The researchers focused on two tyrannosaur specimens known as Jane and Petey. Both were found in the early 2000s at the Hell Creek Formation in the western United States, and are housed at the Burpee Museum of Natural History in Rockford, Illinois. Each is about the size of a horse.
After examining certain aspects of their bones and skulls, some paleontologists have argued that these and other small tyrannosaurs found in the Hell Creek Formation were not young T-rexes, but adult specimens of a separate, contemporaneous species they named Nanotyrannus.
Woodward, who has studied the bone tissue of many dinosaurs as well as other animals, saw the opportunity to make an argument about Nanotyrannus that was “independent of morphology”, she said. While others had scrutinised the shapes and structures of the dinosaurs’ bones, the researchers looked inside them.
They took chips of femur and tibia from both specimens, polished them down until they were less than a millimetre thick, and put them under a microscope.
Bone tissue is made up of small bundles of collagen fibres. The organisation of those bundles can tell you how quickly the bone has grown – if they’re neatly layered, like a stack of logs, it means the growth was slow and even. If they’re haphazard, that signals faster growth.
In the two tyrannosaurs, the fibres “look like pick-up sticks,” said Woodward. “Jane and Petey were growing pretty quickly up until they died.”
The researchers then looked at the bones’ cyclical growth marks. These are the animal equivalent of tree rings – they form during periods of slow growth, and in this way record the passage of years. (For example, the cyclical growth marks in the bones of Svalbard reindeer correspond with polar winters, when the food supply is lowest.)
By counting these growth marks, the researchers found that Jane died at around 13 years old, while Petey was around 15. Experts believed T-rexes reached maturity around 20 years old, and could have lived to about 30.
The marks also indicate how much each dinosaur grew during a particular year. For Jane and Petey, some of the rings are very close together, marking a year of minimal growth. Others are much farther apart, indicating rapid change.
While experts have used morphology to argue both for and against the existence of Nanotyrannus, this internal evidence shows more conclusively that Jane and Petey are not adult specimens of a smaller Tyrannosaur species. Instead, they were “subadults with high growth rates,” which means they were most likely juvenile T-rexes, said Thomas Carr, an associate professor of biology at Carthage College in Kenosha, Wisconsin, who had studied Jane before but was not involved with this research.
They also give us a better idea of how this one carnivore was able to dominate the landscape. The uneven spacing of the growth rings suggests that young T-rexes responded to the amount of resources available, growing quickly when food was plentiful, and stopping growth altogether when times were lean.
In this way, T-rexes could be many different sizes, and “play different roles in the ecosystem as they aged,” said Lawrence M. Witmer, a professor of anatomy and paleontology at Ohio University Heritage College of Osteopathic Medicine who was also not involved in the study.
T-rex babies might eat young herbivores and other small creatures. Teenagers like Jane and Petey caught mid-sized prey. And adults chomped away on herbivore adults.
“This glimpse into the family life of T-rex is really exciting,” said Witmer, from the comfort and safety of 66 million years later.
New York Times