I am doing the skeletal structure in Art, and I have been researching the foot and the hand. In the thumb and the big toe there isn't a middle phalange, and that is basically my question, why don't they have a middle phalange? I have been trying to find the answer everywhere and I just can't find it! It's not something that I desperately need to know for the work persay, but it is annoying me :')

I would really appreciate it if you answered quickly!!!

Thank you :)

Simply, evolutionary history.  The 2-3-3-3-3 pharangeal formula goes back pretty far evolutionarily (I think right back to basal mammalia). It's easier to lose a joint than to add one, so lineages tend to see joint reduction, but then rarely (if ever?) joint increase.

If we go back even further to basal reptiles, the forumal is 2-3-4-4-5.

[side note - apparently the forumla of cetaceans (marine mammals) is 2-12-8-1. No doubt the lack of weight bearing and and the strong selective pressure for increased surface area allowed an increase in phalanges]



Why did it arise in the first place?  Not sure.  If I were guessing I'd say those inside digits (thumb and big toe) were the most weight bearing (which is still the case), and less joints = more stability in the weight bearing digits.  But that is just speculation.