Solving the Mystery of Why Tyrannosaurus Rex Had Such Tiny Arms.

2 Comments

  1. Henry Sharpe
    Henry SharpeMay 20, 2018 at 11:46 pm

    Interesting hypothesis, but I think there are a few factors that may render it less than likely. Firstly, for an organism to develop a trait, there has to be significant pressure. If the arms were small to keep them away from Triceratops, we should have evidence of Tyrannosaurs with not only larger arms, but large arms with numerous pathologies. Also, it should be worth noting that Tyrannosauroids had already developed reduced arms before they began hunting Triceratops-level ceratopsians. Yutyrannus begins to display this condition, despite the fact that it lived with ceratopsians barely the size of a dog. In addition, Triceratops is one of, if not the only ceratopsian for which a defensive horn arrangement has been suggested. The horns of Campanian centrosaurines and chasmosaurines are not well suited to defence (some lacking large horns altogether), yet the tyrannosaurids of this time had extremely reduced forelimbs. Just some food for thought

    1. Max Hawthorne
      Max HawthorneMay 21, 2018 at 9:25 am

      Thank you, Henry. Given the incredibly limited fossil material (fragments) of primitive tyrannosaurs, I think you’ll agree that the pathology point is moot. IN regard to your second point (and forgetting walking Campanian “rhinos” like Styracosaurus) let’s keep in mind that being gored wasn’t the only threat to a tyrannosaur, when facing an irate ceratopsian. Any animal will bite when attacked, and the powerful jaws of a Chasmosaurus or even a Pachyrhinosaurus could easily shear off a smallish front limb, or mangle it badly. Worse, it could be used to drag the tyrannosaur to the ground, where it was extremely vulnerable. (think a stand-up boxer stuck in the UFC). In fact, I should think the tendency to grapple head-on with a ceratopsian sans lethal horns would be more likely, thus leaving the theropod even more vulnerable to an arm bite and subsequent take-down. Hence, the theory is sound. The smaller, more retracted arms made any Tyrannosaur less vulnerable to such defensive strategies, and would cause shorter-armed examples to survive better, passing on their genes, and contributing to the continuance of a short-limbed/large-headed species. Thanks so much. Best, Max

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