Tyrannosaurus rex: the teen years
When we think of dinosaurs, Tyrannosaurus rex – or T. rex – quickly comes to mind. This is hardly surprising, since T. rex is often called the king of dinosaurs and is probably the most famous dinosaur ever to have walked the Earth. An adult T. rex was a huge carnivorous predator with teeth that could crush the bones of any poor victim it caught. Scientists have learned a lot about this mighty and ferocious creature over the years. Now a new study is shedding more light on what T. rex teens were like.
The study was led by Holly Woodward of Oklahoma State University (OSU). It focuses on the bones of two mid-sized T. rex skeletons nicknamed Jane and Petey. The new peer-reviewed paper was published in the journal Science Advances on January 1, 2020.
Jane and Petey’s bones were first discovered and collected in the early 2000s in Carter County, Montana, by Burpee Museum of Natural History in Rockford, Illinois.
As with other dinosaur species, museums and scientists will often focus on the largest and most impressive fossils, including those of T. rex. That’s understandable, especially for public displays, but the OSU team wanted to study younger specimens of T.rex as well. As Woodward explained in a statement:
Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others. The problem is that those smaller fossils may be from younger animals. So, for a long while we’ve had large gaps in our understanding of how dinosaurs grew up, and T. rex is no exception.
By examining the fossils of younger T. rexes, scientists can better understand how they changed as they got older, including their bones and proportions. They can also use paleohistology – the microscopic study of fossil bone structure – to learn about juvenile growth rates and ages. In the case of Jane and Petey, researchers removed thin slices of leg bones from fossils, and examined them under powerful microscopes.
As Woodward pointed out, huge dinosaur bones are also fossilized on a microscopic scale, not just as large bones the way we see them:
To me, it’s always amazing to find that if you have something like a huge fossilized dinosaur bone, it’s fossilized on the microscopic level as well. And by comparing these fossilized microstructures to similar features found in modern bone, we know they provide clues to metabolism, growth rate, and age.
So what were T. rex teens – smaller but still ferocious predators – like?
Woodward and her colleagues found that they grew as fast as modern mammals and birds. It took the juvenile T. rexes about 20 years to mature into adults, these scientists said. They determined the ages of Jane and Petey by counting the number of annular rings in the bones, in much the same way that tree rings are counted. It turned out the two teen dinosaurs were about 13 and 15 years old, respectively – teenage T. rexes. According to Woodward:
The spacing between annual growth rings record how much an individual grows from one year to the next. The spacing between the rings within Jane, Petey, and even older individuals is inconsistent – some years the spacing is close together, and other years it’s spread apart.
Juveniles were fast and had knife-like teeth. Adult T. rexes, though, were slower but had powerful jaws and teeth to crush bones. The researchers also found that if food was more scarce in a particular year for younger T. rexes, then they didn’t grow as much during that time period.
But if there was a lot of food, then they grew a lot.
After Jane and Petey were first discovered, there was speculation that they might not be T. rexes at all, but rather belonged to a pygmy relative species called Nanotyrannus. Studies of the bones, however, confirmed they were juvenile T. rexes.
Our concepts of T. rex have changed over the years, with more recent findings suggesting that fully-grown T. rexes had feathers on the top of their heads, backs and tails. A fully-grown T. rex was fearsome, standing about 12 to 13 feet high at the hip, and about 40 to 43 feet long. Paleontologists have found most T. rex fossils in the northwestern U.S., in states such as Montana and South Dakota, as well as in Alberta, Canada. T. rex lived at the very end of the Late Cretaceous period, about 90 to 66 million years ago.
Bottom line: A new study from Oklahoma State University provides new details about what the famous Tyrannosaurus rex dinosaurs were like during their teenage years.