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Hungry Teen T. Rexes

Why are there so few fossils of medium-sized meat-eating dinosaurs? Thank young T. rexes.

Roger Harris/Science Source

It took about 20 years  for a T. rex to reach its adult length of 12 meters and weight of 6 tons.

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.

After hatching from an egg, a baby T. rex started off small, around 15 kilograms. In 15 to 20 years, it reached adulthood. As T. rexes matured and grew, their behavior, lifestyle, and role in an ecosystem likely changed too. “It would be very, very competitive to be a medium-sized carnivore in the land of T. rex,” says Schroeder.

A baby T. rex was small when it hatched from its egg. It weighed about 15 kilograms. In 15 to 20 years, it reached adulthood. As T. rexes grew, their behavior, lifestyle, and role in an ecosystem likely changed too. “It would be very, very competitive to be a medium-sized carnivore in the land of T. rex,” says Schroeder.

One other explanation could be that medium-sized carnivores are missing from the fossil record because they simply haven’t been discovered yet. Either way, it’s important to understand dinosaurs before they become “big, bad adults,” Schroeder says. “We often ignore what they’re doing as smaller, younger individuals, but they’re actually influencing the ecosystem as much as—if not more than—the adults.”

One other explanation for the missing medium-sized dinosaurs is that their fossils simply haven’t been discovered yet. Either way, it’s important to learn about dinosaurs before they become “big, bad adults,” Schroeder says. “We often ignore what they’re doing as smaller, younger individuals. But they’re actually influencing the ecosystem as much as—if not more than—the adults.” 

Use the line graph above to answer the following questions about T. rexes. Record your work and answers on our answer sheet.

Use the line graph above to answer the following questions about T. rexes. Record your work and answers on our answer sheet.

It took up to 20 years for a T. rex to become an adult. About how much would it weigh?

It took up to 20 years for a T. rex to become an adult. About how much would it weigh?

At what age would a T. rex weigh more than a 4,500-kilogram African Elephant?

At what age would a T. rex weigh more than a 4,500-kilogram African Elephant?

A teenage T. rex called Bucky is on display at the Indianapolis Children’s Museum. Scientists estimate he weighed about 2,984 kg. Roughly how old was Bucky?

A teenage T. rex called Bucky is on display at the Indianapolis Children’s Museum. Scientists estimate he weighed about 2,984 kg. Roughly how old was Bucky?

The Wankel T. rex is on display at the Smithsonian in Washington, D.C. Scientists estimate that it was 18 years old when it died. About how much did it weigh?

The Wankel T. rex is on display at the Smithsonian in Washington, D.C. Scientists estimate that it was 18 years old when it died. About how much did it weigh?

The range of carnivore weights missing from the fossil record is about 100 kg to 1,000 kg. What range in ages of T. rex does that correspond to?

The range of carnivore weights missing from the fossil record is about 100 kg to 1,000 kg. What range in ages of T. rex does that correspond to?

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