Why Did T Rex Have Such Tiny Arms?

Why Did T Rex Have Such Tiny Arms?

It’s easy to assume that every trait – including stubby arms on a terrifying predator – must be beneficial, but the forces of evolution don’t really work like that.

Thanks to the University of Minnesota for sponsoring this video!

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FYI: We try to leave jargon out of our videos, but if you want to learn more about this topic, here are some keywords to get your googling started:
Evolutionary Mechanisms: Forces that change the frequency of genes in populations.
Natural Selection: The process whereby traits that make animals more likely to survive and reproduce take hold in a population.
Vestigiality: The process by which a genetically-determined structure loses some or all of its functionality.
Genetic Drift: A change in the frequency of an allele due to random sampling.


Credits (and Twitter handles):
Script Writer: David Goldenberg (@dgoldenberg)
Script Editor: Kate Yoshida (@KateYoshida)
Video Illustrator: Qingyang Chen (@QCVisual)
Video Director: Emily Elert (@eelert)
Video Narrator: Kate Yoshida (@KateYoshida)
With Contributions From: Henry Reich, Alex Reich, Ever Salazar, Peter Reich
Music by: Nathaniel Schroeder:


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Switek, B. (2013). Paleontology: The Truth about the T. Rex 502 (424-426). Retrieved from:

E. Snively, A. P. Russell, G. L. Powell, J. M. Theodor & M.J.Ryan (2014). The role of the neck in the feeding behaviour of the Tyrannosauridae: inference based on kinematics and muscle function of extant avians. Journal of Zoology 292 (290-303). Retrieved from:

Middleton, K., and Gatesy, S. (2000) Theropod forelimb design and evolution. Zoological Journal of the Linnean Society 128 (149-187). Retrieved from:

Lynch, M., Ackerman, M., Gout, J., Long, H., Sung, W., Thomas, W., and Foster, P. (2016). Genetic drift, selection and the evolution of the mutation rate. Nature Reviews: Genetics 17 (704-715). Retrieved from:

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