What's more, evidence from the fossil site points to Mapusaurus being a pack hunter, the researchers add.
The previously unknown dinosaurs were found together in a prehistoric riverbed 15 miles (24 kilometers) south of Plaza Huincul.
The skeletons showed no signs of disease, Coria says, so the animals were apparently victims of some sudden catastrophic event.
"The burial is formed 100 percent by Mapusaurus bones," he added. "The chances they had been deposited randomly are extremely low."
Coria and Currie say the different sizes of the animals point to the creatures living as a group.
"Most are medium-sized animals, with a very few young and a very few old," Coria said. "It's a normal composition for [a pack] animal population."
The find hints that two-legged, or theropod, dinosaurs such as T. rex might not have been solitary predators as previously thought, but may have hunted in groups.
Other fossil sites found recently in Canada, Mongolia, and the United States suggest such behavior may have been relatively common in late Cretaceous times (65 million to 90 million years ago), Currie said.
Coria and Currie say Mapusaurus might have hunted the greatest dinosaur of all: a 100-ton (102-metric-ton), 125-foot-long (38-meter-long) plant-eater named Argentinosaurus.
Fossils of this massive dino have also been found in hundred-million-year-old rock from Patagonia.
"It would certainly be an advantage to develop some sort of pack-hunting strategy if you are going to catch an adult Argentinosaurus," Coria said.
Don Lessem, an author specializing in dinosaurs, took part in the fossil dig. "This could be the nastiest gang of killers ever found," he said.
Lessem, who helped to fund the excavation, said Mapusaurus was considerably larger than T. rex but probably "not as smart, as fast, as powerful."
Other dinosaur experts say the discovery sheds valuable new light on the most fearsome land predators ever known.
Lowell Dingus is an associate paleontologist at the American Museum of Natural History in New York.
"The remarkable remains of Mapusaurus provide another important example of the spectacular kinds of gigantic carnivorous dinosaurs that roamed South America near the end of the age of the dinosaurs," he said.
Mapusaurus belongs to a group of recently recognized theropod dinosaurs called carcharodontosaurs, which have also been found in Africa.
The new species "increases both our knowledge of the anatomy and the diversity of this peculiar group of theropod," said Ronan Allain of the National Museum of Natural History in Paris, France.
Allain says Mapusaurus is more closely related to the Argentinian Giganotosaurus than to the African species.
"It means a South American [carcharodontosaur] lineage could have evolved regardless of the African forms."
He says the other main contender for the title of biggest ever meat-eating dinosaur is Spinosaurus, whose fossil remains come from North Africa.
Researchers recently calculated that the sail-backed Spinosaurus, also a theropod, may have grown to 56 feet (17 meters) in length and weighed up to 8.9 tons (8.1 metric tons).
But, Allain adds, carcharodontosaurs even larger than Mapusaurus will likely be found in the near future.
"New candidates for the title are expected," he said.
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES