Researchers compared organic molecules preserved in the T. rex fossils with those of living animals, and found they were similar to chicken protein.
The discovery of protein in dinosaur bones is a surprise - organic material was not thought to survive this long.
A US team of researchers have published the finding in Science journal.
The team says their technique could help reveal evolutionary relationships between other living and extinct organisms.
The finding is consistent with the idea that birds can trace a direct evolutionary line to dinosaurs.
It's actually bone collagen that was preserved in trace amounts. Unclear why it should have survived this long and the article doesn't give any specific reason. Maybe anoxic conditions in the mud?
[Please note: No innocent chicken jokes were harmed in the making of this post]