The find belongs to the Abelisaurus family, "the most common carnivorous species in the southern hemisphere during the Cretaceous Period," some 70 to 100 million years ago, paleontologist Diego Pol told AFP on Thursday.
"However the fossils that we found are some 170 million years old," from the earlier Jurassic Period, Pol said.
The creature looks a bit like a scaled-down Tyrannosaurus rex, but with even smaller arms.
The new species, baptized Eoabelisaurus mefi, predates the oldest known member of the Abelisauri lineage by more than 40 million years.
Unlike its descendants, this six-meter (20-foot) long creature creature "has completely reduced arms and tiny claws, which implies that it used only its very sharp teeth to feed itself," Pol said.
Abelisauri remains have been found only in the southern hemisphere.
Experts believe a great desert in the Earth's single land mass at the time, Pangea, could have acted as a geographic barrier, preventing the species from spreading north.
The fossils were discovered on Condor Hill, in the southern Patagonian province of Chubut, some 1,800 kilometers (1,120 miles) southwest of Buenos Aires.
A 25-member team from the Edigio Feruglio Museum of Paleontology in Chubut discovered the creature's cranium and vertebrae during a dig in 2009.
The team was forced to abandon the expedition when winter approached, and returned the next year during the summer, when "we found the animal's whole articulated skeleton," Pol said.
Details of the find appear in an article Wednesday in the Proceedings of the Royal Society of London.
Argentina earned fame as a prime site for dinosaur fossil hunters with several discoveries in the 1980s, including the Argentinosaurus Huinculensis, a giant herbivore more than 40 meters (131 feet) long that lived 98 million years ago.
In 1993, scientists in Argentina found the remains of the Giganotosaurus Carolinii, a T-Rex type creature that is the largest carnivorous dinosaur ever found.