Parents of foot-tailed tyrannosaur couldn’t keep up with their leaner offspring, fossil footprints reveal

Parents of foot-tailed tyrannosaur couldn't keep up with their leaner offspring, fossil footprints reveal

Credit: José Vitor Silva, Author provided

Tyrannosaurus rex is perhaps the most famous of all the dinosaurs. He and his closest relative, a group referred to as “tyrannosors,” are rooted in popular culture as powerful and moving predators.

Consider the scene below of the sprawling 1993 Jurassic Park; a T. rex adult chases a speeding Jeep – much to the delight of the audience.

But fancy Jeeps and theme parks aside, are these drawings realistic?

Our research, published today in the Journal of Vertebrate Paleontology, shows that while young tyrannosians may indeed have been the vision of wrath depicted above, they have probably become wider foot, bulky and less mobile as an adult.

A new perspective

Previous debates about how (or not) tyrannosers ran were either focused on their bones, or relying on computer models to simulate their running capabilities.

Besides being smaller, the skeletons of young tyrannosers are also built lighter than their bulky parents, which suggests they are probably faster and more naive about their body size. Adolescents tended to have relatively longer legs and smaller skulls, and weighed much less than a fully grown adult.

In fact, the size of the leg muscles needed to maintain a fast running speed in a six-ton ​​T. rex adult would probably have been biologically impossible. This would require the dinosaur to gain as much as 86% of its total body weight just like leg muscle!





Must go faster.

Skeletons, however, represent only part of the story. Fossilized footprints provide a unique snapshot of how an animal (or species) moved around its environment – one not provided by skeletons.

Fossilized footprints are an enlarged image of the feet as they once appeared in real life, with the soft parts still intact.

In 2015 and 2018, our team discovered a new collection of rocky tyrannosaur footprints at a lonely outlet in western Canada, which we present for the first time in our newspaper.

These footprints provided a unique opportunity to study how the shapes of tyrannosor footprints were changing from youth to adulthood. If their relative mobility diminished as they grew – as previously assumed from the study of their skeletons – then we would expect this to be expressed in foot shape as well.

Younger and faster animals would have slimmer feet, whereas older individuals would have bulky feet, less suited to speed and agility.

Traces ancient footprints through bear country

The tyrannosaur tracks we found remain preserved in deserts near Grande Prairie in Northwest Alberta, Canada. The region is known for its bitter cold winters, which cause the rivers to flood in the spring as snow melt rolls off the nearby Rocky Mountains.

The footprints themselves are preserved along a bank of the Redwillow River, surrounded by Boreal forest, which today is home to wildlife including brown bears, black bears and wolves.

Parents of foot-tailed tyrannosaur couldn't keep up with their leaner offspring, fossil footprints reveal

A lush forest surrounded the track site where the footprints of the tyrannosaur were found in Alberta. Author provided

Thankfully our close encounters with deer are largely, though flocks of mosquitoes are a constant nuisance.

Tyrannosaur footprints in this area can be identified by the presence of three long and narrow toes, often with sharp pointed claw marks. We saw up to ten footprints, each about 72 million years old, ranging from 30 to 62 centimeters long.

Although no bones were found, it is possible that the footprints belong to Albertosaurus. This tyrannosaur was living in Alberta at the time and was an earlier and smaller relative of T. rex.

My, what big heels do you have!

Using a method called geometric morphometrics, we analyzed the best tyrannosaur footprints from our collection of tracks, along with previously discovered footprints.

This method mathematically eliminates the effect of an overall size difference between each footprint while examining important differences in the shape of a footprint.

Applied to our samples, we found that the main difference across all footprints was the heel area and width compared to the footprint length.

Larger prints had larger proportional heels while smaller tracks had narrower and smaller heels.

Parents of foot-tailed tyrannosaur couldn't keep up with their leaner offspring, fossil footprints reveal

On the left is a picture of a tyrannosaur track likely made by a middle-aged Albertosaurus. The diagram on the right shows how a tyrannosaur’s feet may have changed in shape and size as they got older. Author provided

This difference likely relates to the large size of an adult tyrannosaur, or specifically an adult Albertosaurus, which may have weighed between 1,300 and 2,200 kg.

A wider and more fleshly heel probably helped adults maintain balance and support increased weight, but it probably came at the expense of speed and agility.

Our work on footprints supports the hypothesis that, as oppressors have grown, they have moved from fast, light young to slower, heavy adults.

Slow down by old age

Would this have been a problem for holding food as an adult oppressor? Not likely. The large four-legged herbivores they hunted, like Edmontosaurus regalis (which weighed about 4,000kg), were probably even slower.

So what about that hunting scene from Jurassic Park?

Well, we still can’t be sure how fast an adult T. rex could run. But we can say that heavier and bulkier adults are probably slower in body size than leaner adults.

Maybe he should have been a young oppressor chasing that Jeep instead. Although this would not have been equally scary.

More information:
Exploring potential ontogenetic trajectories in tyrannosaurids using tracks from the Wapiti Formation (upper Campanian) Alberta, Canada, Journal of Vertebrate Paleontology (2021). www.tandfonline.com/doi/full/1… 2724634.2021.1878201

This article has been republished from The Conversation under a Creative Commons license. Read the original article.The ConversationThis story is part of Science X Dialogue, where researchers can report findings from their published research articles. Visit this page for information about ScienceX Dialogue and how to get involved.

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