Dinosaur discovery: Teenage growth spurts saw mighty T-rex grow faster than its cousins

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The mighty T-rex was among the biggest and most fearsome dinosaurs to roam the Earth between 68 and 66 million years ago. And although the dinosaur’s reign was cut short by an asteroid impact at the end of the Late Cretaceous, the T-rex’s popularity has endured. Adult T-rexes measured up to 42ft (12m) from snout to tail and weighed in at an impressive 16,000 pounds.

But the T-rex was not a lone contender in the heavyweight category as several of its theropod cousins reached similar sizes.

Theropods were a family of dinosaurs characterised by their hollow bones and three-toed feat – features carried on by their evolutionary offshoot, birds.

What set the T-rex and its relatives apart from some of their more distant dinosaur species, however, was a series of significant growth spurts in their teenage years.

Palaeontologists have known of these growth spurts from previous T-rex studies, but it has been unclear whether this was true for T-rex as well as other groups.

A team of researchers has now analysed so-called growth lines in fossilised dino bones to determine how the T-rex compared to dinosaurs from the allosauroid group.

Much like a tree, as the dinosaurs grew, they left markings in their bones that roughly indicate their age and how much they grew each year.

The researchers focused on SUE the T-rex – one of the biggest and best-preserved T-rex skeletons in existence.

The study’s findings were published in the journal Proceedings of the Royal Society B.

Lead author Tom Cullen said: “We wanted to look at a wide swath of different theropods, two-legged carnivorous dinosaurs, in order to understand broader patterns and evolution in the group.”

Dr Cullen is a postdoctoral researcher at Chicago’s Field Museum, who worked on the study with Dr Pete Makovicky, Curator of Palaeontology at the institution.

Dr Makovicky said: “We particularly wanted to understand how some of them got so big – is the way T-rex grew the only way to do it?”

He added: “We also wanted to see if we got the same growth record when we sample a variety of different bones from the same skeleton.

“All these questions about how theropods grew could impact our understanding of the evolutions of the group.”

Examples of certain animals growing rapidly in their adolescent years are seen throughout the natural world even today.

Mammals, for instance, tend to grow rapidly in their youth and once they peak, they stay the same size throughout adulthood.

Reptiles, on the other hand, are mature slowly and take their time.

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Dr Cullen said: “Growth rate really varies, there’s no one size fits all.

“Birds have super growth spurts and reach adult size really fast, while reptiles like alligators and various lizards and snakes have extended growth.

“With them, a really, really big individual is probably old.”

By studying SUE’s fossilised bones, the researchers determined the dinosaurs’ growth patterns depend on their families.

The T-rex and its relative coelurosaurs hit massive growth spurts early on.

SUE, for example, lived to the ripe age of 33 and T-rexes most likely reached adult size by the time they were 20-years-old.

And to reach its impressive size of more than 40ft (12m), the teenage dinosaur would have gained between 35 and 45 pounds of weight every week.

Growth like this would have required incredible amounts of energy to be consumed.

Dr Cullen said: “The amount of calories T. rex would have needed during its growth spurt would have been ridiculous.”

The more distantly allosauroids could reach similar sizes to the T-rex but did so over longer periods of time, with the biggest individuals also being the oldest ones.

A Carcharodontosaurus recently discovered in Argentina, for example, reached a size close to SUE but not until its 30s to 40s.

Questions still remain about how other dinosaur species, such as the duck-billed hadrosaurs grew and why.

These omnivorous species also matured at a quick pace in their early days.

Dr Cullen said: “I’m really proud of this work.

“It’s the culmination of many, many years of small projects building towards sort of a central goal of trying to understand growth in these animals and understand the many factors that influence these patterns.

“This doesn’t resolve it, but this is a really big step forward.”

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