If you've ever thought a teenager on a growth spurt was like a Tyrannosaurus – always hungry and with a foul temper – you might be more right than you know. The king of carnivores had a distinctly different growth pattern to other meat-eating dinosaurs, one marked by a period of exceptional growth, during which they were probably hungrier and angrier than ever. Meanwhile, even theropods of similar size were more like the tortoise of Aesop's fable than the hare, reaching their tremendous size through slow and stable growth.
Like the rings of tree trunks, our bones preserve a record of their development in growth layers. Following observations that T.rex and its closest relatives experienced a tremendous increase in size during its teenage years, Dr Tom Cullen of the Field Museum set out to see if this was a universal trait among dinosaurs with similar lifestyles.
"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," Cullen said in a statement. "We particularly wanted to understand how some of them got so big – is the way T. rex grew the only way to do it?"
The answer is no, Cullen and co-authors claim in Proceedings of the Royal Society B. To get there Cullen drilled into the bones of theropods of many sizes, including SUE, one of the largest and most famous T. rexes of all time.
Most of these dinosaurs have bone rings indicating steady growth throughout their life, including allosauroids, fellow theropods some of which approached T. rex size if they lived into their 40s. Tyrannosaurs were the exception. Cullen found SUE stopped growing at around the age of 20, but lived to be approximately 33. To have reached her immense size so fast her teenage years must have involved packing on 15-20 kilos a week.
Birds are the surviving theropods, and do all their growing while young. However, Tyrannosaurs and many other former theropods have more in common with crocodiles and alligators, which keep growing all their lives, rather than reaching a specific size and stopping.
The question the team sought to answer was more complex than simply measuring the rates of different dinosaur species. Just as some people have parts of their bodies grow ahead of others, leading to an awkward stage before the rest catches up, dinosaurs may have done the same thing.
Dr Cullen took a core from SUE's left thigh bone. However, the authors found non-weight bearing dinosaur bones were remodeled through life, and do a poor job of preserving the growth rings. Perhaps T. rex arms grew earlier than the rest of their bodies, and were actually long enough to be useful for juveniles.
Although the study focused on theropods, Cullen noted some of the herbivorous dinosaurs preyed on by T. rexes also had rapid adolescent growth. The allosaurids, on the other hand, preyed on sauropods, which took much longer to reach full size.
"We can't say for sure, but there could be some kind of a 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," Cullen said. However, he admits this is speculative since we lack knowledge of how much of these predators' diets were made up by juvenile prey.