Baby tyrannosaurs were only the size of a Border Collie dog when they took their first steps, a team of palaeontologists has discovered.

Led by Dr Greg Funston, a University of Edinburgh researcher, the team examined fossilised remains of a tiny jaw bone and claw which had been found in Canada and the US.

They were revealed to belong to a baby tyrannosaur – cousin of the fabled T-Rex – in 3D scans and are the first-known fossils of tyrannosaur embryos.

It suggests the creatures which lived more than 70 million years ago were only around three-feet long when they hatched, despite being able to grow to 40ft long and weigh around eight tonnes.

The team has also estimated that tyrannosaur eggs – remains of which have never been found – were around 17 inches long.

Distinctive tyrannosaur features were found in analysis of the three-centimetre long jaw bone, including a “pronounced chin”, which the team say suggests physical traits were also present before they hatched.

We now know that they would have been the largest hatchlings to ever emerge from eggs

Dr Funston, of the university’s School of GeoSciences, said: “These bones are the first window into the early lives of tyrannosaurs and they teach us about the size and appearance of baby tyrannosaurs.

“We now know that they would have been the largest hatchlings to ever emerge from eggs and they would have looked remarkably like their parents — both good signs for finding more material in the future.”

The study is published in the Canadian Journal of Earth Sciences and was supported by the Royal Society, Natural Sciences and Engineering Research Council of Canada, and National Science Foundation.

Researchers from the universities of Alberta, Calgary, Montana State and Chapman were also involved.