I can't believe Laelaps beat me to this (shows how on the ball he is) but he's just noted a paper that I watched getting written, and discussed in detail with Chris Glen, a very smart and talented young paleontologist, before I got to. So I will now, before he goes and does a better job.
Chris and his advisor Michael Bennett have come up with a possible way to test the "trees down or ground up" controversy about the origins of flight. That is, they have some independent evidence that early birds were basically ground dwellers, but that there was, as there is now, a mix of lifestyles - it doesn't follow that a bird today must always live in trees or on the ground. What is needed is a way to identify the actual lifestyles of modern birds, and apply that test to early birds.
So, Chris thought of using the morphology of the middle digit of the bird foot, and in particular, the curvature of the toe claw, as a guide. He studied some 3000 specimen bird claws, if memory serves, from 249 modern (Holocene) birds and identified those that were obligately ground dwelling, those that were obligately arboreal (never came down to the ground) and those that were mixed. It turns out that the lifestyle closely correlates with the degree of curvature of the claw. Hence, it is a reliable guide in early birds.
I just love this kind of scientific reasoning - at its best it puts Sherlock Holmes to shame, and I have seen a few cases first hand. What is really interesting about Chris's work is that it also has wider epistemological application: about how to make historical inferences of the past states of things, about how to test one theoretical issue with independent theory - Chris came up with a theory and tested it extensively.
I don't have Chris' larger essay, part of his PhD, to hand, but he has lovely diagrams that show how to measure curvature without being approximate or subjective - he uses actual anatomical anchor points that are common to all the species he is testing (I think that woodpeckers and ratites might be extremes; I'll have to ask Chris). Apart from anything else, he can draw like a... well, pro.
And the conclusion? Early birds exhibit the morphology of mostly ground dwellers. What that means for the flight issue is a whole nother matter. Watch this guy's work. I I have been privileged to have coffee and chat about this with Chris, and I know he's going to make some more contributions.
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A nice piece of work, but I don't know how much use it will be for the "trees down or ground up" controversy about the origins of flight.
As the paper states, claw curvatures of Mesozoic birds and closely related non-avian theropod dinosaurs, differ significantly from Holocene arboreal birds and more closely resemble those of Holocene ?ground-foraging? birds.
It is to be expected that early birds would have a claw curvature similar to the ancestral condition, as the high curvature of arboreal birds appears to be a derived character. It stands to reason therefore, that early arboreal birds would have the ancestral curvature, since, I assume, the ancestral curvature does not preclude an arboreal lifestyle.
Thus all early birds, whatever their lifestyle choice, would have the ancestral curvature, with arboreals subsequently evolving the high curvature condition.
I think that this is the issue, though. If, as one might expect, lifestyle is an immediate selection force on plastic traits like claw curvature (and if you spend much time in trees, you might expect that claw shape would immediately adapt), the lack of arboreal adaptations in Mesozoic birds indicates that they were cursorial (i.e., mostly ground dwellers). It is of course a primitive character, but that is because their ancestors were ground dwellers.
Plastic traits like tooth structure and claw shape are informative ecologically because they are plastic. They are uninformative phylogenetically for the same reason. So I think Chris's inference is pretty solid.
I'm not 100% sure of this, but foot bone curvature might even be plastic within the lifetime of the individual. Bone can remodel to quite an amazing extent if it's exposed to consistent forces.
As Chris explained it to me, the curvature is not bone, but the keratin sheath of the claw. The bone anchor is the anatomical point used to measure the claw.
Apparently these claws grow in a predictable manner, and are not individually affected except where the sheath is abraded by use. So the extent of the curve indicates the lifestyle of the individual, but the curvature is not.
Ahhh! Too many Chrises here! :-)
Hmmm . . . While I agree that some traits can be rapidly influenced by selection forces [quickly checks to see if Larry Moran is around], these tend to be directly related to survival - beaks on finches for example, where certain beak shapes help access food. But I don't think high curvature rates as one of those, since we are not dealing with high curvature as a prerequisite, or indeed a requirement, for an aboreal lifestyle.
A lot of organisms grip branches quite adequately without the aid of highly curved claws, heck even some kangaroos can climb trees! Having high curvature claws certainly provide grip, but they are not essential for grip.
The question is, how important is having high curved claws to survival in trees? If the answer is "not very" or "nice to have but not essential" then I don't think it follows that high curvature would develop rapidly enough to include the first aboreal birds.
I know similar data has been collected lizards, so does the change in claw curve in lizards mirror that of birds? Where does lizard claw height fit relative to bird claw height?
Has anyone looked at claw development in bird embryos? [quickly checks to see if PZ is around] If cursorial birds started to develop high curved claws and then 'flattened' them, that may provide evidence for an aboreal ancestry.
What an interesting idea. Cool research.
One nit. Your writeup seems to imply something quite like "correlation is causation":
It turns out that the lifestyle closely correlates with the degree of curvature of the claw. Hence, it is a reliable guide in early birds.
(emphasis mine)
I seem to be missing something there. Because in modern birds some single feature is correlated with being ground dwelling or tree dwelling, that's necessarily a guide to the habits of unobserved birds? That seems a little strong a claim to me (though I freely admit I'm no biologist, so feel free to tell me to shut the hell up).
In any case, it's certainly suggestive. Presumably there are more differences between arboreal and ground dwelling birds than claw-curvature that could also be examined. That would improve the claim.
What isn't obvious in my report or the paper is that Chris did a fair bit of work on whether there is a correlation for accidental or phylogenetic reasons or for ecological reasons, and the latter is the case. Since claw shape is causally active in the ability of birds to live in trees or on the ground (if the claw is not recurved, it cannot hold onto branches or trunks and must balance precariously, the way bush turkeys do in Australia), the correlation is a good bet. In any case, it applies in the modern birds, so we have every good reason to think it does in the Mesozoic.
Actually, most things that live in/climb trees have curves somewhere in their feet. Either claws, or more commonly, bones. Including some of the supposedly savanah-roaming hominid fossils... I imagine tree kangaroos have curved foot bones, but I've never seen a skeleton.
to answer Chris Nedin, there is some initial evo-devo type work done on claws and as for arboreality in the other animal groups you've mentioned, it's supportive of our general results (or they're animal groups we're working on... stay tuned). I'm also very interested in working out theoretical stuff in the style of D'arcy Thompson and Reidl, amazingly I think there's some key stones left unturned in their work here (and on one key point Thompson's plain wrong, and that's coming from a fan of the guy).
to efrique, I take your point about correlations - and was aware of it. We have some further work to bolster our take
on the correltion... but paranoia prevents me from saying much further!
PS, thanks John...