In 2017, a Canadian palaeontologist named Gregory Funston found a tiny fossil dinosaur toe near Morrin Bridge, in the Red Deer River Valley of Alberta. Although the team excavating the site found hundreds of small fossils that day, Funston knew that this centimetre-long fossil bone was special. The toe bone was determined to be from a member of the tyrannosaurid family of dinosaurs, a lineage that include the most famous dinosaur of all, Tyrannosaurus rex. That toe bone, along with a tyrannosaurid jaw-bone uncovered from Montana, were identified as the first fossil remains of baby tyrannosaurs.
“It’s always been a pipe dream to find tyrannosaur babies,” says Funston, who is also the study’s lead author and a postdoctoral researcher at the University of Edinburgh. Palaeontologists have slowly been filling in the blanks of what other dinosaurs looked like as babies, he says, but tyrannosaurs have remained elusive.
The toe bone was found in a microsite, an area with an abundance of small fossils. The land is in Treaty 7 Territory, the traditional home of the Kanai (Blood), Tsuu T’ina (Sacree), Siksika (Blackfoot), Piikanki (Peigan), and Naokda (Stoney) First Nations, and Métis Nation.
“Microsites are really exciting to me, because there’s such a diversity of material,” says Funston. At this site, bone fragments from meat and plant-eating dinosaur bones mingle with fossils from smaller animals, like frogs and lizards. Altogether the microsite paints a picture of what the environment looked like at the time.
Searching the microsite for fossils required precision and patience. “You’re unlikely to find things sticking out of the ground,” says Funston. “You’re sorting through little stones, you’ve got your nose to the ground, and you’re looking very closely to pick out the fossils.”
The dedication paid off. The toe bone found at the site didn’t have a fully formed joint yet, indicating that it was from a very young animal. However, it was not possible to determine if the toe bone was from an embryo or a very young hatchling.
The jawbone from Montana was a different story. Measuring only three centimetres in length, the bone had long been too fragile to examine in great detail. But with developments in high resolution scanning, the researchers were able to identify a small peg-like tooth in the jaw. Known as a null generation tooth, this structure can be found in other dinosaurs, alligators, and geckos. Null generation teeth are always replaced by a functional tooth before the animal hatches, says Funston. This points to the jaw being from an embryo still inside the egg.
Funston and the team went further with their findings and estimated the size of tyrannosaurs at birth. Using an immense dataset of tyrannosaur fossil measurements collected by Phil Currie, an eminent palaeontologist and professor with the University of Alberta, the team determined that hatchling tyrannosaurs would have been around a metre in length. That would make tyrannosaurs the largest land-animals ever to hatch from an egg.
“Now that we know that these sites have tyrannosaur babies, we want to go back to them,” says Funston. “Especially the site in Alberta, we’ve barely scratched the surface of that one. We’re going to do a lot more fieldwork and see if we can recover more material.”