Occipital bone (Cranium fragment)
Flat and Irregular bone; part of the axial skeleton (the core frame)
Also known as: Back of the skull, base of the head, poll bone
Classification
Flat and Irregular bone; part of the axial skeleton (the core frame)
Dimensions
In mid-sized mammals, this section is usually several inches wide, forming a sturdy, bowl-like shape that protects the very back of the head and the passage for the spinal cord.
Key Features
Look for a prominent, large circular or heart-shaped opening at the base through which the spinal cord passes. On either side of this hole, you will find two smooth, rounded knobs that resemble rockers on a rocking chair; these allow the head to tilt up and down. The outer surface is often textured where neck muscles once attached.
Similar Bones
The entire back of the skull can sometimes be confused with a pelvis fragment. However, the skull fragment will have the large central hole for the spinal cord and the unique smooth rockers at the base, whereas a pelvis has a deep, distinct round socket for a leg bone.
Across Species
This bone varies dramatically across species based on how the animal holds its head. In humans, the large hole for the spinal cord is at the bottom of the skull to support upright walking. In four-legged animals like deer or dogs, this opening is toward the back of the skull. Birds often have a single rounded nub at the base for incredible head rotation, while mammals typically have two smooth rockers for nodding movements.
Evolutionary History
The back of the skull has evolved from a series of many separate plates in ancient fish and early tetrapods into a fused, heavy-duty protective shield in modern mammals. This fusion provided better attachment points for powerful neck muscles as animals moved onto land and began to support the weight of their heads against gravity.
Photography Tips
To help identify a skull fragment, take photos directly facing the large central opening and the smooth rockers. Include a top-down view to show the curve of the bone and always place a common object like a coin or a ruler next to it to show the size.