When studying comparative anatomy, one of the most basic skills is identifying homologous structures – the same bone or organ in different organisms. This can be tricky, as a structure may become hugely modified in the course of evolution as it is co-opted for new purposes. The bird’s wing provides a classic example.
Birds’ wings are modified forelimbs, and as such have the same basic components as any other tetrapod’s forelimb, including a wrist and hand. A typical tetrapod hand commonly has five bones called metacarpals, that articulate at the proximal ends with a series of small bones called the carpals (wrist bones). In birds there are only three metacarpals, a trait they inherited from their theropod dinosaur ancestors. Moreover, most of the bird carpals and the metacarpals are fused together into a single structure called the carpometacarpus.
At the top of the page is a partial carpometacarpus from the Pleistocene deposits at Diamond Valley Lake. This represents part of the proximal end (the wrist end) of a small right carpometacarpus (the small scale increments are millimeters). The long shaft is the 2nd metacarpal, the hand bone associated with the index finger. Even in such a small fragment several features are visible, such as those indicated below:
The projection marked “1” is the 1st metacarpal, the one associated with the thumb; in birds it it reduced and solidly fused to the carpals. The thumb bone, which is called the alula in birds and supports feathers on the leading edge of the wing, articulates at this point.
Number “2” is the broken base of the 3rd metacarpal, the one associated with the middle finger. Birds don’t have the 4th and 5th metacarpals, the ones associated with the ring and pinky fingers; these were lost in bird ancestors millions of years before the first birds appear in the fossil record.
Number “3” is called the intermetacarpal tuberosity; it’s a bony spur of metacarpal 2 that projects into the gap between metacarpals 2 and 3. In this specimen, the tuberosity is very large relative to the size of the metacarpal. That’s significant, because such a large tuberosity is generally only present in a group of birds called the Galliformes (turkeys, chickens, quails, and their relatives). The size and shape of this particular bone suggests that belongs to a New World quail from the genus Callipepla.
Callipepla is still a common bird in southern California, with two different species considered to be native to the area. Callipepla californica is California’s official state bird, and C. gambelii is also common in this area (below is C. gambelii from the North Carolina Zoo) . Two other species of Callipepla are found in the southwestern United States or northern Mexico but not currently in California. With such a fragmentary specimen, any of these closely related species are a possibilities for this Diamond Valley Lake specimen. We have a number of Callipepla bones in the collection, suggesting that, like today, quail were a common component of the Diamond Valley Lake fauna during the Pleistocene.