Hungry Teen T. Rexes

The age of the dinosaurs is known for its giant reptiles. As an adult, the infamous Tyrannosaurus rex weighed over 6 tons—about as heavy as a city bus. But T. rexes were smaller teenagers before they grew into gargantuan adults.

“If you were to hop into a time machine and go to the Cretaceous period (145 to 65 million years ago), you’d be much more likely to see a teenage T. rex as opposed to an adult,” says Katlin Schroeder, a Ph.D. candidate in biology at the University of New Mexico. Not only were these teenage meat-eating dinosaurs abundant, Schroeder’s team found that they likely played an important role in the food chain.

The age of the dinosaurs is known for its giant reptiles. As an adult, the Tyrannosaurus rex weighed more than 6 tons. That’s about as heavy as a city bus. But T. rexes were smaller teenagers before they grew into giant adults.

T. rexes lived during the Cretaceous period, 145 to 65 million years ago. “If you were to hop into a time machine and go to the Cretaceous period, you’d be much more likely to see a teenage T. rex as opposed to an adult,” says Katlin Schroeder. She’s a Ph.D. candidate in biology at the University of New Mexico. There were many teenage meat-eating dinosaurs back then. Schroeder’s team found that they likely were important to the food chain. A food chain is a  series of organisms related by their feeding habits, where smaller species are eaten by larger ones.

For years, paleontologists have puzzled over a gap in body sizes of meat-eating dinosaurs. There are many small carnivores (10 to 100 kilograms) and large carnivores (over 1,000 kg) in the fossil record, but few in between.

To find out what happened to the medium-sized meat eaters, Schroeder analyzed 43 dinosaur communities that spanned seven continents and existed over 136 million years. This size gap occurred in all communities with megatheropods—the largest carnivorous dinosaurs that weighed 1,000 kilograms or more. Younger megatheropods could have out-competed other medium-sized dinosaurs, Schroeder says.

There have been very few fossils of medium-sized meat-eating dinosaurs found. For years, paleontologists puzzled over this gap in body sizes. There are fossils of many small carnivores (10 to 100 kilograms) and large carnivores (more than 1,000 kilograms). But there are few in between.

So, what happened to the medium-sized meat eaters? Schroeder analyzed 43 different groups of dinosaur communities that lived together and interacted with each other. The communities were from all seven continents. They all existed over a time period of 136 million years. This size gap occurred in all communities with megatheropods. Megatheropods were the largest meat-eating dinosaurs. They weighed 1,000 kilograms or more. Young megatheropods would have eaten the same prey as other medium-sized dinosaurs and caused them to die out.