What Is a Furcula? The Bone That Connects a Roast Chicken to a Theropod
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The wishbone in a chicken is two collarbones fused into a single springy arch. The same bone is found in dinosaurs that never flew, which made it one of the pieces of evidence connecting birds to the rest of the group.
What it is
The furcula forms when the two clavicles, the bones running from the shoulder towards the midline, fuse at their lower ends into a single V-shaped or U-shaped element. It sits at the front of the chest and braces the shoulders apart against the pull of the flight muscles, and in birds it also works as a spring, flexing measurably during the wingbeat and storing and returning energy, which was demonstrated by filming birds in flight with markers on the bone. That springy function is real and is not why the bone exists, since it appears in animals that could not fly. In birds it also forms part of an air sac system, being hollow and connected to the respiratory apparatus in many species, which is another feature shared with the dinosaurs that preceded them.
The evidence it supplied
Its presence in non-flying dinosaurs mattered for a long-running argument:
- •The bird origin question turned partly on whether theropods possessed the features birds needed, and clavicles were once thought absent
- •That supposed absence was used as an argument against the bird connection through much of the twentieth century
- •Fused furculae were subsequently identified in a wide range of theropods, including large ones
- •Earlier specimens were re-examined and the bone found, having been overlooked, misidentified or discarded during preparation
- •The distribution across the group indicates the bone arose well before flight and served the shoulder rather than the wing
- •It joined feathers, air sacs and the wrist joint as features present before the function they are associated with
Exaptation
The pattern the bone illustrates is general and important. A structure that evolves for one purpose and is later put to another is described as having been co-opted, and the history of flight is full of such cases, since feathers are present in animals that could not fly and are associated with insulation and display, the specialised wrist joint that allows a wing to fold appears in ground-dwelling predators where it served prey capture, and the breathing system predates any need for the oxygen demands of flight. The general conclusion is that complex capabilities are assembled from parts that had other jobs rather than being built towards a goal, which answers the objection that an incomplete wing is useless, since the parts were never incomplete wings but functioning structures doing something else.
The rest of the shoulder
The bone sits within a shoulder girdle whose reorganisation was as important as any single element. In a typical reptile the shoulder is built for limbs held out to the side and for a sprawling gait, with the socket facing downward and outward. In theropods the socket rotated to face backward and slightly outward, which permits the arm to swing forward and back in a plane rather than rowing sideways, and that change is what made a flapping motion mechanically available long before anything flapped. The shoulder blade became long and narrow and lay against the ribcage. A separate bone bracing the shoulder against the breastbone completed the arrangement. In flying birds the breastbone itself developed a deep keel anchoring the flight muscles, which is the one major component that genuinely postdates flight and which is absent in the ground-dwelling relatives.
The custom
The practice of two people pulling the bone apart and granting a wish to whoever holds the larger piece is old and traceable. Roman writers refer to divination using the bones of birds, and the specific practice of breaking this bone is documented in Europe from the medieval period, with the English name recorded from the seventeenth century and the wishing association attaching later. Earlier names refer to the bone as a merrythought. Bird divination generally was widespread in the ancient world, with the flight and behaviour of birds read for omens, and using a bone from a bird for the same purpose fits that background. The modern version is entirely detached from any belief and survives as a table ritual, which is the usual fate of such practices, and it has kept the bone familiar to people who would otherwise never have heard of it.
The takeaway
Two collarbones fused into a springy arch that braces the shoulders and, in birds, flexes during the wingbeat and stores energy. Its supposed absence in theropods was used against the bird connection until it was found across the group, having been overlooked in earlier specimens. Like feathers and air sacs it existed before flight, which is how complex capabilities are assembled.