Although the rift at Hell Creek is particularly extreme, most of the megatheropod communities that the researchers studied had no carnivores between 220 and 660 pounds. (For a modern comparison, this is as if the adult carnivores in South Africa’s Kruger National Park were either larger than a lion or smaller than a bat-eared fox, they write.)

The carnivore gaps were more pronounced in these individual communities than those lacking megatheropods, which supported the idea that the young megatheropods filled them, Ms Schroeder said. The gaps did not apply to herbivores either. This suggests that juvenile carnivores’ inability to hunt the same food as adults forced them to carve out their own niche that had a strong impact on the ecosystem, she said. (In contrast, a baby sauropod could chew on the lower branches of a tree while an adult ate the upper ones.)

The new study “represents a tremendous achievement in testing this concept,” said Dr. Clauss. In theory, these dynamics might have made it harder for dinosaurs to repopulate large niches after a mass extinction: when the large dinosaurs died, mammals were better able to take control due to the relative lack of small and medium-sized species.

Comprehensive analyzes like this “are really changing the field of paleontology,” said Lawrence M. Witmer, a professor of anatomy and paleontology at Ohio University who was not involved in the study.

“The idea that adolescents were different types of predators from their monster parents was out there,” said Dr. Witmer. While many paleontologists had focused on one species each in answering this question, this new study instead connects “thousands of points” to show “how entire communities of dinosaurs evolved”.

“It’s a big deal,” he said.

At the same time, the lack of juvenile dinosaur fossils makes it difficult to fully understand the roles of these juveniles, said David Hone, a zoologist at Queen Mary University in London who has used similar methods to study the size distribution of dinosaurs.

“Knowing that teens are filling a general niche space and actually being able to do something with that information are two different things,” he said.