Tour of the Penguin Skeleton: The Phalanges
In this installment of our tour of the penguin skeleton, we will take a look at the phalanges. Phalanx (plural phalanges) is the word anatomists use for a finger bone. We have 14 total phalanges in our own hands, 3 in most fingers but just 2 in the thumb. You can tell where two of these bones meet because the connection forms a knuckle joint. Birds, of course, are not running around with visible fingers (not since the Cretaceous Period anyway), but they do have phalanges embedded within the wing. Most birds have one phalanx remaining in their first digit (equivalent to our “thumb”), two phalanges in their second digit (equivalent to our “pointer finger”), and one phalanx in their third digit (equivalent to our “middle finger”). These are of no use for manipulating objects, but do serve a purpose by anchoring some feathers. In particular, the phalanx of the “thumb”, in birds called the alular phalanx, is important because it anchors the alula, a special feather that is important in controlling flight speed during landings.
In penguins, the phalanges are weird. They look a bit like a normal birds phalanges got run over – this is part of a general pattern of flattening seen in the penguin wing skeleton, which makes it more flipper-like. The third digit is typically tiny in birds, but in penguins it is huge. The phalanx is long and tapers to a pointed tip, and has a sharp backward pointing projection. One of the really cool things about this bone is that you can trace its evolution in the fossil record. In more basal fossils penguins like Icadyptes, the phalanx is large compared to flying birds, but still much smaller than in living penguins, and also lacks the projection. Extending this bone to modern lengths happens about 30 million years into penguin evolution, and results in a slight decrease in aspect ratio of the flipper. Most importantly, penguins lack an alula altogether. This may be related to the lack of differentiation in penguin feathers. Certainly, they don’t need to “land” anymore so a special alula feather is probably superfluous. In the image below, you can see the major differences in shape between the wing of a shearwater (a flying relative of penguins) and an Emperor Penguin.