10 Facts About Eoraptor, the World's First Dinosaur

How Much Do You Know about Eoraptor?

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The earliest identified dinosaur, Eoraptor was a small, speedy omnivore of middle Triassic South America that went on to spawn a mighty, globe-circling breed. On the following slides, you'll discover 10 essential facts about the "dawn thief."


Eoraptor Is One of the Earliest Identified Dinosaurs

Nobu Tamura

The very first dinosaurs evolved from the two-legged archosaurs of the middle Triassic period, about 230 million years ago--precisely the age of the geological sediments in which Eoraptor ("dawn thief") was discovered. In fact, as far as paleontologists can determine, the 25-pound Eoraptor is the earliest identified dinosaur, preceding previous (and comparably sized) candidates like Herrerasaurus and Staurikosaurus by a few million years.

Eoraptor Lay at the Root of the Saurischian Family Tree

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Saurischian, or "lizard-hipped," dinosaurs branched off in two very different directions during the Mesozoic era--two-legged, feathered raptors and tyrannosaurs as well as gigantic, quadrupedal sauropods and titanosaurs. Eoraptor appears to have been the last common ancestor, or "concestor," of these two noble dinosaur lineages, which is why paleontologists have had such a hard time deciding if it was a basal theropod or a basal sauropodomorph!

Eoraptor Only Weighed About 25 Pounds, Max

Nobu Tamura

As befitting such an early dinosaur, at only three feet long and 25 pounds, Eoraptor was nothing much to look at--and to an untrained eye, it might have appeared indistinguishable from the two-legged archosaurs and crocodiles that shared its South American habitat. In fact one of the things that pegs Eoraptor as the first dinosaur is its almost complete lack of specialized features, which made it an excellent template for subsequent dinosaur evolution.

Eoraptor Was Discovered in the "Valley of the Moon"

valle de la luna
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Argentina's Valle de la Luna--the "Valley of the Moon"--is one of the world's most dramatic fossil sites, its stark, arid topography evoking the lunar surface (and harboring sediments dating to the middle Triassic period). This is where the type fossil of Eoraptor was discovered, in 1991, by a University of Chicago expedition led by the noted paleontologist Paul Sereno, who assigned his noteworthy find the species name lunensis ("inhabitant of the moon.")

It's Unclear if the Type Specimen of Eoraptor is a Juvenile or an Adult

A still-embedded Eoraptor fossil. Wikimedia Commons

It's not always easy to determine the precise growth stage of a 230-million-year-old dinosaur. For a while after its discovery, there was some disagreement about whether the type fossil of Eoraptor represented a juvenile or an adult. Supporting the juvenile theory, the bones of the skull were not fully fused, and this particular specimen had a very short snout--but other anatomical characteristics point to a fully grown, or near-fully-grown, Eoraptor adult.

Eoraptor Pursued an Omnivorous Diet

Sergio Perez

Since Eoraptor predated the time when dinosaurs split between meat-eaters (theropods) and plant-eaters (sauropods and ornithischians), it only makes sense that this dinosaur enjoyed an herbivorous diet, as evidenced by its "heterodont" (differently shaped) teeth. Simply put, some of Eoraptor's teeth (toward the front of its mouth) were long and sharp, and thus adapted for cutting into meat, while others (toward the back of its mouth) were blunt and leaf-shaped, and suited to grinding down tough vegetation.

Eoraptor Was a Close Relative of Daemonosaurus

Jeffrey Martz

Thirty million years after the heyday of Eoraptor, dinosaurs had spread across the Pangean continent, including the patch of land destined to become North America. Discovered in New Mexico in the 1980's, and dating to the late Triassic period, Daemonosaurus bore an uncanny resemblance to Eoraptor, to the extent that it occupies a place next to this dinosaur in evolutionary cladograms. (Another close Eoraptor relative of this time and place is the well-known Coelophysis.)

Eoraptor Coexisted with Various Pre-Dinosaur Reptiles

Hyperodapedon, with which Eoraptor shared its territory. Nobu Tamura

One common misunderstanding about evolution is that once creature type A evolves from creature type B, this second type disappears immediately from the fossil record. Even though Eoraptor evolved from a population of archosaurs, it coexisted with various archosaurs during the middle Triassic period, and it wasn't necessarily the apex reptile of its ecosystem. (Dinosaurs didn't achieve full dominion on earth until the start of the Jurassic period, 200 million years ago).

Eoraptor Was Probably a Speedy Runner

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Considering the competition it faced for scarce resources--and also considering that it must have been preyed on by larger archosaurs--it makes sense that Eoraptor was a relatively speedy dinosaur, as evidenced by its slender build and long legs. Still, this would not have set it apart from the other omnivorous reptiles of its day; it's unlikely that Eoraptor was any faster than the small, two-legged crocodiles (and other archosaurs) with which it shared its habitat.

Eoraptor Was Not Technically a True Raptor

James Kuether

By this time, you may have figured out that (despite its name) Eoraptor was not a true raptor--the family of late Cretaceous dinosaurs characterized by the long, curving, single claws on each of their hind feet. Eoraptor isn't the only such theropod to confuse novice dinosaur watchers; Gigantoraptor, Oviraptor and Megaraptor weren't technically raptors, either, and many true raptors of the later Mesozoic era don't even have the Greek root "raptor" in their names!