Costa
Flat bone. Part of the axial skeleton, belonging to the central thoracic cage.
Also known as: Rib
Classification
Flat bone. Part of the axial skeleton, belonging to the central thoracic cage.
Dimensions
In mammals, ribbons of bone vary from short and relatively straight at the top of the chest to long, sweeping curves in the middle. Most are flat and thin, with the width usually being several times the thickness.
Key Features
Often shaped like a boomerang or a C-shaped bow. Look for a 'head' at the upper end featuring small, smooth faces where it meets the spine. Below that, a slight bump or neck usually transitions into a long, flattened shaft. The inner surface often has a shallow groove running along the bottom edge, resembling a small gutter.
Similar Bones
Can be confused with a collarbone (clavicle), but ribs are generally flatter and thinner. While a collarbone has a distinct S-curve when viewed from above, a rib typically has a single, continuous sweeping curve. Curved pieces of a broken pelvis might look similar, but they will usually lack the specific smooth joint surfaces found on the upper end of a rib.
Across Species
The rib cage is a masterclass in adaptation. In tortoises and turtles, the ribs are broad and fused together to form the inner layer of the bony shell. In snakes, ribs are numerous and highly mobile, providing the structural support for their undulating movement across the ground. Birds possess small, backward-pointing hooks on their ribs called uncinate processes, which help strengthen the chest cage during the intense muscular activity of flight.
Evolutionary History
Ribs first appeared in early fish as small protective structures for the internal organs. As early tetrapods moved onto land, ribs became sturdier and longer to support the torso against the force of gravity and to facilitate breathing through expansion. Over millions of years, the variety of rib arrangements has diversified to support specialized lifestyles, from the deep-diving barrels of whales to the flexible cages of burrowing mammals.
Photography Tips
Lay the bone flat on a plain, non-reflective background to show the full curve. Take one photo directly from above to capture the overall bow shape, and a second close-up of the end with the bumps (the head) to show the joint surfaces. Including a common object like a coin or a ruler helps viewers understand the scale of the animal it came from.