The dinosaur family tree just added a

Unearthed in 230-million-year-old rocks in Argentina, in life this bipedal animal would have been as tall as a 7-year-old but as light as a house cat. Paleontologists announced the new dinosaur, dubbed Eodromaeus, or “dawn runner,” in the Jan. 14 Science.

Eodromaeus joins its kin Eoraptor, a similar-sized dinosaur known to have lived in the same time and place. In fact, when researchers first unearthed Eodromaeus they thought the bones belonged to Eoraptor. Yet the two dinosaurs’ superficial resemblance belies a crucial difference: one ate plants while the other ate meat.

“From 20 feet away you’d do a double take — will the animal run from you or take your leg off?” says team member Paul Sereno, a paleontologist at the University of Chicago. Yet each creature eventually led to a separate branch of dinosaur evolution. “There’s no way to look at them and realize that the ultimate descendants of one results in a tyrannosaur, and the other something like [the plant-eating] Diplodocus,” Sereno says.

The Science paper describes not only the newfound remains of Eodromaeus, but also a wealth of other Argentinean fossils that illuminate how the earliest dinosaurs evolved and what their environment was like. “The new dinosaur is special for sure, but we also have the first step of the radiation of the dinosaur story,” says Ricardo Martinez, the paper’s lead author and a paleontologist at the National University of San Juan in Argentina.

Fossil evidence shows that the first dinosaurs appeared around 230 million years ago, as ecosystems were rejiggering themselves after a massive extinction that ended the Permian period, 250 million years ago. Many researchers have thought that as dinosaurs rose to prominence they muscled out other animal groups.

But Martinez’s team analyzed rates of biodiversity in the Argentinean rocks and found the opposite: Other groups of plant-eating creatures underwent a small-scale extinction before dinosaurs began their rise. “Dinosaurs used those empty ecological niches,” Martinez says. From there dinosaurs went on to become Earth’s dominant land animals until going extinct 65 million years ago — save for the avian branch, which thrives today as birds.

Distinguishing early dinos from each other isn’t easy, as their fossils lie so close to the initial evolutionary radiation. “When you find animals that evolved just after the split of major branches, they’re all going to look really similar,” says Randall Irmis, curator of paleontology at the Utah Museum of Natural History and an assistant professor in geology at the University of Utah in Salt Lake City.

Sereno discovered Eoraptor in 1991, and he later described it as a theropod, a group of dinosaurs that mainly ate meat. After doing more detailed studies, the team decided to reclassify Eoraptor in the new paper as an early sauropod, the other main group of dinosaurs, which mainly ate plants.

Unlike Eoraptor’s identification, Eodromaeus’ classification as a theropod is right from the start, researchers believe. It has sharp teeth for cutting meat, says Martinez, as well as long finger bones for grasping, and interlocking mechanisms in its bones that stiffen the tail to help while hunting.

Eodromaeus joins not only Eoraptor but also a host of other early dinosaurs that roamed what is now Argentina, such as the two-legged sauropod Panphagia, reported by Martinez in 2009. All come from the picturesquely named Valley of the Moon in northeastern Argentina and are buried in a fossil mother lode known as the Ischigualasto.

“What we have is this unbelievable graveyard of the earlier dinosaurs,” says Sereno. “We don’t have a lot of places like this.”

Unfortunately, this dinosaur trove ends with the Ischigualasto rocks. The older rocks beneath it, says Martinez, have preserved only fossils of plants.