Cranium
Flat and irregular bones combined, part of the axial skeleton
Also known as: Skull, head bone, braincase
Classification
Flat and irregular bones combined, part of the axial skeleton
Dimensions
Varies drastically by species, ranging from less than an inch in small rodents to several feet in large mammals, generally forming a rounded or elongated housing for sensory organs and the brain.
Key Features
Look for a large central cavity designed to house the brain, large circular openings for the eyes, and a prominent circular hole at the bottom back where the spine connects to the head.
Similar Bones
Pelvic bones can sometimes mimic the curved plates and openings of a cranium, but pelvises lack the distinct eye sockets and the central spine connection opening found at the base of the skull.
Across Species
In mammals, the skull is a fused fortress of plates, whereas in birds it is lightweight with thin, fused struts to reduce weight for flight. Fish skulls feature a complex web of overlapping plates that allow for water pumping over gills, and reptiles often feature rigid boxes with extra joints in snakes and lizards to allow wide gapes.
Evolutionary History
The cranium evolved from the protective head capsules of early jawless fish, incorporating the front parts of the backbone and the gill arches over hundreds of millions of years to better house increasingly complex brains and sensory structures.
Photography Tips
Take photos from the top, profile, and directly from the front. Ensure good natural lighting to cast shadows into the eye sockets and inner cavities to highlight the natural contours and openings. Include a coin or ruler next to the specimen for scale.