Tyrannosaurus Rex (T. Rex), the legendary 40-foot-long predator with bone-crushing teeth inside a five-foot long head, had equally dangerous teens who were not a separate species but kids, a team of researchers has revealed, thus settling a long-standing debate about whether small T. Rex specimens represent a separate genus or rather just "kids" of their kind.
T. Rex specimens at the Burpee Museum of Natural History in Illinois were juveniles that had not yet experienced a major growth spurt before they died, said the authors after analyzing the bones of mid-sized T Rex who were slightly taller than a draft horse and twice as long.
Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others, said Holly Woodward from Oklahoma State University Center for Health Sciences who led the study.
"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," Woodward added.
In the early 2000s, the fossil skeletons of two comparatively small T. Rex were collected from Carter County, Montana and were nicknamed "Jane" and "Petey".
Since it took T. Rex up to 20 years to reach adult size, the tyrant king probably underwent drastic changes as it matured.
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 discovered that growing T. Rex could do a neat trick: if its food source was scarce during a particular year, it just didn't grow as much. And if food was plentiful, it 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 was inconsistent," elaborated Woodward.
Jane and Petey were teenaged T. Rex, 13 and 15 years old, respectively, when they died.
The smaller size of Jane and Petey is what make them so incredibly important.
Not only can scientists now study how the bones and proportions changed as T. Rex matured, but they can also utilize paleohistology-- the study of fossil bone microstructure-- to learn about juvenile growth rates and ages.
"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," Woodward said in a paper appeared in the journal Science Advances.
"By comparing these fossilized microstructures to similar features found in modern bone, we know they provide clues to metabolism, growth rate, and age".
The team determined that the small T. rex were growing as fast as modern-day warm-blooded animals such as mammals and birds.
T. Rex fossils are found in a variety of rock formations dating to the Maastrichtian age of the upper Cretaceous Period, 68 to 66 million years ago.
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