The story
The Ischigualasto Formation in San Juan Province, Argentina, nicknamed the Valley of the Moon, contains some of the oldest and best-known dinosaur fossils. Volcanic ash layers date the rocks to about 231 million years ago. Herrerasaurus, named in 1963 by Osvaldo Reig after the goat herder Victorino Herrera who found the first bones, is a fast hunter up to about 6 m long with sharp curved teeth. Eoraptor lunensis, found by Paul Sereno's expedition in 1991 and described in 1993, is a slender meat-eater about 1 m long, whose name means dawn plunderer.
These animals are not yet the giants of later ages. Dinosaurs make up only a small share of the Ischigualasto fauna, which is dominated by beaked rhynchosaurs, mammal relatives called cynodonts, and crocodile-line archosaurs. Researchers still debate whether Herrerasaurus and Eoraptor sit at the base of the meat-eating theropods or near the root of all dinosaurs. Their bodies show a good compromise: upright hind limbs, grasping hands and a fast, light build that set the pattern for what followed.
Why it mattered
- Ischigualasto shows that dinosaurs started as small, rare animals sharing the world with other archosaurs.
- Their anatomy gives the best picture of what the ancestral dinosaur looked like.
- The site helped show that dinosaurs originated in the southern part of Pangaea, in what is now South America.
Sources
- Localities of the Triassic: Ischigualasto UCMP Berkeley
- Herrerasaurus ischigualastensis Britannica
- Eoraptor Britannica
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