Agence France-PresseFeb 26, 2021 10:37:22 IST
A team of U.S. scientists has proved that the children of giant carnivorous dinosaurs like Tyrannosaurus rex, which has grown from the size of domestic cats to tall monsters, are reshaping their ecosystems by taking out less competitive species. Their study, published in the journal Science on Thursday, helping to answer a lasting mystery about the rule of 150-million-year-old dinosaurs: why there were so many more species compared to small ones, which are contrary to what we see in very terrestrial animals today?
Dinosaurs were not particularly diverse: there are only about 1,500 known species, compared to tens of thousands of species of modern mammals and birds.
“Dinosaur communities were like shopping malls Saturday afternoon, full of teenagers,” said Kat Schroeder, a graduate student at the University of New Mexico who led the research. “They made up a good proportion of the individuals in gender and would have had a huge impact on the resources available in communities.”
Even with so little fossil record, dinosaurs are thought to have been entirely diverse: there are only about 1,500 known species, compared to tens of thousands of species of modern mammals and birds.
Moreover, over the entire Mesozoic period, from 252 to 66 million years ago, there were relatively more species of large-bodied dinosaurs weighing 1,000 kilograms (tons) compared to species with larger weights. less than 60 kilograms (130 pounds).
Some scientists have put forward the idea that since even the craziest dinosaurs start life as tiny hatchlings, they could use different resources as they grew up – take up their place in ecosystems where smaller species would thrive.
To test the theory, Schroeder and her colleagues studied data from fossil sites around the world, including more than 550 dinosaur species, and organized the dinosaurs into herb- edible or carnivorous, in addition to their sizes.
They found a remarkable gap in the presence of medium-sized carnivores in all communities that had megatheropods, or large predators like the T-rex.
“Communities with megatheropods have very few carnivorous dinosaurs between 100-1000 kilograms (200 pounds to one ton),” Schroeder said. “And the youngsters of those megatheropods will fit into that space.”
Treating juveniles as sex
The decision was supported by the way in which dinosaur diversity changed over time. Jurassic communities had smaller gaps (200-145 million years ago) and Cretaceous communities (145-65 million years ago) had large ones.
That’s because Jurassic teens megatheropods were more like adults, and there was a wider variety of long-necked plant sauropods (like the brachiosaurus) to prey on.
“On the other hand, the Cretaceous are completely under the control of Tyrannosaurs and Abelisaurs, which change a lot as they grow,” Schroeder said.
To mathematically test their theory, the team multiplied the size of young megatheropods at certain ages by how many were expected to live each year, based on fossil records.
This statistical method, which effectively treated juveniles as their own genus, quickly swept away the gaps in medium-sized carnivores.
Beyond helping solve a long-term problem, the research shows the value of putting ecological observation on dinosaurs, Schroeder said.
“I think we are moving a little more towards understanding dinosaurs as animals instead of looking at dinosaurs as just cool rocks, and that is where paleontology and its has been there for a long time, “she said.