'Teenage' T. rex fossils shed light on dinosaur's growth rate: Study

Researchers have settled a decades-long debate that two small fossils unearthed in the 2000s, with features like the predatory Tyrannosaurus rex, do not represent a separate genus, but are just T. rex "kids", a finding that sheds light on the early years of the world's most famous dinosaur.

According to the scientists, including those from Oklahoma State University in the US, while T. rex were typically 40-foot-long predators with bone crushing teeth, and a five-foot long head, the two teenage tyrannosaurs, named "Jane" and "Petey," were slightly taller than a draft horse and twice as long.

The study, published in the journal Science Advances, analysed for the first time thinly sliced bones from the two specimens and found that Jane and Petey were juveniles that had not yet experienced a major growth spurt before they died.

By counting the annual rings within the bone, like counting tree rings, the scientists found that Jane and Petey were 13 and 15 years old, respectively, when they died.

"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," said study co-author Holly Woodward from Oklahoma State University.

"So, for a long while we've had large gaps in our understanding of how dinosaurs grew up, and T. rex is no exception," She added.

The team's early assessment revealed that the teenage T. rex were growing as fast as modern-day warm-blooded animals such as mammals and birds.

Due to the smaller size of Jane and Petey, scientists can now study how the T. rex's bones and proportions changed as they matured, the study noted.

Analysing the fossil bone microstructure, the scientists said, can unravel new mysteries about juvenile growth rates across ages in the dinosaur.

"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," Woodward said.

Previously, paleontologists had speculated that the two small skeletons weren't T. rex at all, but a smaller pygmy relative Nanotyrannus.

In the current study, detailed microscopic analysis of the bone tissues led the researchers to conclude that the skeletons were juvenile T. rex, and not a new pygmy species.

Based on earlier studies, the scientists said, T. rex took up to twenty years to reach adult size, and with the current findings they reason that the dinosaur may have underwent drastic changes as it matured.

According to the scientists, juveniles such as Jane and Petey were fast, fleet footed, and had knife-like teeth for cutting, whereas adults were lumbering bone crushers.

Woodward's team also discovered that if T. rex had scarce food during a particular year, they didn't grow as much, and if food was plentiful, they grew a lot.

"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," Woodward explained.

According to the researchers, the study provides evidence that the world's most famous dinosaur assumed the crown of 'tyrant king' long before it reached adult size.