Tyrannosaurus rex, the most famous king of the dinosaurs, was about the length of a medium sized dog when it hatched.
Despite being among the largest predators to ever walk the Earth, experts now believe some baby tyrannosaurs were only the size of a Border Collie when they took their first steps.
Palaeontologists made the discovery by examining the fossilised remains of a tiny jaw bone and claw unearthed in Canada and the US.
Based on the size of the fossils, the dinosaurs were around three feet long when they hatched, the researchers said. When the tiny predators emerged they would have been the largest ever hatchlings to come from eggs, scientists said.
Border Collies on average can grow to around three feet long and stand at just over two feet high.
The bone fragments, which are the first-known fossils of tyrannosaur embryos, belonged to baby tyrannosaurs – cousins of T. Rex.
Tyrannosaurus lived throughout what is now western North America, on what was then an island continent known as Laramidia 68 to 66 million years ago.
These embryo fossils shed light on the early development of the colossal animals, which could grow to 40 feet in length and weigh eight tonnes. The expert team’s findings suggest that tyrannosaur eggs – the remains of which have never been found – were around 17 inches long.
Experts led by a University of Edinburgh researcher, made the discovery by making 3D scans of the delicate fragments to work out the size of the dinosaur.
Dr Greg Funston, of the University of Edinburgh’s School of GeoSciences, who led the study, 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.’
This discovery could aid efforts to recognise tyrannosaurs eggs in the future and gain greater insights into the nesting habits of tyrannosaurs, researchers added.
The analysis also revealed that the just over one inch-long (3cm) jaw bone has distinctive tyrannosaur features, including a pronounced chin, indicating that these physical traits were present before the animals hatched.
Little is known about the earliest developmental stages of tyrannosaurs – which lived more than 70-million-years-ago – despite being one of the most studied dinosaur families. Most tyrannosaur fossils previously studied have been of adult or older juvenile animals, experts said.
The study, published in the Canadian Journal of Earth Sciences, was supported by the Royal Society, Natural Sciences and Engineering Research Council of Canada and National Science Foundation.