Costa
Flat bone; part of the axial (core) skeleton
Also known as: Rib bone, rib cage bone
Classification
Flat bone; part of the axial (core) skeleton
Dimensions
Typically long, slender, and curved, with a length many times its width. Proportions vary depending on its position in the torso, ranging from short and highly curved near the top to long and sweeping in the middle.
Key Features
The most distinctive feature is its long, elegant curve, resembling a bow. One end typically has a rounded 'head' with small flat surfaces where it meets the backbone, while the other end is often flatter where it transitions toward the chest center. Look for a shallow groove running along the inside of the bottom edge, which in life houses soft tissues.
Similar Bones
A rib can sometimes be confused with a collarbone (clavicle), but a collarbone usually has a more distinct 'S' shape rather than a simple arc. It might also be mistaken for a large jawbone fragment, but ribs lack the sockets or specialized surfaces for teeth. Small ribs can also look like the 'wishbone' (furcula) of a bird, though the wishbone is joined at the center while ribs are individual paired sticks.
Across Species
In the animal kingdom, ribs serve as a protective cage for vital organs. While humans typically have 12 pairs, snakes can have hundreds, providing the structural support needed for their undulating movement. In turtles, the ribs have evolved into wide, flat plates that fuse together to form the hard upper shell, or carapace. Marine mammals like whales often have ribs that are not firmly attached to their breastbone, allowing their chests to compress safely during deep dives.
Evolutionary History
Ribs first appeared in early jawed fish as small structures protecting the body cavity. As vertebrates moved onto land, ribs became sturdier to support the weight of the body against gravity and to facilitate breathing. Over millions of years, the connection points between the ribs and the backbone became more complex, allowing for the rhythmic expansion and contraction of the chest seen in modern mammals and birds.
Photography Tips
Lay the bone flat on a plain, contrasting background to capture its full arc. Take one photo from directly above to show the curve, and another close-up of the thicker end (the head) where it attaches to the spine. Including a common object like a coin or a ruler helps show the specimen's true size.