The iguanodontian opposable digit 5

It seems to be common knowledge that many iguanodontids had a spike-like thumb that consisted of just a single conical ungual. What is less well known is that the 5th digit in the hand (the little, or pinky, finger) was also modified. You can see it here on this cast of an Igunaodon hand. The 5th finger is smaller and separated from digits 2-4 which are appressed together and obviously bear the brunt of the weight when the animal walks quadrupedally. This offset angle allows the 5th digit to act something like a thumb – it’s a semi-opposable digit that can be used to grasp and hold things. The thumb might get all the fame, but selection has been acting on the whole hand to produce three distinct morpholgies and functional outputs. That’s pretty good for a manus in what is, effectively, a quadrupedal herbivore. Cows don’t do quite so well in comparison.

2 Responses to “The iguanodontian opposable digit 5”


  1. 1 mattvr 12/11/2010 at 9:39 am

    Any idea how much mobility are we talking about Dave?

    • 2 David Hone 12/11/2010 at 10:19 am

      Not really, or at least I don’t. I have seen reconstructions where it is held right across the palm, but I don’t know if it can actually do thast.


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