Gastornis, a huge ancient 'terror bird' that was believed to be one of earth's most cruel predators snapping necks of mammals with its massive beak, was actually a vegetarian, according to a new study conducted by German researchers.

The two metres (6.5 feet) high, large flightless bird lived in Europe between 40 and 55 million years ago after the extinction of the dinosaurs.

Most of the experts assumed Gastornis to be a meat-eating creature because of its size and threatening appearance - massive beak with a hooked top feature that looked super fierce.

However, the German researchers claimed that the bird was not a meat-eater after examining the fossilised remnants of the beast discovered in a former open-cast brown-coal mine in Saxony-Anhalt, eastern Germany.

"The terror bird was thought to have used its huge beak to grab and break the neck of its prey, which is supported by biomechanical modelling of its bite force. It lived after the dinosaurs became extinct and at a time when mammals were at an early stage of evolution and relatively small; thus, the terror bird was thought to have been a top predator at that time on land," said Dr Thomas Tütken, of the University of Bonn.

Tuetken and his colleagues arrived at the conclusion after measuring the calcium isotope composition in the bones to identify the bird's diet and its position in the food chain.

According to previous experiments, the scientists concluded that the composition becomes 'lighter' as it passes through the food chain.

On paper, the bones of a top predator like Gastornis should have higher levels. But the team found that the isotope levels were identical to those of herbivorous mammals and dinosaurs. Carnivores, including the Tyrannosaurus rex, had stronger levels.

"Tooth enamel preserves original geochemical signatures much better than bone, but since Gastornis didn't have any teeth, we've had to work with their bones to do our calcium isotope assay," Tütken said.

The work was presented Thursday at an annual international meeting of geochemists called the Goldschmidt Conference taking place in Florence, Italy.