T. rex had huge growth spurts, but other dinos grew ‘slowly and steadily’

Tyrannosaurus rex was one of the largest carnivorous dinosaurs of all time – he was up to 42 feet long from snout to tail and is said to have weighed around 16,000 pounds. And that wasn’t the only one – some of his lesser-known cousins ​​could reach about the same height. Scientists have already shown that T. rex got so big going through a huge teenage growth spurt, but they weren’t sure if that was true for tyrannosaurs, just them and their close relatives, or maybe all of the big bipedal dinosaurs. By cutting out dinosaur bones and analyzing the growth lines, a team of researchers got their answer: T. rex and his closest relatives had awkward adolescence in which they grew to be huge, while his more distant cousins ​​in the allosauroid group continued to grow a little taller each year.

“We wanted to look at a wide range of different theropods, carnivorous two-legged dinosaurs, in order to understand 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 dinosaur curator, Pete Makovicky, explains, “We especially wanted to understand how some of them are. become so big. the path T. rex grew up the only way to do it? “

Makovicky, a scientific affiliate of Field and professor of geology at the University of Minnesota and lead 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. of the same skeleton. . All of these questions about the growth of theropods could impact our understanding of the evolution of the group. Makovicky developed the idea for the project and also discovered several of the dinosaurs whose fossils were analyzed in the study.

The question of how an animal grows is surprisingly delicate. Mammals like us tend to go through a period of extreme growth when we are young and then stay the same size once we reach adulthood. In other groups of animals, this is not always the case. “The growth rate really does vary, 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 super spurts of growth and reach adulthood very quickly, while reptiles like alligators and various lizards and snakes have prolonged growth. With them, a very very large individual is probably very old.”

Theropod dinosaurs love T. rex are related to both modern birds and reptiles – in fact, birds are the only living theropods. Scientists weren’t sure whether theropod growth patterns looked more like those of birds or reptiles, and these 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 it makes it harder for other animals to hunt you. On the other hand, a growth spurt takes a lot of energy and resources, and it’s easier to get a little taller every year throughout your life. “The amount of calories T. rex would have needed during his growth spurt would have been ridiculous, ”says Cullen – like a teenager who ate dinosaurs instead of endless bags of bagel bites.

The main difficulty in studying extinct animals is that we can never know exactly what their life was like. Since we can’t directly observe a dinosaur growing up like you can today, it’s hard to know for sure how they grew up. But there are clues in the fossil record that reveal patterns of growth.

“” Inside the bones as an animal grows there are marks like tree rings which roughly record the animal’s age, growth each year and a certain many other factors, “Cullen explains. To find these growth rings, Cullen and his colleagues cut up dozens of dinosaurs into fossils, from those the size of dogs and ostriches to SUE on T. rex, one of the largest predatory dinosaurs ever discovered. Accessing sliced ​​and diced bones from a range of theropods was not an easy proposition, but Cullen and Makovicky reached out to colleagues around the world. In particular, they were able to obtain samples of 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 Argentinian colleagues Juan Canale and Sebastian Apesteguía. The authors also contacted colleagues at the Liaoning Paleontological Museum to obtain samples of small theropods closely related to birds to obtain the large evolutionary sampling needed to determine large-scale models of life history.

“The very first specimen the Field Museum let me sample was SUE on T. rex“Cullen said.” It was pretty scary, because it’s such a famous fossil. He used a diamond-tipped core drill bit to cut a small cylinder out of SUE’s thigh bone. The resulting sample was a cross section of SUE. the bone, 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 stack, was then filled with brown putty – if you go see SUE at the Field Museum and look closely at their left thigh, you might see her, but it’s hard to spot.)

Back in the lab, Cullen cut out bone samples so thin that light could pass through them and examined them under a microscope.

“Most animals have a period each year when they stop growing, which is traditionally suggested to be times like winter when food is scarce. It shows up in the bones like a line, like a tree ring, ”Cullen explains. By analyzing these growth lines and examining the bones for new areas of growth, scientists can get a rough estimate of an animal’s age and growth each year. There are also clues in the bone structure.

“You can see all the little areas where the bone cells have grown and the structure of the blood vessels that have passed through the bone,” Cullen explains. “These vascular channels tell you roughly how fast the bone was growing. If the channels are more organized, the bone settles more slowly, and if the structure is chaotic, it grows faster.”

Cullen discovered that dinosaur growth patterns depended on their families. T. rex and his parents, the coelurosaurs, experienced a period of extreme growth during adolescence and then passed out once they reached adulthood. SUE on T. rex lived to about 33 years old, the oldest T. rex currently known, but reached their adult size at 20 years. To achieve this massive size, SUE probably gained around 35 to 45 pounds per week as a teenager. Their more distant cousins, the allosauroids, could grow to sizes almost as large as T. rex, but they grew slowly throughout their lives, with older individuals reaching the largest sizes. Among the allosauroids they sampled was the new carcharodontosaurus from Argentina. He reached a size close to that of SUE, but did not reach adult size until after 30 to 40 years. It lived to be around 50 years or more, making it the oldest theropod on record, aside from some birds like parrots. Despite its advanced age, it had only stopped growing for 2 or 3 years before it became part of the fossil record.

The discovery raises questions about how these predatory dinosaurs interacted with the animals around them. The herbivorous dinosaurs that lived nearby T. rex were ceratopsians like Triceratops and duck-billed hadrosaurs. They grew extremely quickly as a teenager too. Slow-growing allosauroid carnivores lived with large, long-necked sauropods that also grew rapidly, but appear to have taken a long time to reach full size. These trends can be linked.

“We can’t say for sure, but there could be some sort of selection pressure for coelurosaurs to grow rapidly to follow their prey, or pressure for allosauroids to keep growing in size since their prey was also increasing in size. size. ”Cullen says.“ But that’s pretty speculative. It could be that even though the sauropods were growing all their lives, they had so many offspring that there was always something small to eat. “

But while research hasn’t answered all of the questions about why dinosaurs love T. rex grew up the way they did, says Cullen, “I’m really proud of this work. It’s the culmination of many years of small projects aimed at sort of a central goal of trying to understand how these animals grow. and to understand the many factors that influence these models. That doesn’t solve the problem, but it is a very big step forward. ”