A new study has confirmed that young megatheropod dinosaurs — two-legged carnivores such as Tyrannosaurus Rex — grew up quickly to outcompete adults of smaller dinosaur species, leading to greater diversity among megatheropods than among small or medium-sized carnivores.
This pattern is the opposite of that found in living species, which tend to show more diversity at smaller sizes with few large species, explained the researchers behind the paper published Thursday in Science.
“When you look at the global biomass distribution — or the number of large dinosaurs versus small dinosaurs everywhere on Earth — you see this backwards pattern to what you find in modern ecosystems,” said Katlin Schroeder, a Ph.D. candidate in biology at the University of New Mexico and first author of the study. “We wanted to understand if there was some sort of ecological driver causing that in these dinosaurs.”
While these patterns in carnivore diversity have been well-documented, their drivers have not been carefully studied. And while many small species are missing from the fossil record because these specimens are harder to find, the researchers did not believe this could account for the sheer number of missing species.
“For perspective, if the modern mammal carnivore assemblage of Kruger National Park were similarly structured, there would be no carnivores between the size of an African lion (190 kilograms) and a bat-eared fox (four kilograms),” the study reads.
The researchers hypothesized that the cause for this large gap in diversity was related to how megatheropods grew and matured, particularly the fact that they hatched from eggs.
Because there is a biological limit on the size of eggs, megatheropod hatchlings were much smaller than their adult counterparts. They had to grow rapidly to reach adulthood and reproduce, shifting through several different ecological niches as they grew, a phenomenon known as ontogenic niche shift.
As they grew and changed, young megatheropods perfectly filled the same ecological niches that small- to medium-sized carnivores would have filled. The result of this “grow fast, die young” lifestyle is that megatheropods dominated the landscape at all sizes, while the mesocarnivores — species weighing 100 to 1,000 kilograms — died out when they weren’t able to keep up with the competition.
“One of the things we did in our analysis was treat these small theropods as morphospecies, which means we treated them as their own species in an ecosystem because they’re so different in size and ecological role from the adults,” Schroeder told The Academic Times. “When we did that, they perfectly filled this gap that we were seeing in the number of smaller carnivore species.”
To confirm their hypothesis, the researchers used data sourced from past paleontology papers and from the Paleobiology Database, a massive collection of paleontology data from around the world that includes known species occurrences and records of which bones have been found for a given species.
Their analysis encompassed 43 dinosaur communities across seven continents, spanning 136 million years and including more than 550 species of dinosaur. Having such a large dataset to work with is part of what made this analysis feasible for the researchers, and also allowed them to confirm their hypothesis as a global phenomenon, not one isolated to a specific community or region.
“Nowadays we’re getting into the realm of having really good, international datasets,” Schroeder said. “Researchers don’t have to spend years looking through data to get this kind of information anymore. It’s starting to be much more centralized.”
Looking at dinosaurs from this type of large-scale ecological standpoint is a somewhat newer approach to paleontology, though a 2018 review from Oxford University shows that researchers are beginning to explore these methods. Historical approaches tended to focus only on finding and identifying species, because for many years the driving force of paleontology research was fieldwork.
“We have to remember that a lot of paleontology stems from geology,” said Schroeder. "So historically it was a bunch of old dudes in the desert collecting bones. Now we’re starting to shift more into looking at dinosaurs as biological entities rather than just cool rocks.”
The researchers hope that studies like this will encourage more paleontologists to look at dinosaur diversity from a macroecological standpoint, examining the interactions between them in the ecosystem rather than just studying them as individual species.
And while this research looks toward a past that can never return, the researchers maintain that the insights of ancient ecology can provide valuable contributions to the understanding of modern ecosystems.
“The fact that we can take the principles that we understand from modern mammalian ecology, even though those animals are physiologically very different, and look back up to 200 million years to apply those principles to dinosaurs," Schroeder said, "really reasserts our understanding of the fundamental building blocks of communities and modern ecology."
The paper “The influence of juvenile dinosaurs on community structure and diversity” was published Feb. 25 in Science. The authors of the study were Katlin Schroeder and Felisa A. Smith, University of New Mexico; and S. Kathleen Lyons, University of Nebraska.