The fossil of a giant carnivorous bird has been found in the Patagonia region in Argentina. The flightless predator was about 10 feet tall and ate dog-sized mammals.

The new fossil was discovered in 15 million-year-old rocks in Patagonia, Argentina.

The specimen includes a limb bone and a horse-sized, virtually complete, skull of a gigantic Phorusrhacid with a vicious eagle-like bill.

Apart from the fossil being evidence of the largest known bird to date, the find is significant because it suggests important changes that occurred during the evolution of terror birds, notably in their agility.

"It is an unbelievable creature," said paleontologist Luis Chiappe, director of the museum's Dinosaur Institute, who documented the find in the journal Nature. "This is the largest known bird, with a skull bigger than a horse's head."

Measuring more than 28 inches long, the fossil skull is at least 10% bigger than the largest previously known species, Chiappe and his colleagues reported.

An Argentine high school student, Guillermo Aguirre-Zabiala, found the fossil two years ago among the rock outcrops between two houses by the railroad station in his village east of Bariloche.

The young man was so galvanized by his discovery that he changed his course of study from psychology to paleontology and Earth science.

During much of the time that terror birds existed, South America was an island continent where species evolved in isolation from the rest of the world. With no large carnivorous mammals to compete with, the terror birds became the top predators.

"The new fossil adds significantly to our knowledge of the evolution of terror birds", said Dr Chiappe.
"Future discoveries are likely to reveal a much greater diversity of these fascinating birds."

Until now, scientists thought that these unusual flightless birds had become more portly and less agile as they evolved into bigger and bigger carnivores.

The slender leg and foot bones found with the immense skull, however, closely resemble those of a typical running bird, the scientists reported.

"It was a speedy bird," Chiappe said. "I am not saying this animal ran as fast as an ostrich, but it was clearly a good runner."