Hold your arms out with your palm oriented vertically, as if you were trying to shake someone's hand. Now without moving your forearm, bend your hand downwards towards the floor. Unless you are freakishly flexible, you will only have managed to a measly acute angle. But if you were a bird, you could bend your wrist so that your hand pointed back towards your body. These incredibly flexible wrists allow birds to fold their wings and they help with flying. And many dinosaurs could do something similar.
Many older depictions of small raptors, including the Jurassic Park films, have them holding their arms in a zombie-like stance - arms out at the front and hands palms-down. More recently, artists have portrayed them with more bird-like postures, with their hands bent back towards the forearm. How far the hand could actually bend has been an open question, so Corwin Sullivan from the Chinese Academy of Sciences decided to answer it by piecing together the evolution of the raptor wrist together with Dave Hone (who blogs at Archosaur Musings), Xing Xu and Fucheng Zhang,
Sullivan examined the hands of several dinosaurs, from large hunters like Allosaurus, to smaller, more bird-like species like CaudipteryxandDeinonychus, to living examples like the turkey. He showed that the asymmetric wrists first appeared in this dynasty of predators. They gradually became increasingly asymmetric and backward-bending, culminating in the flexible versions of early birds. This is particularly a tale of two wrist bones - the radiale, which became increasingly wedge-like, and the semilunate carpal, which developed a rounded, convex dip.
These changes to the raptor wrist were already well underway before the group developed powered flight and possibly even before the evolution of long feathers on the arms. For the moment, it's not clear what advantage the dinosaurs would have gained from their slightly more flexible wrists.
Some scientists have suggested that it was an adaptation for hunting but Hone says, "It's hard to see how [that] could have worked. Certainly we still see the wrist change occurring in herbivorous and omnivorous lineages like oviraptorosaurs and ornithomimids, which doesn't really help the 'predatory' angle."
The later benefits are clearer, especially in smaller dinosaurs with arms covered in long feathers. In most species, these almost-wings were totally incapable of flight. They were probably used for display, or other roles like keeping warm or sheltering eggs. In any case, Sullivan thinks that the bendy wrists were adaptations that allowed the dinosaurs to protect these precious plumes by holding them off the ground or preventing them from snagging on vegetation. What's less clear is whether the longer feathers fuelled the evolution of the more flexible wrists or if the latter created an opportunity to develop the former.
Either way, birds inherited these adaptations and put them to another purpose entirely - flying. After the powerful downward flap, birds fold the wing slightly inwards during the upstroke. This reduces the drag on the wing and it would be impossible without the anatomical heirlooms they inherited from their dinosaur forebears.
Understanding the evolution of the dinosaur wrist is an important part of piecing together the origins of birds and the evolution of flight. "This seems to be a much under-studied area," says Hone. "We can now see that this bird-like wrist started to evolve well before birds took to the air, and it opens up the possibility that feathers were more important to some dinosaurs than perhaps previously thought."
Reference: Sullivan, C., Hone, D., Xu, X., & Zhang, F. (2010). The asymmetry of the carpal joint and the evolution of wing folding in maniraptoran theropod dinosaurs Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.2281
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