Two nasty looking little critters that roamed a remote valley of Argentina 230 million years ago point to a key fork in the road of early dinosaur evolution, when some predatory dinosaurs turned away from meat and went down a path to become plant eaters, according to the latest issue of the research journal Science.

CHICAGO — Two nasty looking little critters that roamed a remote valley of Argentina 230 million years ago point to a key fork in the road of early dinosaur evolution, when some predatory dinosaurs turned away from meat and went down a path to become plant eaters, according to the latest issue of the research journal Science.

These two creatures, like all early dinosaurs, were pipsqueaks in a world dominated by bigger non-dinosaur reptiles, according to research published Thursday by a scientific group that includes University of Chicago paleontologist Paul Sereno.

Sereno and other scientists unearthed fossils of the dinosaurs in an arid area of Argentina called Ischigualasto in the 1990s, but only recently found that two similar looking specimens were actually two separate species. One shows evidence or being a plant eater, while the other favored meat.

"We think we're getting a huge part of the story of early dinosaur evolution from this one valley," Sereno said. "This is the only place in the world so far that we have a window into this early dinosaur time."

The new research unveils a previously unknown, 230 million-year-old pint-sized carnivorous dinosaur species that the scientists have dubbed Eodromaeus, or dawn runner.

The paper also re-examines another early dinosaur Sereno found in 1991 in the same rock strata, a creature he called Eoraptor, or dawn plunderer. That discovery solidified Sereno's reputation as a world-class paleontologist.

"These two are the earliest dated dinosaurs known to science," Sereno said early this week as he worked with bone casts of Eodromaeus and Eoraptor in his Hyde Park, Ill., fossil laboratory.

"We're looking at the dawn of the dinosaur era where the fork in the road is still very narrow in the divergence of plant eaters from meat eaters," Sereno said. "That is why Eoraptor and Eodromaeus look so much alike."

Both Eoraptor and Eodromaeus were swift, slender, two-legged animals with short arms and grasping claws. They measure 4 feet from nose to tip of tail and would have weighed 10 to 15 pounds.

The genetic lineages of both continued for the next 165 million years, evolving into the largest land animals in history, including the biggest carnivore, Tyrannosaurus rex, and the even bigger plant eaters, like Diplodocus and Apatosaurus. Almost all died out in the great dinosaur extinction 65 million years ago, though one line of dinosaurs — birds — survive to this day.

Fossils of both Eodromaeus and Eoraptor were found in the bottom layer of sedimentary and volcanic rock that over a 5 million year period beginning 230 million years ago filled up a 2,000-foot deep valley in the Ishigualasto region.

Since paleontologists began excavating in Ischigualasto more than 50 years ago, they have found more than 1,000 fossil specimens of ancient animals. Most are non-dinosaurs, among them large, pre-dinosaurian crocodile-like serpents that probably fed upon on newcomers such as Eodromaeus and Eoraptor.

"The main significance of the Ischigualasto community of dinosaurs is that it is as close as we have so far to the common origin of all dinosaurs," Thomas Holtz, a University of Maryland paleontologist who did not have a role in the study said in an e-mail interview.

"It was probably only a few million years earlier that there was only a single species of dinosaur, the common ancestor to all Dinosauria," Holtz said, concurring with the paper's findings.

He and Sereno both cautioned that the findings do not suggest Ischigualasto is where dinosaurs first arose, but simply is the place with the earliest fossil record has been discovered so far.

Sereno and Argentinean colleagues found Eoraptor in 1991. It was the most primitive dinosaur yet found and it proved to be a sensation after Sereno published its initial description. At the time Sereno and his colleagues were convinced Eoraptor was a carnivorous theropod.

Ricardo Martinez, one of Sereno's Argentine colleagues who is lead author of Thursday's Science paper, first excavated Eodromaeus in 1996 at the same bottom level of the valley that produced Eoraptor's remains. He, Sereno and others at the time said they were certain it was another Eoraptor specimen. Sereno brought the Eodormaueus fossil to Chicago to remove its bones from the rock.

That took a couple of years, and as he examined the recovered bones, Sereno said he shocked his partners in the Ischigualasto excavations with an e-mail.

"Hey, you guys, we got a new dinosaur here," he said he told them. "It isn't another Eoraptor."

It forced the team to take a closer, more measured look at their Eoraptor bones, he said, and they began see tiny differences — including rows of blunt, grinding teeth of a plant eater — indicating it was already an herbivore or an omnivore evolving toward becoming an herbivore.

The team decided to inventory every dinosaur and non-dinosaur fossil found in the valley over the last 50 years.

Their findings buttress newer theories about how long it took dinosaurs to dominate life on Earth. For decades the theory was that soon after their evolutionary emergence dinosaur populations exploded and they ruled the land. In recent years that theory has been challenged.

"The vast majority of fossil species we recovered at the deepest level of deposits were not dinosaurs," said Sereno. "Dinosaurs only made up about 10 percent of the diversity in that area at that time.

"When we got to the top of the valley, the ratio is almost exactly the same, dinosaurs still only about 10 percent of the species living there 5 million years later."

"Their evolution is much more complicated than previously thought, one in which they remained in the shadow of earlier, more dominant species for millions of years," Sereno said.

"Dinosaurs probably stayed more or less under the radar until 200 million years ago, when there was a large extinction event, and they emerged on the other side, while many of the large non-dinosaur species that dominated the landscape up until then did not."