One of the earliest known dinosaurs, only 1 meter long, from 231 million years ago. Its primitive features help scientists understand what the common ancestor of all dinosaurs looked like.
Rjcastillo, CC BY-SA 4.0, via Wikimedia Commons
Discovery in the Valley of the Moon
In 1991, paleontologist Ricardo Martínez was working in the Ischigualasto Formation of northwestern Argentina when he spotted a rock containing a shiny fossil tooth. This discovery led to the excavation of a nearly complete and articulated skeleton, one of the earliest dinosaurs ever found. The arid, otherworldly landscape of the Ischigualasto Formation is known as the Valle de la Luna, or Valley of the Moon.
A team led by American paleontologist Paul Sereno formally described the fossil in 1993. They named it Eoraptor lunensis, which translates to "dawn plunderer from the Valley of the Moon". The name references its primitive nature and the location of its discovery. The holotype specimen, cataloged as PVSJ 512, dates to the Carnian stage of the Late Triassic period, approximately 231 to 228 million years ago. At that time, the region was a river valley with abundant ferns and large conifers. This dinosaur was found in the same geological formation as another important early dinosaur, Herrerasaurus.
A blueprint for dinosaurs
Eoraptor was a small, bipedal animal, measuring about 1 meter (3.3 feet) in length and weighing an estimated 10 kilograms (22 pounds). Its skeleton, PVSJ 512, shows a combination of traits that has made its exact placement on the dinosaur family tree a subject of debate. Initially classified as a basal theropod, subsequent analyses have suggested it may be a basal sauropodomorph, the group that would later include giant long-necked herbivores.
This classification debate stems from its unspecialized anatomy. Eoraptor possessed a heterodont dentition, meaning it had teeth of different shapes. The front teeth in the upper jaw were recurved and serrated like those of a carnivore, while some teeth in the lower jaw were leaf-shaped, similar to those of herbivores. This suggests an omnivorous diet of small animals, insects, and plants. Its hands had five digits; the three longest ended in large claws for grasping, while the fourth and fifth were very small. The presence of hollow bones is another important dinosaurian feature exhibited by the specimen. This mosaic of primitive characteristics shows the anatomical starting point for the great dinosaur lineages.
💡Fun Facts
The name *Eoraptor lunensis* means "dawn plunderer from the Valley of the Moon."
The holotype skeleton is so complete that it took nearly 12 months to collect.
While it had five fingers on each hand, the two smallest were likely too tiny to be useful for hunting.
The area where *Eoraptor* lived 231 million years ago was a lush river valley, not the desert badlands it is today.
The holotype specimen (PVSJ 512) is housed at the Museo de Ciencias Naturales in San Juan. Check the official website for current hours.
Admission
Check the museum's website for current admission prices.
Accessibility
The museum building is accessible. The original find site in Ischigualasto Provincial Park involves rugged, desert terrain and visits are typically done by vehicle tour.
