Herrerasaurus ischigualastensis - Reig, 1963

Remains: Two complete skulls and several partial skeletons (Paul, 2010) including juveniles and adults (Paul, 1988).

Size: 4.5 meters (15 feet) long and 200 kilograms (450 pounds).

Location: Ischigualasto Formation of San Juan, Argentina (Weishampel et al., 2004).

Biology: Herrerasaurus ischigualastensis is the key species for all the Herrerasauridae. It is far more complete than other species and can be considered the “typical” herrerasaur. Like others of the family, H. ischigualastensis had four toes on its hind feet (though only three of these bore weight), giving better traction and stability but less speed and maneuverability compared to other theropods, but of these things it had no need, because all its prey animals and the predators that might have hunted it were slower still.

It preyed on the smaller cynodonts and rhynchosaurs of its environment. To dispatch these victims it probably used its large backward facing teeth. It would make a lunge, perhaps with its five-fingered hand or perhaps with its deep jaws. Once the teeth had been pushed into the flesh, there would be no escape even for the most determined prey. It would have made short work of any soft-bodied animal such as a cynodont by utilizing a hinge on the lower jaw that allowed for a slicing action of the serrated teeth.

The hands and jaws probably worked in coordination with each other to maneuver prey. The arms were long and three of the fingers were long and clawed (the other two were short and provided more of a grasping palm).

H. ischigualastensis was the most common predator in its environment and had little to fear most of the time. However, it was not the top predator. Saurosuchus was a huge rauisuchian that could have easily taken the life of a herrerasaur given the chance. Luckily, rauisuchians are top heavy and move about on all fours so, as long as Herrerasaurus kept its wits about it, it could out maneuver and out pace the larger carnivore.

Herrerasaurus led a relatively quite life for a theropod. The bones show no sign of stress so the animals would have done little active movements that involved bashing, bruising, or other activities that might stress the bones (Rothschild et al., 2001). This is supported by the scleral rings (eye bones) which bear similarities to animals that are only active for short periods throughout the day (Schmitz et Motani, 2011). However, some Herrerasaurus bones do have puncture wounds that match the teeth of other Herrerasaurus, so inter-species fights of a violent nature must have been fairly common. The wounds had been infected, but healed successfully (Molnar, 2001). A herrerasaur predator may have left some of these wounds, particularly those on the skull (Sereno et Novas, 1993). Saurosuchus is a likely culprit.

The habitat in which H. Ischigualasto lived experienced both a wet and dry season but it would have been at least fairly moist all year round. Ferns, horsetails, and giant conifers would have been common, as in any Triassic-type habitat and would have been thick across the forest floor. Signs of volcanic activity and heavy rainfall indicate that these animals died during the global flood in a situation probably involving volcanic activity.

Selected Organisms:

Aetosauroides schagliai

An aetosaur of fairly plain features, having short legs that hold its body close to the ground and no spines on the margins of its dorsal plates. It was about 3 meters (10 feet) long. It was a likely candidate for a Herrerasaurus ischigualastensis menu though, being so heavily armored, it would not have been the easiest option.

Exaeretodon frenguellii

An herbivorous cynodont about 1.8 meters (5.9 feet) long, having large incisor-like teeth at the front of its jaws. It was one of the largest traversodonts and a likely food source for Herrerasaurus ischigualastensis.

Hyperodapedon sanjuanensis

A strange, squat reptile with a beak and broad, flat head, it was only 1.3 meters (4.3 feet) long. It was chunky and heavily built. Though it might have used its narrow, sharp beak protruding form its upper jaw as a defensive mechanism, it was also a very likely food source for Herrerasaurus ischigualastensis, along with Exaeretodon frenguellii.

            Ischigualastia jenseni

A large dicynodont which grew close to 4 meters (13.2 feet) in length and weighed more about 2.5 tonnes. It was the largest animal in the Ischigualasto Formation. It sported a frill-like structure around the back of its head to protect the neck and had a horny beak with downward protruding edges, almost like tusks. Its overall bulk of body is reminiscent of a hippopotamus. It would have been a difficult task for a Herrerasaurus ischigualastensis to bring down, but so much meat was likely worth the effort.

Saurosuchus galilei

At 9 meters (30 feet) long, this was the top predator of the Ischigualasto and likely provided a consistent source of trouble for Herrerasaurus ischigualastensis. It was a rauisuchian, with a large skull and deep jaws filled with large, serrated and curving teeth. While it was certainly capable of hunting and likely targeted the large dicynodonts (above) it would not have been above steeling kills from a begrudging Herrerasaurus.

Eoraptor lunensis

A small dinosaur similar in proportion to Herrerasaurus ischigualastensis, but judging from its teeth, it was more of a generalist taking invertebrates and even plants. It would likely have fallen prey to Herrerasaurus, given an encounter.

Notes: Synonymous with Ischisaurus cattoi and Frenguellisaurus ischigualastensis.

References:

Nesbitt, Sterling J., Randall B. Irmis, et William G. Parker. March, 2007. “A Critical Re-evaluation of the Late Triassic Dinosaur Taxa of North America”. Journal of Systematic Paleontology. Volume 5, Issue 2. http://www.tandfonline.com/doi/abs/10.1017/S1477201907002040#.UcjA66XNvao

Molnar, R. E. 2001. “Theropod Paleopathology: A Literature Study”. From Mesozoic Vertebrate Life. Darren H. Tanke and Kenneth Carpenter (editors). Indiana University Press, Indiana. Pages 337-363.

Olsen, Paul E. and Donald Baird. 1986. “The Ichnogenus Atreipus and its Significance for Triassic Biostratigraphy”. From The Beginning of the Age of Dinosaurs: Faunal Change Across the Triassic-Jurassic Boundary. K. Padian (editor). Cambridge University Press, New York. Pg 61-87. http://www.ldeo.columbia.edu/~polsen/nbcp/olsen_baird_86.pdf

Paul, Gregory S. 1988. Predatory Dinosaurs of the World: A Complete Illustrated Guide. Simon and Schuster, New York.

Paul, Gregory S. 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press, Princeton.

Rothschild, Bruce, Darren H. Tank, and Tracy L. Ford. 2001. “Theropod Stress Fractures and Tendon Avulsions as a Clue to Activity”. From Mesozoic Vertebrate Life. Darren H. Tanke and Kenneth Carpenter (editors). Indiana University Press, Indiana. Pages 331-336.

Safran, J. et E. C. Rainforth. 2004. “Distinguishing the Tridactyl Dinosaurian Ichnogenus Atreipus and Grallator: Where are the Latest Triassic Ornithischia in the Newark Supergroup?”. Abstracts with Programs. Geological Society of America. 36(2): 96. https://gsa.confex.com/gsa/2004NE/finalprogram/abstract_70257.htm

Schmitz, Lars and Ryosuke Motani. 2011. “Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology”. Science. Volume 332, Number 6030, pages 705-708. http://www.sciencemag.org/content/332/6030/705

Sereno, Paul C. and Fernando E. Novas. 1993. “The Skull and Neck of the Basal Theropod Herrerasaurus ischigualastensis”. Journal of Vertebrate Paleontology. Volume 13, Number 4, pages 451-476. http://www.tandfonline.com/doi/abs/10.1080/02724634.1994.10011525#.UdZzQqVuDao

Weishampel, David B., Peter Dodson, et Halszka Osmolska (editors). 2004. The Dinosauria. University of California Press, Berkeley.