Though the gap in Hell Creek is particularly extreme, most of the communities with megatheropods 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 all 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 in those that lacked megatheropods, supporting the idea that the young megatheropods were filling them, Ms. Schroeder said. The gaps also didn’t apply to herbivores. This suggests that the juvenile carnivores’ inability to hunt the same food as adults forced them to carve out their own niche, which had a strong influence on the ecosystem, she said. (A baby sauropod, in contrast, could munch on the bottom branches of a tree while an adult ate the top.)
The new study “represents an enormous feat in testing this concept,” Dr. Clauss said. Theoretically, he said, these same dynamics might have made it harder for dinosaurs to repopulate large niches after a mass extinction event: When the big dinosaurs died, the relative lack of small and medium-sized species meant that mammals were better positioned to take over.
Broad analyses like this one are “truly transforming 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 notion that youngsters were different kinds of predators than their monster parents was out there,” Dr. Witmer said. But while many paleontologists had been focusing on one species at a time in addressing this question, this new study instead connects “thousands of dots,” he said, to show “how whole communities of dinosaurs evolved.”
“It’s a big deal,” he said.
At the same time, the paucity of juvenile dinosaur fossils makes it difficult to precisely understand the roles those youths played, said David Hone, a zoologist at Queen Mary University of London who has used similar methods to study dinosaur size distribution.
“Knowing that juveniles filled a general niche space and actually being able to do anything with that information are two different things,” he said.