Tyrannosaur Hatchlings Were as Big as Medium-Sized Dog
An international team of paleontologists has examined the fossilized remains of baby tyrannosaurid dinosaurs found in Alberta, Canada, and Montana, the United States.
These dinosaurs have garnered considerable interest since their discovery, both in the popular and scientific realms.
As a result, they are well studied and much is known of their anatomy, diversity, growth, and evolution.
In contrast, little is known of the earliest stages of their development.
Tyrannosaurid eggs and embryos remain elusive, and juvenile specimens — although known — are rare.
“Tyrannosaurs are represented by dozens of skeletons and thousands of isolated bones or partial skeletons,” said co-author Mark Powers, a Ph.D. student at the University of Alberta.
“But despite this wealth of data for tyrannosaur biology, the smallest identifiable individuals are aged three to four years old, much larger than when they would have hatched.”
In the new research, Powers and colleagues examined 75 to 71.5-million-year old fossilized jaw bone, tooth and claw of baby tyrannosaurids.
“A perinatal tooth and an embryonic claw from the Horseshoe Canyon Formation of Alberta are probably attributable to Albertosaurus sarcophagus,” they said.
“From the Two Medicine Formation of Montana, an embryonic jaw with teeth probably pertains to Daspletosaurus horneri.”
“They provide information on the size of tyrannosaurid hatchlings and some preliminary clues to the nesting habits of tyrannosaurids.”
The paleontologists found that baby tyrannosaurs were around 0.9 m (3 feet) long when they hatched.
“There were two surprising results,” Powers said.
“The first is that the small tyrannosaur teeth were distinct from the teeth of older individuals — having not yet developed true serrations along the cutting edge of its teeth, as is iconic of the juveniles all the way through to adults.”
“The second was the estimated size of the embryos. The specimen belonging to the claw was estimated to be about 1.1 m (3.6 feet) long, while that of the jaw bone was about 71 cm (2.3 feet).”
Their findings also suggest that tyrannosaur eggs — the remains of which have never been found — were around 43 cm (17 inches) long. This could aid efforts to recognize such eggs in the future and gain greater insights into the nesting habits of tyrannosaurs.
“The discovery of embryonic material is a huge find in our efforts to understand how some of the most popular and charismatic dinosaurs began their life, and grew to immense sizes,” Powers said.
“It provides a much-needed — and until now, missing — data point depicting the starting point for tyrannosaur growth.”
The researchers also found that the 3-cm- (1.2-inch) long jaw bone of Daspletosaurus horneri had distinctive tyrannosaur features, including a pronounced chin, indicating that these physical traits were present before the animals hatched.
“These bones are the first window into the early lives of tyrannosaurs and they teach us about the size and appearance of baby tyrannosaurs,” said lead author Dr. Greg Funston, a researcher at the University of Edinburgh.
“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.”
A paper on the findings was published in the Canadian Journal of Earth Sciences.
Gregory F. Funston et al. Baby tyrannosaurid bones and teeth from the Late Cretaceous of western North America. Canadian Journal of Earth Sciences, published online January 25, 2021; doi: 10.1139/cjes-2020-0169