Fossils that some scientists thought to be a separate species were likely adolescent Tyrannosaurus rexes, a new study says. A full-grown T. rex was probably the most fearsome sight of the Cretaceous: It stood two stories tall, stretched the length of a city bus and boasted jaws that could crush the bones of a triceratops. But Tyrannosaurus Rex Teenagers inspired their own form of terror. Fast and light, they could catch prey their parents couldn’t. The T. rex was “basically king of its environment from the get-go,” said Holly Woodward, an associate professor of anatomy and paleontology at the Oklahoma State University Center for Health Sciences.
In a paper published Wednesday in Science Advances, Dr. Woodward and her colleagues used bone samples from juvenile T. rexes to shed light on the years before the dinosaurs grew to adulthood. Their findings suggest that variable growth rates allowed teen tyrannosaurs to terrorize the landscape just as effectively as their adult counterparts.
The team also helped to advance one side’s argument in a long-running debate over the Tyrannosaurus family tree that has divided paleontologists for decades. While some researchers have argued that small tyrannosaurus remains found in the fossil record were a separate species, Dr. Woodward and company say that their evidence reveals that these small tyrannosaurs were likely not fully grown.
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, Ill. 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.
Dr. 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 scrutinized the shapes and structures of the dinosaurs’ bones, the researchers looked inside of them.
They took chips of femur and tibia from both specimens, polished them down until they were less than a millimeter thick, and put them under a microscope.
Bone tissue is made up of small bundles of collagen fibers. The organization 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 fibers “look like pickup sticks,” said Dr. 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 “sub-adults 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, Wis., 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 Dr. Witmer, from the comfort and safety of 66 million years later.