Tour of the Penguin Skeleton IV: The Patella
One of the most difficult penguin bones to identify in isolation is the patella. This element looks like a misshapen cube, with one smooth surface and several rough faces. If found by itself outside a box of penguin bones, or eroding out of the surface in fossil form, it would be difficult to be sure that a patella was even a bone. The bizarre appearance of the patella is in part due to the fact that it is a sesamoid, or a bone that is embedded within a tendon. In life, the patella sits at the joint between the femur and tibiotarsus. One of its main functions is to help guide the tendon of the ambiens muscle, which either travels through a hole in the patella (in most extinct penguins and in the living stiff-tailed penguins of the genus Pygoscelis) or across a groove in the surface (in most living penguins).
Humans have a patella too, and it is sometimes referred to as the kneecap. This is a fairly apt name, as the bone looks somewhat like a smashed lid. It sits between the same two bones in humans (although we have a plain tibia, rather than a tibiotarsus). Presence or absence of a patella varies in birds – some families have a large patella like penguins, others have a very tiny version, and some have none at all. Perhaps the most interesting patella is that of the loon, which is very large and helps these birds with their unique kick-diving mode of locomotion.