Are there irreconcilable differences between the skeletal digits of birds and the fossilized traces of dinosaur “hands”? It’s a complex issue, involving genetics and developmental biology as well as paleontology.
PALEONTOLOGISTS AND SOME ornithologists consider the three digits in a bird’s “hand” to be equivalent to a human’s thumb, index and middle finger (1-2-3). Other ornithologists and many developmental biologists consider the digits to be index, middle and ring fingers (2-3-4). If a coelurosaur’s hand is 1-2-3, and a bird’s hand is 2-3-4, you couldn’t go from one to the other.
However, what may be going on is a phenomenon known as frame shift, in which the genetic coding for one configuration is actually expressed as a different configuration. As a bird embryo develops, its digits are equivalent on a developmental system to 2-3-4, but in terms of what’s actually formed, the hand is structurally 1-2-3.
It’s a peculiar system, but it’s not unique to birds. Some evidence suggests this developmental phenomenon still operates in the reduced hand of the kiwi bird, which sometimes forms what look to be digits 1-2, and other times 2-3.