Have teenage “tyrants” outperformed other dinosaurs?

Paleoecologists at the University of New Mexico and the University of Nebraska-Lincoln have demonstrated that the offspring of huge carnivorous dinosaurs, like the Tyrannosaurus rex, may have fundamentally reshaped their communities by outperforming smaller rival species. .

The study, published this week in the journal Science, is the first to examine the diversity of dinosaurs at the community level while treating juveniles as their own ecological entity.

“Dinosaur communities were like shopping malls on a Saturday afternoon – packed with teenagers,” said Kat Schroeder, a graduate student in the UNM Department of Biology who led the study. “They were an important part of the individuals of a species and would have had a very real impact on the resources available in the communities.”

Because they are born from eggs, dinosaurs like T. rex are necessarily born small, about the size of a house cat. This meant that as they grew to the size of a city bus, these “megatheropods”, weighing between one and eight tons, would have changed their hunting habits and their prey. Paleontologists have long suspected that giant carnivorous dinosaurs would change their behavior as they grow older. But how this could have affected the world around them remained largely unknown.

“We wanted to test the idea that dinosaurs could take on the roles of multiple species as they grew, limiting the number of actual species that could coexist in a community,” Schroeder said.

The number of different types of dinosaurs known around the world is low, especially among small species.

“Dinosaurs had surprisingly low diversity. Even taking into account fossilization biases, there really weren’t as many dinosaur species,” said Felisa Smith, professor of biology at UNM and graduate advisor to Schroeder .

To address the issue of diminishing dinosaur diversity, Schroeder and his co-authors collected data from well-known fossil localities around the world, including over 550 species of dinosaurs. Organizing the dinosaurs according to their mass and diet, they looked at the number of small, medium, and large dinosaurs in each community.

They found a remarkably clear pattern:

“There is a gap – very few carnivorous dinosaurs between 100 and 1000 kg [200 pounds to one ton] exist in communities that have megatheropods, “Schroeder said.” And the juveniles of those megatheropods fit perfectly into that space.

Schroeder also notes that it was essential to examine the diversity of dinosaurs across time. Jurassic communities (200 to 145 million years ago) had smaller gaps and Cretaceous communities (145 to 65 million years ago) had large ones.

“Jurassic megatheropods don’t change as much? Adolescents look more like adults, which leaves more room in the community for several megatheropod families as well as some small carnivores,” Schroeder explained. “The Cretaceous, on the other hand, is completely dominated by Tyrannosaurs and Abelisaurs, which change a lot as they grow older.”

To find out if the gap was really caused by juvenile megatheropods, Schroeder and his colleagues reconstructed communities with the considered adolescents. By combining the growth rates of lines found in cross sections of bone and the number of infant dinosaurs surviving each year based on mass-dead assemblages of fossils, the team calculated what proportion of a species of megatheropod would have been juvenile.

Schroeder explained that this research is important because it elucidates (at least partially) why dinosaur diversity was lower than expected based on other fossil groups. It also explains why there are many more very large species of dinosaurs than small ones, which is the opposite of what one would expect. Most importantly, she added, it demonstrates the results of growing from very small infants to very large adults on an ecosystem.

“Dinosaurs have been a lifelong passion. I was and still am a “child of dinosaurs”. My interest in the diversity of dinosaurs was born when I realized that no one really looks at dinosaurs the way we look at modern mammals and birds, “said Schroeder. “There is a ton to be gained from applying modern and paleoecological methods to dinosaurs. Fortunately, we are now in the age of dinosaur research where a lot of information is available digitally, so questions of ecology data intensive are now becoming more plausible for dinosaur paleontology. ”