Eoraptor

The Quest for the “Oldest Dinosaur”

Fossils of Eoraptor were discovered relatively recently, in 1991.
This discovery provided a massive clue for unraveling how dinosaurs evolved.

While Eoraptor is often considered the “oldest dinosaur,” the quest and debate surrounding this title continue.
The larger Herrerasaurus was discovered in the same geological formation as Eoraptor, and fossils of the supposedly even older Nyasasaurus have been found in Africa.
Because the fossils of Nyasasaurus are fragmentary, opinions are divided on whether it is a “true dinosaur.”

Given this context, Eoraptor holds the highly crucial position of being “one of the oldest living species among those that can definitively be called dinosaurs.”

An Agile Body: Theropod or Sauropodomorph?

Eoraptor was a small dinosaur, roughly the size of a fox, but its long hind legs and lightweight build allowed it to move with great agility.
Standing on its hind legs and using its long tail for balance, it was capable of running quickly.

Following its formal description in 1993, Eoraptor was classified as a theropod, a group of carnivorous dinosaurs.
However, the study of the new dinosaur genus Eodromaeus in 2011 sparked a massive debate over its classification.

Theropod Characteristics

The teeth in the back of its jaw were sharp, a characteristic typical of carnivorous dinosaurs.

Sauropodomorph Characteristics

The teeth at the front of its jaw were leaf-shaped, similar to those of primitive sauropodomorphs.
The shape of its external nostrils and the fingers on its forelimbs also resembled those of sauropodomorphs.

Today, it is increasingly likely that Eoraptor will be classified as an early sauropodomorph.
This debate is significant because Eoraptor’s taxonomic placement is a key to solving the great mystery of “what the common ancestor of all dinosaurs looked like.”

Agile Body and Primitive Features

Eoraptor’s long jaw, complete with sharp teeth, did not possess a strong bite force.
It freely moved its long neck to catch fast-fleeing prey or eat plants such as ferns and cycads.
Its forelimbs had five fingers, three of which were long and equipped with sharp, powerful claws.
It used these claws to pin down prey, grasp plants, and protect itself from enemies.

Eoraptor is also characterized by “heterodonty”—having teeth of different shapes in the front and back of its mouth.
The front teeth were leaf-shaped and suited for a plant-based diet, while the back teeth were curved and suited for a carnivorous diet.
It is believed that by making full use of these teeth and claws, it could prey on animals roughly its own size.

Ecological Mysteries and Fossil Discovery

Eoraptor fossils were discovered in 1991 by a joint expedition team led by Argentine paleontologist Ricardo N. Martínez and Paul Sereno of the University of Chicago.
It took about a year to excavate an almost complete skeleton, and in 1993, it was formally described as a new genus in a scientific paper.
The location where it was discovered is called the “Valley of the Moon,” from which the specific name “lunensis” is derived.

Because no skin fossils have been found, it remains unknown whether it was covered in scales or had feathers.
However, since it lived in a warm region, it might not have needed feathers to keep its body warm.

Eoraptor is a truly fascinating creature, holding the great mysteries of dinosaur evolution within its tiny body.