A team of American scientists has shown that it likes the offspring of huge carnivorous dinosaurs Tyrannosaurus rex, growing from the size of domestic cats to towering monsters, altered their ecosystems by defeating smaller rival species.
Their study, published in the journal Science on Thursday, helps answer an enduring mystery about the 150 million-year rule of dinosaurs: why were there many more large species compared to small ones, which is the opposite of what we see in land animals today?
“Dinosaur communities were like shopping malls on a Saturday afternoon, packed with teenagers,” said Kat Schroeder, a graduate student at the University of New Mexico who led the study.
“They constituted a significant portion of the individuals in a species and would have had a very real impact on available resources in communities.”
Even given the limitations of the fossil record, it is believed that dinosaurs were generally not particularly diverse: there are only about 1,500 known species, compared to tens of thousands of modern mammal and bird species.
Additionally, throughout the Mesozoic Era, from 252 to 66 million years ago, there were relatively more large-body dinosaur species weighing 1,000 kilograms (a ton) compared to species weighing less than 60 kilograms (130 pounds).
Some scientists put forward the idea that since even the most gigantic dinosaurs begin to live as little youngsters, they could use different resources as they grow up – taking up space in ecosystems where smaller species could otherwise thrive.
To test the theory, Schroeder and her colleagues examined data from fossil sites around the world, including more than 550 dinosaur species, and organized the dinosaurs by herbivores or carnivores, and by their size.
They discovered a striking gap in the presence of medium-sized carnivores in any community with megatheropods or giant predators such as the T. rex
“There are very few carnivorous dinosaurs between 100 and 1000 kilograms (200 pounds to a ton) in megatheropod communities,” said Schroeder.
“And the juveniles of those megatheropods fit right in that room.”
Treat juveniles as a species
The conclusion was supported by the way dinosaur diversity changed over time. Jurassic communities (200-145 million years ago) had smaller gorges and Cretaceous communities (145-65 million years ago) had large ones.
That’s because teens of Jurassic megatheropods look more like adults, and there was a wider variety of long-necked herbivorous sauropods (like the brachiosaurus) that they could hunt for.
“The Cretaceous, on the other hand, is completely dominated by tyrannosaurs and abelisaurs, which change a lot as they grow,” said Schroeder.
To test their theory mathematically, the team multiplied the mass of juvenile megatheropods at certain ages by the number expected to survive each year, based on fossil data.
This statistical method, which effectively treated juveniles as their own species, neatly dispelled the observed gaps of medium-sized carnivores.
In addition to helping solve a long-standing question, the research shows the value of applying ecological considerations to dinosaurs, Schroeder said.
“I think we’re shifting a little bit more towards understanding dinosaurs as animals, rather than seeing dinosaurs as just cool rocks, and that’s where paleontology started and has been around for a long time,” she said.
© Agence France-Presse