A loaf-sized fragment of ancient faeces, which scientists describe as "king size", shows that T rex crushed the bones of its victims into tiny fragments before swallowing each mouthful.
The fossil, called a coprolite, has also revealed that T rex was unlikely to have suffered from indigestion. Bone fragments in the faeces were only partially dissolved, indicating that the acids in the dinosaur's stomach were relatively weak.
It is the first time that scientists have been able to study fossilised faeces that they know have come from a T rex.
The fossil was found in the Canadian province of Saskat-chewan in 1995 and the analysis is published in the journal Nature.
T rex - considered to be the king of the dinosaurs - left behind equally impressive calling cards. The "tremendous size" of the faecal mass meant it could only have come from the world's largest predator, the scientists said.
Timothy Tokaryk, a palaeontologist at the Royal Saskat-chewan Museum in Regina, said the coprolite is the largest from any dinosaur and the first from a meat-eating species. "For the first time it gives us a greater insight into the internal workings of a creature that died more than 65 million years ago," he said.
"Before this coprolite was found we were still guessing from the front end of the animal what it did with its prey. Now we have confirmation from what it left behind."
T rex lacked grinding molar teeth, and so could not chew its food. The research confirms that it managed to pulverise each mouthful into tiny pieces with the immense force of its bite acting on its huge peg-like teeth.
Analysis of the bones in the coprolite show T rex's victim was probably a young herbivorous dinosaur that would have been about the size of a cow.
Dr Gregory Erickson, a palaeontologist at Stanford University in California, said the victim was probably an adolescent, which suggests that T rex was an active predator of healthy prey rather than a lumbering scavenger of old and sick animals, which some dinosaur experts have suggested.
Much of the crushed bone in the coprolite escaped damage, suggesting digestive juices of dinosaurs were very different from those of crocodiles, which dissolve the bones they eat.
Peter Andrews, a coprolite researcher at the Natural History Museum in London, believes the bone in the faecal matter is in such pristine condition that its DNA might be sufficiently preserved for molecular analysis.
"The study of fossil faeces may seem rather a strange subject, but it can reveal much about dinosaur behaviour," Dr Andrews said.Reuse content