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Tyrannosaurus rex was one of the largest carnivorous dinosaurs of all time – it measured up to 42 feet in length from snout to tail and would have weighed around 16,000 pounds.
And he wasn’t alone: some of his lesser-known cousins could grow to nearly the same size. Scientists have already proved this T. rex they got that big going through huge teenage growth, but they didn’t know if this was only true for tyrannosaurs, just them and their close relatives, or maybe all of the large bipedal dinosaurs. By cutting dinosaur bones and analyzing growth lines, a team of researchers came up with the answer: T. rex and his closest relatives had a clunky adolescence during which they grew huge, while his more distant cousins in the allosauroid group continued to grow a little each year.
“We wanted to examine a wide range of different theropod, two-legged carnivorous dinosaurs, in order to understand the broader patterns of growth and evolution in the group,” says Tom Cullen, lead author of a new study in Proceedings of the Royal Society B.
Cullen, a scientific affiliate of the Field Museum in Chicago who worked on the study as a postdoctoral researcher at the Field with the museum’s then dinosaur curator, Pete Makovicky, explains, “We specifically wanted to understand how some of them became like this. large – is the way T. rex grew up the only way to do it? “
Makovicky, a Field science affiliate and professor of geology at the University of Minnesota and senior author of the paper, says, “We also wanted to see if we got the same growth record when we sampled a variety of different bones from the same skeleton. All these questions about how theropods grew up could affect our understanding of the evolution of the group. ” Makovicky developed the idea for the project and also discovered many of the dinosaurs whose fossils were analyzed in the study.
The question of how an animal grows up is surprisingly complicated. Mammals like us tend to go through a period of extreme growth when we are young and then remain the same size once they reach adulthood. In other groups of animals, this is not always the case. “The growth rate really varies, there is no one size fits all,” says Cullen, who is now a postdoctoral researcher at the North Carolina Museum of Natural Sciences. “Birds have a super growth spurt and reach adult size very quickly, while reptiles like alligators and various lizards and snakes have extensive growth. With them, a really, really great individual is probably very old. “
Theropod dinosaurs like them T. rex they are related to both modern birds and reptiles, in fact birds are the only living theropods. Scientists didn’t know if theropods’ growth patterns were more like those of birds or reptiles, and those different growth patterns can make a big difference in how an animal fits into its ecosystem. Getting big quickly can be a competitive advantage – it makes it easier for you to hunt other animals and harder for other animals to hunt you. On the other hand, a growth spurt requires a lot of energy and resources, and it’s easier to get a little bigger every year throughout your life. “The amount of calories T. rex needed during his growth spurt would have been ridiculous, ”says Cullen – like a teenager eating dinosaurs instead of endless bags of bagel bites.
The central struggle in the study of extinct animals is that we can never know exactly what their lives were like. Since we cannot directly observe a dinosaur growing as we can a living animal today, it is difficult to know for sure how they grew up. But there are clues in the fossil record that reveal growth patterns.
“” Inside the bones as an animal grows, there are marks such as tree rings that record roughly how old the animal is, how much it grows each year and a number of other factors, “says Cullen. these growth rings, Cullen and his colleagues cut fossils of dozens of dinosaurs, from those the size of dogs and ostriches up to SUE il T. rex, one of the largest predatory dinosaurs ever discovered. Getting access to slice and dice bone from a range of theropods wasn’t an easy proposition, but Cullen and Makovicky reached out to colleagues from around the world. Notably, they were able to obtain samples from a new species of giant carcharodontosaurid from Argentina as a direct counterpoint to T. rex–This specimen was discovered and excavated by Makovicky in collaboration with his Argentine colleagues Juan Canale and Sebastian Apesteguía. The authors also contacted colleagues from the Liaoning Museum of Paleontology for samples of small theropods closely related to birds to obtain the evolutionarily large sampling needed to determine large-scale patterns in life history.
“The first specimen that the Field Museum allowed me to sample was SUE the T. rex“Says Cullen.” It was pretty nerve-wracking, since it’s such a famous fossil. “He used a diamond-tipped core drill bit to cut a tiny cylinder from SUE’s femur. The resulting sample was a cross-section of SUE’s femur. bone from SUE, with lines like tree rings showing where the new bone had grown year after year. (The missing piece of bone, the size of a D battery, was then filled with brown putty – if you go see SUE at Field Museum and look closely at their left thigh you might see it, but it’s hard to spot.)
Back in the lab, Cullen cut bone samples so thin that light could pass through them and examined them under a microscope.
“Most animals have a period each year where they stop growing, traditionally suggested to be in periods like winter when food is scarce. It appears in the bones as a line, like a ring in a tree, “says Cullen. By analyzing these growth lines and examining the bones for new growth regions, scientists can get a rough estimate of an animal’s age and how much it has grown every year.There are also clues in the bone structure.
“You can see all the small areas where the bone cells have grown and the structure of the blood vessels that have passed through the bone,” says Cullen. “These vascular channels tell you more or less how fast the bone was growing. If the canals are more organized, the bone was deposited more slowly, and if the structure is chaotic, it grew faster. “
Cullen found that dinosaur growth patterns depended on their family. T. rex and its relatives, the celurosaurs, have shown a period of extreme growth during adolescence, only to fade once they reach adulthood. SUE on T. rex lived to about 33 years, the eldest T. rex currently known, but reached adult size at the age of 20. To reach this massive size, SUE probably gained around 35-45 pounds a week as a teenager. Their more distant cousins, the Allosauroids, could grow to nearly as large a size T. rex, but they have grown slowly throughout their lives, with older individuals reaching the largest size. Among the allosauroids they sampled was the new carcharodontosaur from Argentina. It reached a size close to that of SUE, but did not reach adult size until the age of 30-40. It lived to be around 50 years or more, making it the oldest individual theropod ever recorded apart from a few birds such as parrots. Despite his advanced age, he had only stopped growing 2 or 3 years before becoming part of the fossil record.
The discovery raises questions about how these predatory dinosaurs interacted with the animals around them. The plant-eating dinosaurs that lived together T. rex they were ceratopsians like the triceratops and duck-billed hadrosaurs. They grew very quickly even in adolescence. Slow-growing allosauroid carnivores lived with large long-necked sauropods that also grew rapidly, but they appear to have taken a long time to reach full size. These trends could be related.
“We can’t say for sure, but there may be some sort of selective pressure for celurosaurs to grow rapidly to keep up with their prey, or pressure for allosauroids to continue growing in size as their prey also increases in size,” he says. Cullen. “But it’s pretty speculative. It could be that even though sauropods continued to grow throughout their lives, they had so many children that there was always something small to eat. “
But while the research didn’t answer all the questions about why dinosaurs like them T. rex they grew up the way they did, says Cullen, “I’m really proud of this work. It is the culmination of many, many years of small projects developing towards some sort of central goal of trying to understand the growth in these animals and understand the many factors that influence these patterns. This doesn’t solve it, but this is a really big step forward. “
CAMPO MUSEUM
Image credit: Steveoc 86 – CC BY-SA 4.0
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