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Herrerasaurus ischigualastensis (Osvaldo A. Reig, 1963)

Name Means: "Herrera's Lizard" Length:  
Pronounced: huh-Rare-uh-Sore-us Weight:  
When it lived: Late Triassic - 225 MYA    
Where found: San Juan Province, northwestern Argentina    
    Herrerasaurus is one of the oldest dinosaurs ever found. It is close to 230 million years old! It is one of three dinosaurs found in South America that represent the earliest form of meat-eating dinosaurs. In fact, Herrerasaurus may be the oldest of the three. It was a fast, ferocious hunter and it set the stage for large predatory dinosaurs to become the most dominant animals on earth for more than 150 million years.
    Unlike later dinosaurs, Herrerasaurus had a wide variety of non-dinosaur creatures from which to chose for its menu. In the Triassic there were many types of creatures still roaming the land that would later disappear in an extinction event at the end of the Triassic, which the dinosaurs survived. One of the main types of creatures upon which Herrerasaurus would have fed were the mammal-like reptiles. For many millions of years the mammal-like reptiles were the dominant land creatures until dinosaurs appeared on the scene. They were slower and less adaptable than the dinosaurs and could not survive the competition. Herrerasaurus would have also eaten lizards, amphibians and possibly some of the large insects that lived in the Triassic.
    Herrerasaurus, being an early dinosaur, seems to have had some characteristics that confuse scientists trying to find its place in dinosaur evolution. It shows traits that show up in much later Jurassic dinosaurs and traits that are found in different classifications of dinosaurs, making it difficult to fit it into a specific family tree. Its teeth are more conical in shape than later or contemporary dinosaurs, and they have serrations like most of the later carnivorous dinosaurs. It also appears that the Herrerasaurus jaw was somewhat flexible in order to fit more firmly around its prey and prevent it from escaping. Its arms show proportions more similar to later predatory dinosaurs in that they were much shorter than its hind limbs and clearly designed for prey capture.  It is estimated that Herrerasaurus could have reached lengths of up to 15 feet (4.5 m), making it as large as dinosaurs that did not come along until well into the Jurassic.
   Herrerasaurid anatomy is unusual and specialized, and they are not considered to be ancestral to any later dinosaur group. They often present a mixture of very primitive and derived traits. The acetabulum is only partly open, and there are only two sacral vertebrae, the lowest number among dinosaurs. The pubic bone has a derived structure, being rotated somewhat posteriorly and folded to create a superficially tetanuran-like terminal expansion, especially prominent in H. ischigulastensis. The hand is primitive in having five metacarpals and the third finger longer than the second, but theropod-like in having only three long fingers, with curved claws. Herrerasaurids also have a hinged mandible like theropods, but this may have evolved independently.
    Found in the Late Triassic of the Ischigualasto Formation of northwestern Argentina, Herrerasaurus ischigualastensis is an early archosaur and on the verge of being a dinosaur proper. The first specimen was found in 1958 by Victorino Herrera, for whom the fossil was named. This skeleton was incomplete, but the discovery of a complete skull in 1988 and additional fragments have provided enough information to make a complete reconstruction; this has also permitted Paul Sereno at the University of Chicago to redescribe Herrerasaurus properly in a series of papers published in the Journal of Vertebrate Paleontology. Material found thus far suggests that it was a large carnivore about three to four meters long.
    Also in the Middle to Late Triassic of South America, other dinosaur relatives have been found which may be closely related to Herrerasaurus. These include the incompletely known Staurikosaurus pricei from southern Brazil and northwestern Argentina and Ischisaurus cattoi, which is very similar to Herrerasaurus and may even be the same species. The North American Chindesaurus briansmalli, from the Chinle Formation, may also be related.
     Systematic relationships of Herrerasaurus and its relatives are far from certain. While some analyses suggest they are sister to the dinosaurs, others consider them saurischian or even theropods. The importance of this group is that they give us some idea of the time at which dinosaurs evolved (towards the end of the Triassic) and what the earliest dinosaurs would have looked like.


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