The great-grandfather of the vicious T. Rex weighed only 10 pounds

About 230 million years ago, a tiny meat-eating dinosaur roamed South America looking for prey. It measured just four feet from long from head to tail, but it was the ancestor of all meat-eating dinosaurs, including the fearsome Tyrannosaurus rex.

This recently discovered dinosaur is Eodromaeus, which rather poetically means "dawn runner." This ancient beast was a lanky creature, more neck and tail than anything else. Partially because of that, it was an incredibly light creature, weighing just ten to fifteen pounds, far less than you might expect for a four-foot-long creature. Eodromaeus dates right back to the dawn of the dinosaurs 230 million years ago, in the beginnings of the Triassic Period.

University of Chicago paleontologist Paul Sereno says that this creature is the closest we've gotten to discovering the common ancestor of all meat-eating dinosaurs, meaning this tiny scavenger was the evolutionary forefather of later, far more intimidating creatures like the Tyrannosaurus rex. He explains:

"It really is the earliest look we have at the long line of meat eaters that would ultimately culminate in Tyrannosaurus rex near the end of the dinosaur era. Who could foretell what evolution had in store for the descendants of this pint-sized, fleet-footed predator?"

The great-grandfather of the vicious T. Rex weighed only 10 pounds

The dinosaur was discovered in a valley in what is now western Argentina, located near the foothills of the Andes. 230 million years ago, this area was a rift valley in the southwestern corner of the supercontinent Pangaea. The valley was constantly covered by volcanic ash and other sediments, preserving an unusually high quantity and quality of fossils under thousands of feet of earth.

Eodromaeus was discovered near another tiny dinosaur forerunner, the plant-eating Eoraptor. Discovered in 1991, this equally small dinosaur would eventually spawn all the gigantic sauropods that later roamed the Earth. It's rather cool to think that, over a hundred million years, the basic interactions between carnivorous and herbivorous dinosaurs never really changed, but the scale of their conflict increased a dozen times over.

The great-grandfather of the vicious T. Rex weighed only 10 pounds

Of course, the earlier version of this story hadn't yet become the main attraction. Eodromaeus and Eoraptor lived before dinosaurs ruled the planet, as both large lizard-like creatures and mammal-like reptiles dominated the first dinosaurs. Over several millions of years, these competitors declined in numbers and ultimately disappeared, leaving the door open for dinosaurs to become the masters of Earth.

How that happened is still a mystery, but we're getting a lot closer to answers, as chief researcher Ricardo Martinez explains:

"Two generations of field work have generated the single best view we have of the birth of the dinosaurs. With a hike across the valley, you literally walk over the graveyard of the earliest dinosaurs to a time when they ultimately dominate. The dawn of the age of dinosaurs is coming into focus."

[Science; images by Todd Marshall, Ricardo Martinez, and Mike Hettwer.]