its femurs attached to its pelvis was not as open, or "window-like", as it is with later dinosaurs.  It also only had two vertebrae over its hips, called sacral vertebrae, whereas most dinosaurs typically have three.  Lastly the bone int its hip, called a pubis, was angled behind the body, which is typical in ornithischian dinosaurs, dromaeosaurs, and birds, the last two wouldn't evolve until millions of years after Herrerasaurus.

Herrerasaurus mounted skeleton on display at the Field Museum in Chicago.

Herrerasaurus is one of those fossil creatures that has gone through a few identity crises over the years since its discovery.  Because it lived so early on in the Mesozoic, before dinosaurs started branching off into more diverse forms, it has proven tricky to place, genetically.  At first, because of its teeth and long legs, it was classified as a very early theropod.  However, some suggested it had more in common with early sauropodomorphs, like Plateosaurus.  Some have suggested that Herrerasaurus, despite its meat-eating qualities, was actually closest to ornithischian dinosaurs, because of its backwards-facing pubis bone.  It was even proposed to be not a dinosaur, at all, placed just outside the dinosaur family tree as something more basal to the group.  However, as more and more fossils from the Triassic are being unearthed, the latest analysis of Herrerasaurus places it back as a dinosaur,  almost at the very base of the dinosaur line, as a kind of very early saurischian dinosaur, related to theropods and sauropods.  Further study may yet again change this!

References

Benedetto, J.L. (1973). "Herrerasauridae, nueva familia de saurisquios triasicos" (PDF). Ameghiniana. 10 (1): 89–102.

Bittencourt, J.S.; Arcucci, A.B.; Marsicano, C.A.; Langer, M.C. (2014). "Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chan~ares Formation, Argentina), its inclusivity, and relationships amongst early dinosauromorphs". Journal of Systematic Palaeontology. 0 (3): 1–31. 

Gauthier, J.A.; Nesbitt, S.J.; Schachner, E.R.; Bever, G.S.; Joyce, W.G. (2011). "The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion" (PDF). Bulletin of the Peabody Museum of Natural History. 52 (1): 107–126. doi:10.3374/014.052.0102. Archived from the original (PDF) on 2015-09-24.

Gilmore, Charles W. (1920). "Osteology of the carnivorous dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus". Bulletin of the United States National Museum. 110 (110): 1–159. 

Nesbitt, S. J.; Smith, N. D.; Irmis, R. B.; Turner, A. H.; Downs, A. & Norell, M. A. (2009). "A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs". Science. 326 (5959): 1530–1533.

Novas (1993). "New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina". Journal of Vertebrate Paleontology. 13: 400–423.

 Novas, F.E. (1994). "New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina". Journal of Vertebrate Paleontology. 34 (4): 400–423.

Padian, K.; May, C.L. (1993). "The Earliest Dinosaurs". In Lucas, Spencer G.; Morales, M. The Nonmarine Triassic. New Mexico Museum of Natural History & Science Bulletin. 3. pp. 379–381.

Sereno, P.C.; Novas, F.E. (1993). "The skull and neck of the basal theropod Herrerasaurus ischigualastensis". Journal of Vertebrate Paleontology. 13 (4): 451–476.