First tyrannosaur embryo fossils revealed

New scans of a tiny Cretaceous jaw and claw show the tyrant dinosaurs started out the size of a small dog.

Illustration by Julius Csotonyi
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An illustration shows what Tyrannosaurus rex hatchlings may have looked like. The newly described embryonic fossils were not from T. rex, but an earlier species of related tyrannosaur that has not been identified.

Illustration by Julius Csotonyi

First tyrannosaur embryo fossils revealed

New scans of a tiny Cretaceous jaw and claw show the tyrant dinosaurs started out the size of a small dog.

The first known fossils of baby tyrannosaurs reveal that some of the largest predators ever to stalk the Earth started life about the size of a Chihuahua—with a really long tail.

The fossils—a foot claw and a lower jaw—are from tyrannosaurs still in the embryonic stage, when the developing dinosaurs would have been snugly wrapped up in their eggs. Found at different fossil sites in western North America, both date to about 71 to 75 million years ago, when tyrannosaurs had just become the apex predators of their environments.

The itty-bitty claw was uncovered at a site in the Horseshoe Canyon Formation in 2018, within First Nation lands in Alberta. The jaw was also found on indigenous land, in the Two Medicine Formation of Montana in 1983. They were described last week by University of Edinburgh paleontologist Gregory Funston at the virtually held annual meeting of the Society of Vertebrate Paleontology.

The significance of the bones wasn’t immediately clear when they were discovered. While a graduate student in Alberta, Funston was researching the identity of the toe claw when his advisor, Philip Currie, showed him the piece of little jaw encased in stone, too delicate to be removed from the rock. “I wasn’t convinced that it was a tyrannosaur at all,” Funston says. But after a 3D scan and reconstruction to show the jaw in full detail, Funston changed his mind.

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A 3D reconstruction of the embryonic tyrannosaur jaw, at the bottom of the image, compared to jaw bones from other known tyrannosaurs. The bottom-most image is the jaw at 10x the scale of the other images for comparison, and the little silhouette shows how large the specimen is compared to the larger jaws.

Oklahoma State University paleontologist Evan Johnson-Ransom, who was not involved in the research, agrees that the bones are “diagnostic and discernible” from other dinosaurs. The jaw, in particular, very much resembles those of known tyrannosaurs.

“We knew then that we had a chance to learn a lot about baby tyrannosaurs, which had been entirely mysterious until then,” Funston says. Most of the tyrannosaur fossil record consists of adult or older juvenile animals. Paleontologists have made speculative reconstructions of tyrannosaur babies, but no one really knew what they looked like. The toe and the jaw finally allowed the experts to check the fossil record against their expectations.

A baby tyrant

The new fossils reveal tyrannosaur babies were tiny compared to the adults—only about a tenth as long as grown tyrannosaurs. By contrast, a baby African elephant is about a fourth the height of the adults. The jaw came from a tyrannosaur that was about two and a half feet long, and the toe claw belonged to an animal a little over three feet long.

While a three-foot-long baby might sound pretty big by our standards, the dinosaurs would have hatched to be incredibly small next to adults that reached 30 feet in length and nearly three tons. The jaw has tiny teeth that match up with what experts call “null generation teeth,” or the very first teeth that are soon replaced by a fully functional set of choppers as the animals grow.

With their little, blade-like teeth and small jaws, hatchling tyrannosaurs probably dined on insects and lizards. The prey options continued to change as the dinosaurs grew. Specimens of T. rex, for instance, indicate that these carnivores were preying on small dinosaurs by the time they were 11 years old—and by 22, they could crush the bones of large herbivores and even other tyrannosaurs.

“The embryonic tyrannosaurids give us an idea on not only the size of a baby, but also the size of tyrannosaurid eggs,” Johnson-Ransom says. No one has yet positively identified tyrannosaur eggs or hatchlings, but the size of the new embryonic dinosaurs matches with large, elongated eggs that paleontologists have found before. From the size of the embryos, researchers think tyrannosaurs were curled up in eggs that measure about 17 inches long.

Searching for little fossils

The new fossils give researchers a few clues to go searching for more embryonic and hatchling tyrannosaurs. Until now, researchers were mystified as to why young tyrannosaurs hadn’t been found. It wasn’t clear whether T. rex mothers was nesting in different places or if something else was obscuring signs of their offspring, even as paleontologists discovered eggs and babies from other species such as duckbilled dinosaurs.

Now it seems that the tyrannosaurs were hiding in plain sight. Both the claw and the jaw were discovered at sites where the eggs and bones of other species have been uncovered.

“Considering that we found both of the embryonic bones at localities where we’ve found embryonic bones of other kinds of dinosaurs, it gives us a good clue that tyrannosaurs were nesting in the same areas” as other species, Funston says.

Paleontologists often collect more specimens on one foray than they have time to study in detail at that point, so there may be other baby tyrannosaur fossils hiding in museum collections. “Hopefully these new bones will help to refine the search” for additional fossils, says Funston, who is also planning to investigate known sites for more material with his team.

“We’ve barely scratched the surface at these sites,” he adds, “and we’re finding more each year.”