Image: Illustration of Eodromaeus
Illustration by Todd Marshall
Weighing only 10-15 pounds and about 4 feet in length from snout to tail tip, pint-sized Eodromaeus may be the oldest ancestor to date of all meat-eating dinosaurs, including T. rex.
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updated 1/13/2011 2:33:12 PM ET 2011-01-13T19:33:12

What may be the earliest known relative of T. rex and all meat-eating dinosaurs has been discovered. The dog-sized mini-predator would've made its future relatives proud as it fed on small dinosaurs and the young of other reptiles, and is now shaking up what scientists had previously learned about the evolution of those extinct giants.

The small, lanky, two-legged carnivore named Eodromaeusmurphi — Eodromaeus being Greek for "dawn runner," murphi in honor of field volunteer Jim Murphy — weighed only 10 to 15 pounds and measured about 4 feet in length from snout to tail. The skeletons of two specimens were discovered side-by-side in 230-million-year-old iron-rich stone in the "Valley of the Moon" at the foothills of the Andes in northeastern Argentina, which was once the southwest corner of the supercontinent Pangaea.

"With a hike across the valley, you literally walk over the graveyard of the earliest dinosaurs to a time when they ultimately dominate," said researcher Ricardo Martinez, a paleontologist at the National University of San Juan in Argentina.

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Dino graveyard
The predator came from the dawn of the age of dinosaurs, a time we know relatively little about. The long-necked, long-tailed predator was armed with saber-shaped upper cheek teeth and sharp-clawed five-fingered hands. [ Images: Dinosaur Fossils ]

"Seeing what a modest beginning this lineage had, no one would have ever thought it would evolve to giant predators like T. rex," researcher Paul Sereno, a paleontologist at the University of Chicago, told LiveScience.

Sereno, Martinez and their colleagues concluded Eodromaeus was likely a very close relative of the ancestor of the theropod lineage, which includes all predatory dinosaurs. Key features it had in common with theropods include long finger bones tipped with claws that enhance grasping, and pockets in the sides of the neck vertebrae for air sacs from the lungs. [ Image of Eodromaeus' tiny skull ]

"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," Sereno said. "Who could foretell what evolution had in store for the descendants of this pint-sized, fleet-footed predator?"

Image: Eodromaeus skull (made of replica bones)
Photo by Mike Hettwer
The delicate skull of Eodromaeus (made of replica bones) shows the dinosaur's slender saber-shaped upper teeth. Skull is being held by paleontologist Paul Sereno.

Eodromaeus' plant-eating twin
The area was once a verdant valley, judging by all the petrified wood found there. Sediments covered the skeletons over a period of 5 million years, eventually building up in thickness to more than 2,000 feet. Volcanoes associated with the nascent Andes Mountains occasionally spewed volcanic ash into the valley, allowing the researchers to use radioactive elements in the ash layers to help determine the age of the sediments.

The rocks in which these fossils were found have proven a treasure trove of discoveries from the earliest days of dinosaur evolution, including Eoraptor, or "dawn thief," another creature from very close to the root of the dinosaur family tree. This reptile, which Sereno and his colleagues discovered in 1991, also ran on two legs and was similar in size, and until recently, they believed Eoraptor was a very primitive theropod as well.

However, with these new fossils for comparison, the researchers see the plant-eating Eoraptor was actually an early ancestor of the sauropod lineage, which includes the giant, long-necked herbivores that were the largest animals to ever walk the Earth. Eoraptor lacked theropod-like features seen in Eodromaeus, such as an opening near the end of the snout, and had more sauropod-like features, such as enlarged nostrils and an inset first lower tooth.

"That will be a surprise for a lot of people," Sereno said.

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Evolutionary experimentation
When it came to mistakenly classifying Eoraptor as more like a theropod than a sauropod, Sereno apologetically said, "What can I say, I was young." He did note, however, "that if you were transported back 230 million years, and you turned your head as they ran by, you would be really hard-pressed to tell them apart. The differences at the root of the dinosaur family tree are really subtle. And no one else saw it, either."

The general similarities between the two dinosaurs suggest that the three main groups of dinosaurs — sauropods, theropods and "bird-hipped" ornithischians such as stegosaurs, ankylosaurs and Triceratops — did share an overall body plan.

"We're looking at a snapshot of early dinosaur life," Sereno said. "Their storied evolutionary careers are just unfolding, but at this point they're actually quite similar."

"The dawn of the age of dinosaurs is coming into focus," Martinez said.

In the red cliffs on the far side of the Valley of the Moon, larger plant- and meat-eating dinosaurs from further back in time had evolved to be many times the size of Eoraptor and Eodromaeus, but it would be even later when dinosaurs overcame their rivals to dominate all land habitats in the succeeding Jurassic and Cretaceous periods, beginning some 200 million years ago.

