As the title has already given away, I’m going to talk about mammals a bit on here for once (well, OK, they have cropped up occasionally before) but as a set-up to a point about convergence and evaluating evidence so it will hopefully be instructive and not overly extant mammal-y for those who like their extinct archosaurs. So onto, as also already given away by the title, spines in mammals.
Not backbone-spine, but spikey-spines such as those of hedgehogs and porcupines. While both these examples are likely familiar, there are plenty more like them. Tenrecs and spiny mice for starters, plus of course echidnas so these have cropped up six times at least that I can think of (twice for the spiny mice as several mice called ‘spiny mice’ have this) and there’s likely another one or two that I have missed. Importantly these six different acquisitions are independent of each other – each evolved separately from the existing hair. The presence of these spines in the various clades are therefore an example of convergence – independent evolution of a characteristic. Six times is quite a number really and as I note there may be a few more floating around that I have missed or have yet to be discovered (it’s not hard to imagine a new rat or mouse being discovered with spines of some kind) and of course there are plenty of extinct fossil lineages for which we have little or no information on their integument and thus might have also borne spines of some kind or another. It you want to stretch things a bit there are also plenty more mammals with less than furry fur, and while not exactly spiny, peccaries, platypus and anteaters are all quite spiky.
So onto the point about convergence. Convergence is a simple fact of evolution – sometimes (or perhaps that should be often) two different organisms independently evolve the same general solution to a problem. If you want to drink nectar from long flowers, you really need a long tongue. If you want to open up a termite nest, you’ll probably be wanting big claws. And if you want to grip things an opposable thumb of some sort is really going to help.
Of course what this means at a practical level for taxonomists and systematicists is that there are lots of features out there that can mislead – just because two organisms share a feature (like spines) does not necessarily mean that they are close relatives. Hence the point about evaluating evidence (which of course applies to science as a whole and not just taxonomy, but it is I think, especially relevant here) – all the evidence must be assessed and not just one, or a few, features. Looking at porcupines, spiny mice and echidnas it should be pretty clear that they don’t have much in common *apart* for the normal mammalian characteristics and the spines. The teeth, skull shapes, posture and so on are all pretty different (and let’s not start on echidnas, you know, laying eggs) and once the weight of evidence is brought to bear their differences trump their similarities and their true relationships become clear.
Take the whole of the evidence available and not just one subset or you run the risk of falling foul of convergence and being fooled into assuming that just a few characters define relationships when they do not.
I’m away for the next few days so not much going on here till next weekend I suspect.
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Dave Hone's Archosaur Musings 
You can count “porcupines” twice as well (Erethizontidae and Hystricidae). Actually within rodents alone in addition to your two “spiny mice” (Acomys and Heteromys ?) and the aforementioned porcupines there are at least two distinct types of “spiny rats” (Echimyidae and Maxomys) and the African ground squirrels (Xerinae) although whether they are really “spiny” in the same way that hedgehogs or porcupines are might be debatable. So that might be six or seven right there. I’m not certain that all have evolved spines independently but I suspect that at least most of them have.
Plus there are some other nominative spiny mammals – spiny bandicoots, a spiny shrew etc. – though I’m uncertain exactly how well developed the spines are in these.
Which raises an interesting question, why aren’t there any spiny birds? Or, are there?
Thanks for this Neil – I am rather lacking in my knowledge of small mammals as this rather demonstrates, though of course handily it also makes my point rather well!
@Neil: All birds are spiny! They just have stuff along the spines (=quills) as well.
I was about to say that it’s interesting that marsupials don’t seem to have evolved a spiny species, but what’s this about spiny bandicoots?
Well Mike I’m not ure kiwis or cassowaries are quite as spiky as hedgehogs etc. – the rachis on their feathers is pretty soft and flexible. But yes, typically that is a very stiff shaft they have going on there.