Bird-like dinosaurs long believed to be carnivorous predators were in fact plant lovers, with the notable exception of dedicated hunters such as T. rex, US paleontologists said Monday.

Lindsay Zanno and Peter Makovicky of the Field Museum in Chicago used statistical analysis to concluded that 90 species of theropod dinosaurs ate a plant-based diet, especially among coelurosaurs, the most bird-like dinosaurs.

The results were in sharp contrast to a widespread belief among paleontologists who say theropod dinosaurs hunted their prey, especially those closest to the ancestors of birds.

"Most theropods are clearly adapted to a predatory lifestyle, but somewhere on the line to birds, predatory dinosaurs went soft," Zanno explained.

Zanno and Makovicky found nearly two dozen anatomical features were statistically linked to direct evidence of plant eating among coelurosaurian dinosaurs, such as the loss of teeth or a long neck.

"Once we linked certain adaptations with direct evidence of diet, we looked to see which other theropod species had the same traits... Then we could say who was likely a plant eater and who was not," added Zanno.

Through their analysis, the researchers found that 44 theropod species distributed across six major lineages ate plants and that the ancestor to most feathered dinosaurs and modern birds had probably already stopped eating meat only during the Cretaceous Period, some 145-65 million years ago.

In light of the large number of plant eaters during that period, the carnivorous diet of T. rex, Velociraptor and other meat-eating coelurosaurs should be viewed "more as the exception than the rule," Zanno said.

"Its time to start seeing these animals in a new evolutionary context," Zanno said.

The researchers also suggested that these big predators and their close relatives may have in fact evolved from omnivorous ancestors.

The study was published in this week's edition of the Proceedings of the National Academy of Sciences.