A curious teenager in Argentina has discovered the fossil skull of the biggest bird ever found -- a swift, flightless predator 10 feet tall that pursued its prey across the steppes of Patagonia 15 million years ago, researchers at the Natural History Museum of Los Angeles County announced Wednesday.

The skull, tapering to a cruel beak curved like a brush hook, belongs to a previously unknown offshoot of extinct birds known as phorusrhacids -- “terror birds.”

Weighing perhaps 400 pounds, the bird most likely preyed on rodents the size of sheep that once grazed on the South American savanna.

“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, Chiappe said. “This discovery has shaped his life.”

The fossil also is altering how scientists understand the evolution of South America’s largest prehistoric terror 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.”

*


lee.hotz@latimes.com