Costa
Flat bone; part of the axial (core) skeleton
Also known as: Rib
Classification
Flat bone; part of the axial (core) skeleton
Dimensions
These vary greatly in length depending on the species and their position in the body. They are generally much longer than they are wide, following a curved, slender arc like a bow or a crescent moon.
Key Features
Look for a long, thin, curved shape. One end usually has a small, rounded 'head' for connecting to the spine, followed by a slight neck and a bump where it meets the back. The main body of the bone is flattened and often has a shallow groove running along the inner bottom edge.
Similar Bones
Beginners may confuse a rib with a clavicle (collarbone). A rib is typically more uniformly curved and flatter, whereas a collarbone often has a distinct 'S' shape and thicker, more blocky ends. Small ribs can also be mistaken for fish spines or long, thin teeth, but ribs lack the enamel coating found on teeth.
Across Species
In mammals, these bones form a protective cage for vital organs. Whales have very flexible connections to allow their chest to change shape during deep dives. Birds have additional small spikes called uncinate processes on their ribs to help stabilize the body during flight. In snakes, ribs are nearly the entire skeleton, providing the structural framework for locomotion.
Evolutionary History
Ribs first appeared in early fish as small protective structures. Over millions of years, as animals moved onto land, these bones lengthened and curved to support the body's weight against gravity and to facilitate more efficient breathing. Some ancient reptiles even used specialized ribs to support wing-like membranes for gliding.
Photography Tips
To help with identification, photograph the bone lying flat against a plain background with a coin or ruler for scale. Take one shot looking straight down and another from the side to show the degree of curvature and the shape of the ends.