Scientists are learning more about what appears to be one of the biggest meat-eating dinosaurs known, a two-legged beast whose bones were found several years ago in the fossil-rich Patagonia region of Argentina.
One expert called the discovery the first substantial evidence of group living by large meat-eaters other than tyrannosaurs like T. rex.
The creature, which apparently measured more than 40 feet long, is called Mapusaurus roseae.
The discovery of Mapusaurus included bones from at least seven to nine of the beasts, suggesting the previously unknown animal may have lived and hunted in groups. That hunting strategy might have allowed it to attack even bigger beasts, huge plant-eating dinosaurs.
The find was reported in 2000 by The Associated Press. It is described in the latest issue of the journal Geodiversitas by paleontologists Rodolfo Coria of the Carmen Funes Museum in Plaza Huincul, Argentina, and Philip Currie of the University of Alberta in Canada. They oversaw the excavation of the dinosaur's remains about 15 miles south of Plaza Huincul from 1997 to 2001. Mapusaurus is estimated to have lived about 100 million years ago.
Currie, in an e-mail, said it's hard to say how long the biggest specimen was because no complete skeleton was found. He estimated it may have measured about 41 feet from the snout to the tip of the tail.
It may have been about a foot longer than Giganotosaurus, also found in Patagonia, but without a complete skeleton "you will never know," he wrote.
The Field Museum in Chicago says its T. rex skeleton, Sue, is 42 feet long.
Thomas Holtz Jr., a University of Maryland dinosaur expert, said that Mapusaurus clearly joins Giganotosaurus, T. rex and a huge African beast called Spinosaurus as among the biggest carnivorous dinosaurs. But he said it's impossible to know exactly how they rank in overall size. The fossil record is too fragmentary, and unlikely to capture the biggest individual of each species, he said.
Spinosaurus was probably the longest species, but length is a poor indicator of overall size because tails can be shorter or longer without affecting a creature's weight very much, he said. Still, Spinosaurus was probably the biggest in overall bulk as well, he said.
Coria noted the dig showed evidence of social behavior in Mapusaurus. The excavation found hundreds of bones from several Mapusaurus individuals but none from any other creature. That suggests the animals were together before they died, Coria said.
Perhaps they hunted in packs, though there is no direct evidence for that, he said in an e-mail. Currie, in a statement from his university, speculated that pack hunting may have allowed Mapusaurus to prey on the biggest known dinosaur, Argentinosaurus, a 125-foot-long plant-eater.
Holtz called the finding the first substantive evidence of group living by giant two-legged carnivores other than tyrannosaurs. It's not clear whether the animals cooperated in hunting, as wolves or lions do, or simply mobbed their prey or just gathered around after one of them made a kill, he said.
"Mapusaurus" comes from the word for "Earth" in the language of the Mapuche tribe of western Patagonia, while "roseae" refers both to the rose-colored rock that yielded the specimens and to the name of a sponsor of the excavations.