Tyrannosaur arms were still functional and dangerous

New findings suggest the T-rex dinosaur’s arms were strong enough to slash prey at close quarters, despite being small for its body size.

Many paleontologists have viewed the small arms of T. rex as having been vestigial. At ~1m long, these arms were not as tiny as often portrayed, and derived traits indicate that they were actually functional. The few previous suggestions of possible functions for the arms are all problematical. Six of the arms’ derived traits indicate that they were adapted for slashing at close quarters:
(1) The shortness of the arms would actually have been advantageous for this activity.
(2) A large coracoid indicates that the arms were very strong: not only slightly longer than the leg of a six-foot man but also of similar girth.
(3) The arm bones were quite robust and would readily have sustained the impact of slashing.
(4) The unusual reduction of the number of fingers from three to two would have resulted in 50% more pressure being applied to each claw.
(5) The humoral head was part of an unusual quasi-ball-and-socket joint that would have provided considerable mobility for slashing.
(6) The huge (8-10cm-long) sickle-shaped claws would have caused deep wounds.

Its short, strong forelimbs and large claws would have permitted T. rex, whether mounted on a victim’s back or grasping it with its jaws, to inflict four gashes a meter or more long and several centimeters deep within a few seconds — and it could have repeated this multiple times in rapid succession. Infliction of damage by slashing was widespread among other theropod taxa, so in light of its formidable weaponry, why should T. rex not have engaged in this activity?

Tyrannosaur ancestors used long arms primarily for grasping. These atrophied during the evolution that led to the tyrannosaurids because the jaws took over their grasping function. No longer being selected for, the arms were selected against: the expansion of the head deprived them of nutrition in a zero-sum game. Then, as the arms approached their final size, natural selection kicked in opportunistically and put them to good use for slashing at close quarters.

7 thoughts on “Tyrannosaur arms were still functional and dangerous”

  1. If they had a role in grabbing prey or mates, the arms would indeed not be vestigial but strongly selected. Vestigial limbs are rather rare transitional occurrences, because something either helps and is therefore strongly selected or it doesn’t and it’s strongly discouraged.

    The biggest apparent exception are purely sexual display attributes, but the cost of such things is not always obvious, while the benefit (displaying health and a good genome at a single glance) is clear.

    • So, if I get where you are going with this… the T Rex would flex his arm muscles to impress the ladies.

      But it didn’t work, and that’s why they are extinct.

  2. Ok First the fact that you picked two trex mating is hilarious.

    And really this makes sense. Look at a lions kill. The head holds the prey still while the claws keep a tight hold. Nothing is getting out of a trex jaw. However this way even if the prey was at a bad angle the claws could have eviscerated the prey.

    Though I do wonder. Did Trex have longer arms proportionately to their body when they were younger and they decreased in size as they aged/ It would really help a Juvenile to be able to jump on prey then slash till it died.

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