Skeletal Similarities in Mammals

What a piece of work is the mammalian skeleton. I don't mean it is beautiful in itself, although I think it is. I mean the fact that we can talk about 'the' mammalian skeleton at all: the fact that such a complicatedly interlocking thing is so gloriously different across the mammals, in all its parts, while simultaneously being so obviously the same thing throughout the mammals. Our own skeleton is familiar enough to need no picture, but look at this skeleton of a bat. Isn't it fascinating how every bone has its own identifiable counterpart in the human skeleton? Identifiable, because of the order in which they join up to each other. Only the proportions are different. The bat's hands are hugely enlarged (relative to its total size, of course) but nobody could possibly miss the correspondence between our fingers and those long bones in the wings. The human hand and the bat hand are obviously - no sane person could deny it - two versions of the same thing. The technical term for this kind of sameness is 'homology'. The bat's flying wing and our grasping hand are 'homologous'. The hands of the shared ancestor - and the rest of the skeleton - were taken and pulled, or compressed, part by part, in different directions and by different amounts, along different descendant lineages.

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The skeletons of all mammals are identical, but their individual bones are different. The resolution of the paradox lies in my calculated usage of 'skeleton' for the assemblage of bones, in ordered attachment one to the other. The shapes of individual bones are not, on this view, properties of the 'skeleton' at all. 'Skeleton', in this special sense, ignores the shapes of individual bones, and is concerned only with the order in which they join up: 'bone to his bone' in the words of Ezekiel, and, more vividly, in the song that is based upon the passage:

Your toe bone connected to your foot bone,
Your foot bone connected to your ankle bone,
Your ankle bone connected to your leg bone,
Your leg bone connected to your knee bone,
Your knee bone connected to your thigh bone,
Your thigh bone connected to your hip bone,
Your hip bone connected to your back bone,
Your back bone connected to your shoulder bone,
Your shoulder bone connected to your neck bone,
Your neck bone connected to your head bone,
I hear the word of the Lord!

The point is that this song could apply to literally any mammal, indeed any land vertebrate, and in far more detail than these words suggest. For example your 'head bone', or skull, contains twenty-eight bones, mostly joined together in rigid 'sutures', but with one major moving bone (the lower jaw*). And the wonderful thing is that, give or take the odd bone here and there, the same set of twenty-eight bones, which can clearly be labelled with the same names, is found across all the mammals.

There are corresponding bones across species, evolved into other functions.