For all its appearance as a ferocious predator, Tyrannosaurus rex was just an overgrown chicken, researchers have found.

Analysis of protein preserved within a fossil bone has provided molecular evidence to support the theory that dinosaurs evolved into birds.

Similarities between bone structure and the discovery of feather-like remains on dinosaur fossils have previously been cited, but scientists have now found the first molecular link.

Collagen preserved within a T. rex bone was broken down and analysed with a mass spectrometer for its amino acid content, which could then be compared to other species. Researchers then compared the molecular profile of chickens and ostriches to that ofT. rexand found a close match. They now hope to carry out further tests to establish which type of bird is most closely linked to the predator.

Alligators were found to be closely related to the predatory dinosaurs but more distantly than birds and farther back in the family tree.

“We determined thatT. rexgrouped with birds – ostrich and chicken – better than any other organism that we studied,” John Asara, from the American Beth Israel Deaconess Medical School in Boston, said.

“We also show that it groups better with birds than modern reptiles, such as alligators and green anole lizards. Is it an overgrown chicken? Maybe it is.”

The study, reported in the journal Science, strengthens and expands initial findings last year that linked the dinosaur to birds. “We have now done robust calculations and have a very, very high confidence in this relationship,” Dr Asara added.

The collagen extracted from the 68-million-year-old fossil was analysed alongside traces extracted from a 500,000-year-old bone from a mastodon – a prehistoric woolly mammoth-like creature that lived in North America.

The samples are the oldest and second-oldest examples of preserved animal protein, and the extract from the mastodon confirmed the assumption that the animals were most closely related to modern-day elephants.

Researchers are confident that they have opened up a new field of study that should enable the animal family tree to be drawn more accurately and to solve riddles that cannot be answered by just observing body shapes.

“These results match predictions made from skeletal anatomy, providing the first molecular evidence for the evolutionary relationships of a nonavian dinosaur,” Chris Organ, of Harvard University in the US, said.

Members of the team added in a statement that the protein analysis was “putting more meat on the theory that dinosaurs’ closest living relatives are modern-day birds”.

They said: “Analysis of a shred of 68-million-year-old Tyrannosaurus rex protein – along with the proteins of 21 modern species – confirms that dinosaurs share common ancestry with chickens, ostriches and, to a lesser extent, alligators.”

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