Revved-Up Rex: How T.rex Outpaced Other Giant Theropods
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Theropod dinosaurs were the largest bipedal creatures to ever roam the planet. Some of them could reach lengths of up to 13 meters (42 feet) and weigh as much as 9 tons. While Tyrannosaurus is a prime example, paleontologists recognize that several theropods achieved comparable dimensions.
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What scientists were uncertain about was whether these enormous creatures exhibited similar growth patterns beyond their size. Researchers from Chicago's Field Museum aimed to uncover this mystery.
The Study
In their investigation, the team analyzed various theropod dinosaur skeletons from museums in the U.S., Canada, China, and Argentina. They received permission to slice into the bones of different theropod genera, including one of the most renowned T. rex fossils, known as Sue. Other examined species included the late Jurassic predator Allosaurus and the carcharodontosaurid Acrocanthosaurus from North America, along with a newly discovered carcharodontosaurid species from Argentina that rivaled T. rex in size.
The findings of the research were unexpected.
Supercharged T.rex
Prior studies indicated that Tyrannosaurus grew at a rate akin to that of modern mammals and birds. Researchers observed that T. rex and its closest relatives experienced significant growth spurts during their teenage years. This rapid growth led to dramatic changes in their appearance, speed, and even their choice of prey.
Between the ages of 14 and 18, the ‘tyrant lizard king’ transformed from a nimble predator into a colossal hunter that could simply approach its prey and deliver a powerful bite. Estimates suggest that during peak growth, a typical T. rex could gain an astonishing 4.6 pounds (2.1 kilograms) daily!
Growing rapidly provides a competitive edge, allowing predators to hunt more effectively while making it harder for other predators to catch them. However, such growth spurts demand considerable energy and resources.
Thanks to this remarkable growth rate, Tyrannosaurus could attain its full adult size relatively quickly, within approximately two decades. Nonetheless, scientists were uncertain if other large bipedal theropods shared the same growth patterns observed in tyrannosaurs.
Tom Cullen, a scientific affiliate at Chicago's Field Museum, states, “We particularly wanted to understand how some of them got so big — is the way T. rex grew the only way to do it?”
A Different Approach
The study revealed that this was not the case. It found that other large predatory theropods exhibited more gradual growth rates over extended periods. This specific growth strategy was noted among dinosaur families that originated earlier in the Mesozoic era, including allosaurs, which were apex predators of their ecosystems long before tyrannosaurs emerged.
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The study's findings raise questions about how these predatory dinosaurs interacted with their prey.
The herbivorous dinosaurs coexisting with T. rex were ceratopsians like Triceratops and duck-billed hadrosaurs. These animals also experienced rapid growth during adolescence. In contrast, the slower-growing allosauroid carnivores lived alongside gigantic long-necked sauropods, which, despite growing quickly, appeared to take a considerable amount of time to reach full size.
“We can’t say for sure, but there could be some kind of selection pressure for the coelurosaurs to grow quickly to keep up with their prey, or pressure for the allosauroids to keep growing in size since their prey were also increasing in size. But it’s pretty speculative. It could be that even if the sauropods kept growing their whole lives, they had so many offspring that there was always something small to eat,” explains Tom Cullen.
Epilogue
Theropod dinosaurs were the largest bipeds to have ever existed. While various predatory theropods achieved significant sizes, they did so through two different growth strategies. Tyrannosaurus and its nearest relatives experienced rapid growth during adolescence, while their more distant cousins grew at a steadier rate year after year.
These varied growth patterns lead scientists to conclude that there were no strong mechanical or physiological constraints affecting the size evolution of these incredible predators.
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T.rex: The Reign of The King
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References
Field Museum. “T. rex had huge growth spurts, but other dinos grew slow and steady: By cutting into fossils and examining growth rings, scientists learned how predatory dinosaurs got so big.” ScienceDaily. ScienceDaily, 24 November 2020. **Link:* https://www.sciencedaily.com/releases/2020/11/201124190527.htm*
Thomas M. Cullen, Juan I. Canale, Sebastián Apesteguía, Nathan D. Smith, Dongyu Hu, Peter J. Makovicky. Osteohistological analyses reveal diverse strategies of theropod dinosaur body-size evolution. Proceedings of the Royal Society B: Biological Sciences, 2020; 287 (1939): 20202258 DOI/ **Link:* 10.1098/rspb.2020.2258*
Holly N. Woodward, Katie Tremaine, Scott A. Williams, Lindsay E. Zanno, John R. Horner and Nathan Myhrvold, (Jan 2020), Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy “Nanotyrannus” and supports ontogenetic niche partitioning in juvenile Tyrannosaurus, Science Advances, **Link:* https://advances.sciencemag.org/content/6/1/eaax6250*
Aylin Woodward, (Jan 2020), Paleontologists are unraveling the mysteries of young T. rexes. Creatures they thought were 2 species turned out to be kids and adults., Business Insider, **Link:* https://www.businessinsider.com/tyrannosaurus-rex-teenage-growth-spurt-2019-12*