The baby tyrannosaurs were the size of a Border Collie

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A study focused on the earliest fossils of tyrannosaur embryos ever identified suggests that at birth, these predators were likely as large as a medium-sized Border Collie-type dog. Details of this work are published in the journal Canadian Journal of Earth Sciences.

Tyrannosaurs are an extinct genus of theropods that lived approximately 68 to 66 million years ago in what is now North America. With a length of over thirteen meters, a height of four meters at the level of the hips and a weight of up to eight tons, the most emblematic representative of these dinosaurs is none other than the T-Rex. But how tall were these animals when they were born? An unprecedented analysis of fossils allows us to see a little more clearly.

As big as a medium dog

As part of this work, paleontologists from the University of Edinburgh (Scotland) and Alberta (Canada) examined the earliest known fossils of Tyrannosaurus embryos ever identified. As such, these remains provided a rare opportunity to shed light on the early life stages of these ancient predators.

The researchers focused their study on a section of the jaw (three centimeters) of an embryonic tyrannosaur of the species Daspletosaurus isolated in the Two Medicine formation in Montana and on an “embryonic claw” belonging most likely to an Albertosaurus specimen. found in the Horseshoe Canyon Formation, Alberta.

Thanks to 3D scans, the researchers were able to estimate that these dinosaurs did not measure no more than 0.9m long when they hatch. They compared it to the Border Collie, a breed of herding dog known for its intelligence. The analysis also revealed that the jaw bone possesses distinctive tyrannosaur characteristics, including a pronounced chin, indicating that these physical traits were present before the animals hatched.

The researchers were also able to determine that the eggs they were curled up in whose remains were never found were measuring about 432 millimeters. This information is important as it could aid efforts to recognize these eggs in the future, thus allowing researchers to better understand the nesting habits of tyrannosaurs.

These bones are the first window into the early life of tyrannosaurs and they tell us about the size and appearance of baby tyrannosaurs.“Says Dr Greg Funston, who led the study.

The size of an adult Albertosaurus compared to that of two baby tyrannosaurs (determined by their fossilized remains) and researcher Greg Funston. Credit: Greg Funston

The growth of the T-Rex

Remember that previous studies have also determined that newborn T-Rex had longer “arms” in proportion to their body in comparison to adulthood.

On the growth side, the juveniles also gained about three kilos per day, for thirteen years. At that age, they were measuring about 6.4 m long and were still very slender. In other words, they didn’t quite look like adults yet.

The first big physical changes then took place around the age of fourteen or fifteen. The head and jaw would get deeper then, the teeth would get thick, and basically they looked like real T-Rexes then. Then a second wave of changes took place around the age of eighteen. During this period, the T-Rex took about three tons, becoming heavier than any other tyrannosaurid.

Finally, remember that each baby T-Rex was also covered with a layer of downy feathers. Whether the adult was still wearing his feathers is still debated. According to recent analyzes, it is likely that some feathers were still visible along the animal’s head and tail.

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