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Rex relative, dog-sized

A terrifying tyrannosaur – a cousin of T. rex – may have started its life about the size of a border collie.

Artist’s impression of a juvenile tyrannosaur. Credit: Julius Csotonyi

A team of palaeontologists led by Greg Funston, of the University of Edinburgh, Scotland, found that a tyrannosaur embryo fossil with a tiny jawbone and claw would have been around 90 centimetres long when hatched.

The adult version of the tyrannosaur is estimated to have grown to 12 metres in length; the team suggests that the eggs it laid were around 40 cm long. Currently, no egg remains have been found, but this estimation could assist palaeontologists in identifying them in the future, they suggest in their paper, published in Canadian Journal of Earth Sciences.

“These bones are the first window into the early lives of tyrannosaurs and they teach us about the size and appearance of baby tyrannosaurs,” says Funston.

“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.”

Their analysis of the fossil also revealed that the 3 centimetre jawbone had distinct tyrannosaur characteristics such as a pronounced chin, which mean these distinguishing features were developed before they hatched instead of while they grew to adulthood.

This discovery is particularly exciting because previous tyrannosaur fossils were adults or older juveniles, so this sheds light on the early developmental stages of these dinosaurs.

Silhouette imaging showing baby tyrannosaurs to scale. Credit: Greg Funston