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Eoraptor & the first dinosaurs (Argentina)
c. 231 million years ago· Fossil or site· Animals
Well establishedStrong fossil, genetic or geological evidence.
A small, early dinosaur from Argentina's Ischigualasto rocks, one of the oldest well-known dinosaurs.
See it on the Life map →What happened
Eoraptor ("dawn plunderer") was found in 1991 in the Ischigualasto Formation of northwestern Argentina, badlands nicknamed the Valley of the Moon, and named in 1993 by Paul Sereno and colleagues. About 1 m long, light and two-legged, it had both leaf-shaped and blade-like teeth, suggesting a mixed diet. The same 231-million-year-old rocks hold Herrerasaurus and Eodromaeus, yet dinosaurs were still a minor part of that fauna. First called an early theropod, it was later moved to the sauropodomorphs.
Why it matters
It shows that the first dinosaurs were small and unspecialized, and that the three great branches split soon after dinosaurs appeared.
The evidence (fossils, genes, rocks)
A nearly complete skeleton plus many other dinosaurs from Ischigualasto, with volcanic ash layers dated by radioactive decay to about 231 million years ago.
Still debated
Whether Eoraptor is an early sauropodomorph, an early theropod or outside both is still argued, because the earliest dinosaurs look so alike.
Connections (2)
▸Eoraptor & the first dinosaurs (Argentina) is among the earliest DinosaursWell established
The connection
Eoraptor lived about 231 million years ago, near the start of dinosaur history. It shows the early dinosaur starter kit: small size, two-legged walking, grasping hands and an open hip socket. At the time, dinosaurs were still a minor part of their ecosystems, outnumbered by other reptiles and mammal relatives.
When and how
Ischigualasto Formation, San Juan Province, northwestern Argentina; found by Ricardo Martínez in 1991 and named in 1993 by Paul Sereno and colleagues.
The evidence
Its hip, ankle and skull features match those of later dinosaurs, and volcanic ash in the same rocks gives a radiometric age.
Debates and open questions
Possibly older dinosaurs, such as Nyasasaurus from Tanzania, about 243 million years old, may push the origin back, but their status is debated.
▸Eoraptor & the first dinosaurs (Argentina) may resemble the ancestors of Sauropods: the long-necked giantsDebated
The connection
A 2011 reanalysis placed Eoraptor on the sauropodomorph branch, the line that later produced the long-necked giants. If correct, sauropods trace back to small, two-legged, partly plant-eating ancestors much like Eoraptor, and only grew huge over the following 40 million years.
When and how
Proposed by Ricardo Martínez and colleagues in Science (2011), in the paper describing Eodromaeus, also from Argentina's Ischigualasto Formation.
How sure are we?
Features of its teeth and skull hint at sauropodomorph links, but early dinosaurs share so many traits that the placement is uncertain.
Debates and open questions
Other analyses place Eoraptor as an early theropod or near the base of all dinosaurs; its position shifts as new fossils are added.