When baby tyrannosaurs were taking their first steps they were the ‘size of a border collie dog’ according to a new study of ancient 70 million year old fossils.
Palaeontologists from the University of Edinburgh examined the fossilised remains of a tiny jaw bone and claw found in Canada – dating back millions of years.
The remains belonged to a baby tyrannosaur – cousin of the fabled T-Rex – and are the first-known fossils of tyrannosaur embryos, according to the researchers.
Through 3D scans they found the creature would have been about 3ft long when it first hatched – about the same size as a border collie dog – despite it growing to a mammoth 40ft long when it reached full adult size.

An artist’s illustration of a baby tyrannosaurus which were the size of Border Collie dog when taking first steps, a team of palaeontologists has discovered

This is part of the fossilised remains of a baby tyrannosaur. Researchers say they underwent considerable skeletal changes throughout their life

Through 3D scans they found the creature would have been about 3ft long when it first hatched – about the same size as a border collie dog – despite it growing to a mammoth 40ft long when it reached full adult size
Study lead author Dr Greg Funston, said the bones were the first window into the early lives of tyrannosaurs – teaching us much about their size and appearance.
The team has estimated that tyrannosaur eggs – remains of which have never been found – were around 17 inches long.
The jaw bone was about 1.2 inches long and included evidence of a pronounced chin – suggesting some physical tyrannosaur traits were present before they hatched.
‘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, said Funston.
He added that the size of the eggs and the similarity to their parents were ‘both good signs for finding more material in the future’.
Tyrannosaurs lived throughout what is now western North America, on what was then an island continent known as Laramidia 68 to 66 million years ago.
‘Tyrannosaurids were the apex predators of Late Cretaceous Laurasia, and their status as dominant carnivores has garnered considerable interest since their discovery, both in the popular and scientific realms,’ the team wrote.

The remains were found in the Horseshoe Canyon Formation in Alberta, Canada and the Two Medicine Formation stretching from northwestern Montana to southern Alberta

The remains were found in the Horseshoe Canyon Formation in Alberta, Canada and the Two Medicine Formation stretching from northwestern Montana to southern Alberta
‘As a result, they are well studied and much is known of their anatomy, diversity, growth, and evolution.’
However, only adult and juvenile fossils have so far been found by experts, leaving a black spot in understanding when it comes to their early development.
These new embryo fossils shed light on the early development of the colossal animals, allowing for a much deeper understanding of what they were like when they first hatched.
The team created a series of 3D scans of the delicate bone fragments from the jaw and claw fossils – allowing them to get a picture of the size of the baby dinosaur.

Researchers compared the size of the dinosaurs that the fossils belonged to with a human and an adult Albertosaurus
This discovery could aid efforts to recognise tyrannosaurs eggs in the future and gain greater insights into the nesting habits of tyrannosaurs, researchers added.
‘These immature specimens are essential because it is now recognised that the tyrannosaur skeleton undergoes dramatic changes as the creature ages,’ according to the team.
However, as the small pieces of bone fragments were found fairly isolated, not surrounded by other similar fragments, there is a limit to what the team can determine.
‘The fragmentary and isolated nature of the specimens makes it difficult to determine their development stages with certainty,’ the team wrote.
However, there were enough indications, as well as comparisons to other embryo fossils from other dinosaurs, to determine they were from pre-hatched infants.
‘Some of the characteristics distinguishing tyrannosaurids from other theropods later in life are already present during the earliest stages of development,’ the authors wrote.
‘The rarity of tyrannosaurid embryonic material and the absence of eggshell at nesting sites is perplexing and requires further investigation.
‘But current evidence suggests that tyrannosaurid perinates should be present in nesting assemblages of other dinosaurs.’
The remains were found in the Horseshoe Canyon Formation in Alberta, Canada and the Two Medicine Formation stretching from northwestern Montana to southern Alberta.
These areas have resulted in the discovery of a range of dinosaur and dinosaur-like species.
The study is published in the Canadian Journal of Earth Sciences.
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