"The story from this valley suggests there was no single advantage or lucky break for dinosaurs, but rather a long period of evolutionary experimentation in the shadow of other groups," Sereno said. "We now hope to find things that predate the first dinosaurs — that is where the gap is now."

The scientists detailed their findings in the Jan. 14 issue of the journal Science.

You can follow LiveScience on Twitter @livescience.

© 2012 LiveScience.com. All rights reserved.

Explainer: The world's seven deadliest dinosaurs

  • Copyright 1985 Mark Hallett, "Awakening of Hunger"

    Yeah, it's cliche to say Tyrannosaurus rex was deadly. But the tyrant king was likely true to the billing. Its bone-crushing jaws could splinter prey like toothpicks, after all. And the beast was big, up to 40 feet long, 20 feet tall, and may have topped the scales at nearly 16,000 pounds.

    The king walked on two legs over a vast territory in western North America at the end of the Cretaceous, about 65 million years ago. Scientists wonder whether T. rex was more a lumbering scavenger or a quick and agile predator, but dead or alive, its meals were big, meaty and bloody.

    T. rex wasn't the only deadly dino, however. The globe was filled vicious killers. Click the arrows above to see more.

    — John Roach, msnbc.com contributor

  • Family diner

    Demetrios M. Vital

    Family meals for Majungasaurus crenatissimus were often bloody affairs of the grimmest sort: Kin were the main course. The evidence of its cannibalistic ways comes from telltale tooth marks on Majungasaurus bones that match up perfectly with the size and spacing of teeth in its jaws.

    The 20-foot-long dinosaur stalked the plains of Madagascar about 70 million years ago. At that time, the crime-scene investigators said, pickings were often slim. Their data indicate Majungasaurus fed on dried out "dino jerky" from its plant-eating compatriots as well as members of its own species. Cannibalism was likely just as common among dinosaurs as it is among living animals, but the evidence is rare.

  • Brow beater

    Todd Marshall

    Eocarcharia dinops' brow was swollen into a massive band of bone. The menacing head piece may have been used as a battering ram against rivals and to attract potential mates. Its blade-shape teeth were reserved for disabling live prey and severing their body parts.

    The 110-million-year-old beast and its snout-nosed, gut-and-carcass-scavenging relative Kryptops palaios were discovered in Africa's Sahara Desert. The approximately 7-foot-tall and 25-foot-long duo likely teamed up with a third carnivore and feasted on the long-necked plant-eater Nigersaurus.

  • Vicious rocker

    Parsons

    The toothy Masiakasaurus knopfleri likely speared prey with its forward projecting front teeth and then sliced and tore the captives into chewy chunks with its bladelike rears. This type of tooth arrangement is otherwise unknown in predatory dinosaurs.

    Scientists believe the German Shepherd sized beast feasted on fish, lizards, and other critters on the southern supercontinent Gondwana in the Late Cretaceous period, about 65 to 70 million years ago.

    The first part of this dinosaur's name means "vicious lizard" and the second part is derived from the Dire Straits singer and guitarist Mark Knopfler, whose music inspired the discoverers as they toiled on the African island nation of Madagascar.

  • Biggest carnivore?

    Prof. Rodolfo Coria / Ap

    At about 40 feet long and weighing an estimated 6 tons, Mapusaurus roseae was one of the biggest meat-eating dinosaurs to ever stalk the Earth. Bones of several individuals were discovered in one place, suggesting the giants may have hunted in packs that could have toppled perhaps the largest dinosaur that ever lived — a 100-foot-long plant eater called Argentinosaurus.

    Mapusaurus lived about 100 million years ago and was bigger than well known Tyrannosaurus rex and possibly larger than its older cousin, Gigantosaurus. Its teeth were narrow and blade like, made for slicing its prey. The bones were discovered in the Patagonia region of Argentina.

  • Going green

    MWS

    Falcarius utahensis is the living image of a vicious meat eater trying to go vegetarian. Scientists aren't sure if the bird-like relative of Velociraptor had fully kicked its taste for flesh, but its meat-cutting teeth had shrunk to leaf-cutting size and its gut had expanded sufficiently to ferment plants.

    Falcarius walked on two legs and stood about 4.5 feet tall. Head to tail, it was about 13 feet long and wielded sharp, curved, four-inch long claws — perhaps to grab a bite when it fell off the wagon? The dinosaur lived during the Early Cretaceous, about 125 million years ago, in what is today Utah.

  • Plant ripper

    utah.edu

    No plants were safe from Gryposaurus monumentensis, a big boned, duck-billed dinosaur that could have eaten any vegetation it stumbled across. Its massive skull packed more than 300 teeth for slicing up fibrous greens. Hundreds more replacement teeth rested in its jawbone for the call-up to action.

    The dinosaur lived in the Late Cretaceous, about 75 million years ago on the western side of a giant, shallow ocean that split North America at that time. The plant-munching beast may have reached 30 feet long as an adult and had a 3-foot-long head.

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