The tyrannosaurus rex (T-rex) was one of the biggest meat-eating dinosaurs of all time, frequently measuring up to 42 feet (12.8 metres) in length from snout to tail. It would also have weighed in at a staggering 16,000 pounds (about 7.2 tonnes).
And it was not alone, as recent scientific research has shown that some of its lesser known cousins could have reached nearly the same size.
Scientists have previously shown that T-rex got so big by going through a huge teenage growth spurt. What they did not know, however, is if other bipedal dinosaurs underwent the same sort of growth spurt, or whether this was just true for the tyrannosaurs and their close relatives.
By cutting into dinosaur bones and analysing the growth rings, a team of researchers have received their answer: the T-rex and its closest relatives went through an awkward adolescence during which they underwent a massive growth in height and length. At the same time, whilst its more distant cousins in the allosauroid group kept on growing, this occurred at a lesser and more measured rate every year.
“We wanted to look at a wide swath of different theropods, two-legged, carnivorous dinosaurs, in order to understand broader patterns of growth and evolution in the group,” says Tom Cullen, the lead author of a new study in the Proceedings of the Royal Society B.
Cullen is also a scientific affiliate of Chicago’s Field Museum who worked on the study as a postdoctoral researcher in the field with the museum’s then-curator of dinosaurs, Pete Makovicky.
He added, “We particularly wanted to understand how some of them got so big, [and asked] is the way T-rex grew the only way to do it?”
Makovicky, a scientific affiliate of the field and professor of geology at the University of Minnesota and the paper’s senior author, 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.”
He added, “All these questions about how theropods grew 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 analysed in the study.
The question of how an animal gets big is a surprisingly tricky one. Mammals like humans tend to go through a period of extreme growth when young, and then stay the same size once in adulthood. In other animal groups, this is not always the case.
“Growth rate really varies, there’s no one size fits all,” says Cullen, who is now a postdoctoral researcher at the North Carolina Museum of Natural Sciences, “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 really old.”
Theropod dinosaurs, like T-rex, are related to both modern birds, the only living theropods still around, and reptiles. Scientists did not know whether the growth patterns exhibited by theropods were more like those of birds or reptiles, with those different growth patterns making a big difference to how an animal fits into its ecosystem.
Getting big quickly can be a competitive advantage, as it makes it easier for the animal to hunt, and harder for other animals to hunt you. On the flip side, a growth spurt takes a lot of energy and resources, and it is easier to get just a little bigger every year over the course of that animal’s life.
“The amount of calories the T-rex would have needed during its growth spurt would have been ridiculous,” says Cullen, comparing it to that of a teenage boy.
The central struggle in studying extinct animals is that we can never know exactly what their lives were like. Since we cannot directly observe a dinosaur growing the way one can with a living animal today, it is hard to know for sure how they grew. But there are clues in the fossil record that reveal growth patterns.
“Inside the bones as an animal grows, there are markings like tree rings that record roughly how old the animal is, how much it’s growing each year, and a number of other factors,” says Cullen.
To find these growth rings, Cullen and his colleagues sliced into fossils from dozens of dinosaurs, from ones the size of dogs and ostriches all the way up to SUE the T-rex, one of the biggest predatory dinosaurs ever discovered.
Getting access to slice and dice bones from a range of theropods was not an easy proposition, but Cullen and Makovicky reached out to colleagues across the globe.
In particular, they were able to get 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 Argentinean colleagues Juan Canale and Sebastian Apesteguía.
The authors also reached out to colleagues at the Palaeontological Museum of Liaoning for samples of small theropods closely related to birds. This was to gain the broad evolutionary sampling needed to determine large scale patterns in life history. Cullen sliced samples of bone so thin that light could pass through them and examined them under a microscope.
“Most animals have a period every year when they stop growing, traditionally suggested to be in times like winter when food is more scarce,” Cullen says, “It shows up in the bones as a line, like a tree ring.”
By analysing these growth lines and examining the bones for new regions of growth, scientists can get a rough estimate of an animal’s age and how much it grew every 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 passed through the bone,” says Cullen, “These vascular canals tell you roughly how fast the bone was growing. If the canals are more organized, the bone was being laid down more slowly, and if the structure is chaotic, it grew more quickly.”
Cullen found that dinosaur growth patterns depended on their family. T-rex and its relatives, the coelurosaurs, showed a period of extreme growth during adolescence, and then they petered out once they reached adulthood.
SUE the T-rex lived to about 33 years of age, and is the oldest T-rex currently known for which there is physical evidence. The T-rex tended to reach their adult size by age 20.
To reach this massive size, SUE probably gained around 35-45 pounds (16-20 kg) per week as a teenager. The T-rex’s more distant cousins, the allosauroids, could reach sizes almost as big as the T-rex, albeit at a slower pace staggered throughout their whole lives, with the oldest individuals reaching the biggest sizes.
Among the allosauroids the researchers sampled was the new carcharodontosaur from Argentina. It reached a size close to that of SUE, but did not reach adult size until its 30s to 40s.
This species lived up to about 50 years of age or older, making it the oldest individual theropod on record aside from some species of birds such as parrots. Despite its advanced age, it had only stopped growing two or three years before becoming part of the fossil record.
The discovery opens up questions about how these predatory dinosaurs interacted with the animals around them.