Occipital bone (part of a skull)
Flat and Irregular bone; part of the axial (core) skeleton
Also known as: Back of the skull, braincase fragment, base of the head
Classification
Flat and Irregular bone; part of the axial (core) skeleton
Dimensions
In many mammals, this bone forms a curved, bowl-like plate at the base of the skull, often featuring a large circular opening about the width of a finger or more depending on the species size.
Key Features
The most striking feature is a large, central circular hole. Surrounding this hole are usually two smooth, kidney-shaped bumps that look like small rockers; these are the surfaces where the first neck bone plugs in. The bone itself is often thick and heavy compared to the thinner plates on the top of the head.
Similar Bones
It can be confused with a pelvic bone because both have large openings. However, the skull base is usually more compact and symmetrical, whereas a pelvic bone is often wider, flatter, and lacks the specific smooth 'rocker' surfaces found around the skull's main opening.
Across Species
The shape of this bone tells a story about how an animal moves. In mammals like deer or dogs, the large opening for the spinal cord faces toward the back, lining up with a horizontal spine. In a bird, the base of the skull often has a single rounded knob for turning the head, whereas mammals usually have two smooth, rocker-like surfaces for nodding.
Evolutionary History
This bone is an ancient piece of the vertebrate puzzle, evolving from a series of separate plates in early fish to a fused, protective anchor for the brain and neck in modern mammals. It represents the crucial junction where the brain connects to the rest of the body, a design that has remained remarkably stable for hundreds of millions of years.
Photography Tips
Lay the bone down and take a direct photo looking straight into the large circular opening. Then, flip it over to show the outer curved surface. Placing a coin or a ruler next to it helps viewers understand the size of the animal it came from.