One thing worth bringing up on the wrists front never quite made it into the paper (or even the supplementary material) in quite the way I would have liked. It gets a mention but not the emphasis required at least in terms of palaeoart but also in terms of the actual biology of the animal in hand (since there is only limited space and you can’t really go off on a tangent for a few hundred words even if you want to).
Microraptor, as we know, has very long arm feathers, far longer than anything else we looked at in fact. Those keeping up will also note that it has really rather strongly rooted feathers and thus, what you see in the holotype is what you get. Take another look at that and you can see that the big flight feathers on the hand are stick out at a fair angle from the hand, but still act as a kind of angled extension to the arm. They were also probably rooted firmly in place and could not move much, even if the animal wanted to move them. But what does this mean?
Well, if you take a look at most palaeoart of Microraptor and even academic reconstructions and drawings of the animal in published papers, people seem to have been cheating. Not in a dishonest sense I hasten to add, more that, whether they meant to or not, the image just does not match the anatomy of the specimen. I’ve even checked a few of these and the feathers are either not at the right angle to the hand (which would be fixed), or are much shorter than as are preserved. The feathers are so long, and indeed the arms are so long that, [important note looming] assuming the arms are held such that the palms face inwards (the ‘clapper’ position) the feathers will be stuck in the ground. They can’t not be, they are just too long and can’t be moved out of the way. Microraptor has a serious feather issue.
Even with a flexed wrist, Microraptor's feathers would be stuck in the ground. Even if the arm were held out straight and parallel to the ground, the problem would remain. Image from Sullivan et al., 2010.
There are of course solutions to this problem and we illustrate and describe them. First of all, if you pronate the hands a little (i.e. rotate the hands inwards towards a ‘slapper’ position) then of course the feathers will now point at least partially to the side rather than down, and while this makes them stick out awkwardly, they won’t drag on the ground. Secondly, if you raise the arm right back so that the humerus is parallel to the spine, and then crook the elbow and / or the wrist a lot, you can get the feathers to point backwards or even up. The arm is in an odd looking position and might even get in the way of the legs (though of course birds, with an ever more abducted wrist, pull this off quite neatly) but again the feathers are at least clear of the ground.
Bringing the humerus back to be subparallel to the spine and then bending the elbow and the wrist finally brings the feathers free of the ground. Image from Sullivan et al., 2010.
Now Microraptor has some really big feather and clearly led a rather unusual life. Most, if not all of these things would be unnecessary for pretty much every other feathered dinosaur, but the general point is important. Feathers (whatever their purpose) were very important for the animals that had them, and as we can see from the fossils, they evolved to have lots of them and big one, and were kept in good condition. You don’t see dromaeosaurs or oviraptorosaurs with frayed feather tips and ragged edges, they looked after them and this includes not scragging them on vegetation or abrading them on the ground. Whatever else Microraptor was doing, it was looking after its feathers. This means it did not hold its arms in a way that would have led to profound damage to the feathers (or may just have even been physically impossible if those feather shafts were especially inflexible), and thus one of these, admittedly unusual, postures must have been adopted. You can’t manipulate the anatomy and you can’t contradict the evidence, so anyone thinking of drawing Microraptor in an old-style ‘Velociraptor’ pose with the hands held in front of the animal below the head needs to think again.
Biologically this is also interesting and potentially important. After all if the hands of Microraptor (or any similarly feathered dinosaur) tried to bring its hands to its mouth while holding something or to strike at prey in front of it, then it won’t be able to without dragging its feathers across the floor. This puts rather a limit on what the arms can actually do, especially when brought forwards. This isn’t necessarily and issue for the animal, such a penalty would clearly be a trade off against having those enormous flight feathers. But it does mean that if we are to successfully consider and test ideas about how the hands and arms of maniraptorans evolved and were used, we have to take such issues into account and not consider the bones in isolation – those feathers were important too.
(A few extra caveats / details. Yes, feather shafts are at least a bit flexible, and could probably be crushed against the ground without breaking them or doing too much damage, and yes lots of preening would help keep them in good shape. However, doing this every time the animal wanted to bring the arms forwards would in the long term risk damaging them, especially if done often, and preening also brings its own costs in terms of time and effort. On balance, it is far easier to just keep the feathers off the ground the bend shafts, ‘unzip’ barbs, and break tips that it is to go through that rigmarole and risk constantly. It’s also possible that the feathers were less firmly fixed / more motile than in modern birds, but they do have to take the forces of flight [presumably] and we do see things like quill knobs that argues for a very secure and fixed attachment).
I guess I should add, in the best traditions of SV-POW! and Tet Zoo, massive ‘Musings bonus points’ will be available to anyone who can find a ‘correct’ Microraptor online or produce one themselves.