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Herrerasaurus: Dinosaur or Archosaur?

© Beverly Eschberger

Herrerasaurus was discovered in Argentina in 1958 by a man named Victorino Herrera, for whom the specimen was later named. The specimen was incomplete, but a skull of the species was found in 1988. At that time, paleontologist Paul Sereno of the Field Museum of Natural History in Chicago, Illinois rediscribed the species to incorporate new information about the species. (Be sure to read my article, "Brontosaurus versus Apatosaurus" about how scientists give animals scientific names.)

Until recently, Herrerasaurus was believed to have been one of the earliest dinosaurs, living in the late Triassic Period (230 to 195 million years ago). It was originally thought to be a theropod, one of the saurischian dinosaurs. It is now recognized to be an archosaur ("ruling reptile"), a group of animals which included the crocodilians (alligators and crocodiles), pterosaurs, and dinosaurs. The archosaurs got their name because they were the dominant group of animals throughout the Mesozoic Era; only the crocodiles survived to our modern times.

Originally, Herrerasaurus was considered to be a dinosaur that was too primitive to be classified as either a saurischian or ornithischian dinosaur. It was believed to be a representative of the common ancestry of both lineages of dinosaurs. The structure of Herrerasaurus's skull, which resembled that of early theropod dinosaurs, resulted in it being classified as a theropod.

There was a problem with Herrerasaurus being classified as a theropod, its hip structure did not really resemble that of a saurischian dinosaur. (Be sure to read my article "Dinosaur Primer" for information about the saurischian and ornithischian dinosaurs.) Instead, Herrerasaurus's pelvic structure was more similar to that of the archosaurs. The pelvic structure was a hotly debated topic: was Herrerasaurus a theropod with a primitive pelvic structure, or an archosaur with a theropod-style skull?

Currently, Herrerasaurus is classified as an archosaur. Perhaps as more specimens are discovered and modern paleontological research techniques improve, we will learn where Herrerasaurus really belongs on the family tree of ancient animals.

Before you complain, "What's with these paleontologists that they can't even decide what kind of animal this is?" remember that even the classification of extant (still living) animals is still being fine tuned. As more DNA research is conducted on modern animals, it is often found that long-held beliefs in the relationships between animals are incorrect. Biologists have discovered that animals long believed to be closely related are actually much more distantly related, and the converse as well, animals believed to be only distantly related often turn out to be much more closely related.

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