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How Little Tyrants Grew Up

A new study finds that Tyrannosaurus truly had "thunder thighs." Juveniles were likely more agile than adults

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Virtual, fleshed-out models of the Tyrannosaurus specimens "Sue" (left) and "Jane" (center) compared to a human. Image by Julia Molnar.

I think science writer Alexandra Witze put it best in her succinct summary on Twitter yesterday: “T. rex = thunderthighs.” No, she wasn’t suggesting that the mighty sauropod Brontomerus (translated: “thunder thighs”) is synonymous with Tyrannosaurusthe current trend of lumping multiple dinosaur genera and species into growth series of single taxa has not gone that far yet—but was instead referring to some of the conclusions presented in a new PLoS One paper by paleontologists John Hutchinson, Karl Bates, Julia Molnar, Vivian Allen and  Peter Makovicky.

As Hutchinson and colleagues point out, Tyrannosaurus has rapidly become “an exemplar taxon for palaeobiological studies” because the Cretaceous predator is big, popular and known from a number of relatively complete specimens from various growth stages. We have a better fossil dataset for Tyrannosaurus than many other giant theropod dinosaurs. In the case of this study, the abundance of specimens allowed Hutchinson and co-authors to virtually reconstruct the body of the animal during juvenile and adult growth stages. Based on skeletons alone, paleontologists have recognized that Tyrannosaurus was rapidly transformed from a lanky juvenile to a bulky adult, but the point of the new study was to investigate how these changes affected the way the dinosaur moved as its body proportions changed.

Through examination of the reconstructed skeletons and virtual models, Hutchinson and collaborators found that as they grew from juveniles to adults, Tyrannosaurus‘ arms became lighter; the animal’s torso increased in length and weight, and the dinosaur’s thighs became heavier despite the hindlimbs becoming lighter overall. This doesn’t mean that the dinosaurs were svelte, though—in the models, the four adult Tyrannosaurus specimens were estimated to weigh more than six metric tons, with the famous specimen “Sue” being more than nine tons. Tyrannosaurus was a heavy-hitter. The authors recognized that subjective differences and the incomplete nature of some of the specimens might have produced some errors in their models, but overall, the trends seen in the skeletons and the virtual, fleshy dinosaurs were in accord.

The changes in body shape and mass would have undoubtedly influenced the way Tyrannosaurus moved, but exactly how these changes would have manifested themselves in terms of locomotion is unclear. A few inferences about how fast Tyrannosaurus could move can be made, though. For example, Hutchinson and co-authors were not able to confirm a recently-published study suggesting that one of the large muscles that attached from the dinosaur’s tail to the upper part of the femur increased in size as Tyrannosaurus grew. The opposite actually appeared to be the case, and a decrease in the size of this muscle may have adversely affected the running ability of Tyrannosaurus. Juveniles were likely more agile animals. At the same time, the virtually restored Tyrannosaurus specimens had massive hip and thigh muscles that were as large as, if not larger than, those in any living animal. So even if the dinosaur had less “junk in the trunk” we could still call it “thunder thighs.” Just make sure you’re at a safe distance when you do that, should you ever happen across a Tyrannosaurus.

For additional details, see the official press release from the Field Museum and the open-access paper, referenced below.

References:

Hutchinson, J., Bates, K., Molnar, J., Allen, V., & Makovicky, P. (2011). A Computational Analysis of Limb and Body Dimensions in Tyrannosaurus rex with Implications for Locomotion, Ontogeny, and Growth PLoS ONE, 6 (10) DOI: 10.1371/journal.pone.0026037

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About Brian Switek
Brian Switek

Brian Switek is a freelance science writer specializing in evolution, paleontology, and natural history. He writes regularly for National Geographic's Phenomena blog as Laelaps.

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