Researchers learn more about teen-age T.Rex
A study of two juvenile T.rex skeletons show how the large predator grew up
Without a doubt, Tyrannosaurus rex is the most famous dinosaur in the world. The 40-foot-long predator with bone crushing teeth inside a five-foot long head are the stuff of legend. Now, a look within the bones of two mid-sized, immature T. rex allow scientists to learn about the tyrant king's terrible teens as well.
In the early 2000s, the fossil skeletons of two comparatively small T. rex were collected from Carter County, Montana, by Burpee Museum of Natural History in Rockford, Illinois. Nicknamed "Jane" and "Petey," the tyrannosaurs would have been slightly taller than a draft horse and twice as long.
The team led by Holly Woodward, Ph.D., from Oklahoma State University Center for Health Sciences studied Jane and Petey to better understand T. rex life history.
The study "Growing up Tyrannosaurus rex: histology refutes pygmy 'Nanotyrannus' and supports ontogenetic niche partitioning in juvenile Tyrannosaurus" appears in the peer-reviewed journal Science Advances.
Co-authors include Jack Horner, presidential fellow at Chapman University; Nathan Myhrvold, founder and CEO of Intellectual Ventures; Katie Tremaine, graduate student at Montana State University; Scott Williams, paleontology lab and field specialist at Museum of the Rockies; and Lindsay Zanno, division head of paleontology at the North Carolina Museum of Natural Sciences. Supplemental histological work was conducted at the Diane Gabriel Histology Labs at Museum of the Rockies/Montana State University.
"Historically, many museums would collect the biggest, most impressive fossils of a dinosaur species for display and ignore the others," said Woodward. "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."
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. Woodward and her team removed thin slices from the leg bones of Jane and Petey and examined them at high magnification.
"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. "And 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. Woodward and her colleagues also found that by counting the annual rings within the bone, much like counting tree rings, Jane and Petey were teenaged T.rex when they died; 13 and 15 years old, respectively.
There had been speculation that the two small skeletons weren't T. rex at all, but a smaller pygmy relative Nanotyrannus. Study of the bones using histology led the researchers to the conclusion that the skeletons were juvenile T. rex and not a new pygmy species.
Instead, Woodward points out, because it took T. rex up to twenty 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. Not only that, but 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 is inconsistent - some years the spacing is close together, and other years it's spread apart," said Woodward.
The research by Woodward and her team writes a new chapter in the early years of the world's most famous dinosaur, providing evidence that it assumed the crown of tyrant king long before it reached adult size.
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NEWS RELEASE
Bone analysis suggests small T. rexes were not a separate genus; they were kids
Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy 'Nanotyrannus' and supports ontogenetic niche partitioning in juvenile Tyrannosaurus
Settling a decades-long debate about whether small Tyrannosaurus rex specimens represent a separate genus or rather just "kids" of their kind, a new examination of thinly sliced bones from two specimens at the Burpee Museum of Natural History in Illinois suggests the latter. The specimens were juveniles that had not yet experienced a major growth spurt before they died, the authors say. "That's even cooler [than their being a separate genus,]" said co-author Scott Williams in a related video," because that tells us they go through a drastic change when they grow up from these sleek, slender, fleet-footed T. rexes with these wonderful knife-like teeth to these big, monster, plodding crushing tyrannosaurs that we are familiar with. It also tells us these animals probably dominated their ecosystems at all ages," even as juveniles. The results support the hypothesis that T. rex experienced a period of exponential growth late in their development. They also support that a skull specimen at the Cleveland Museum of Natural History, which was classified as a separate genus in 1988 (Nanotyrannus), is actually a young T. rex. Although most specialists now reject the idea that the specimen belongs to a separate classification, previous studies used characteristics of one of the Burpee Museum skulls to justify the classification. To assess the age and growth rate of the T. rex specimens, Holly Woodward et al. compared the organization of bone fibers and other microstructures in the two Burpee specimens, finding that they appeared to have been growing, as evidenced by growth rings in the bone in a spaced-out pattern not typically seen in adults. The bones also lacked the closely spaced series of lines present in adults that signals growth is complete. The researchers estimated the specimens' ages at the time of death by counting their cyclical growth marks, a series of lines in the femur and tibia that, like tree rings, that record periods of development. Woodward and colleagues suggest reaching full size after a period of prolonged adolescence may have meant that juveniles and adults fulfilled different roles in the ecosystem, such as feeding on different prey.
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