Cranium
Flat and irregular bones forming part of the axial skeleton
Also known as: Skull cap or braincase
Classification
Flat and irregular bones forming part of the axial skeleton
Dimensions
Varies widely by species, ranging from a few centimeters in small rodents to over a meter in large mammals, featuring a domed roof and hollow interior chamber.
Key Features
Rounded top surface, prominent zigzag suture lines where separate bone plates meet, and a smooth inner cavity that once protected the brain.
Similar Bones
Pelvic bones or turtle shells, which can be told apart from a cranium by looking for the distinctive puzzle-like suture lines and the smooth hollow interior chamber designed to hold a brain rather than internal organs.
Across Species
Mammal craniums house complex brains and feature specialized interlocking puzzle-piece seams called sutures, whereas bird skulls fuse into a single lightweight structure and reptile skulls feature extra flexibility.
Evolutionary History
The cranium evolved from ancient bony armor plates in early fish ancestors, gradually expanding to encase and protect the growing brain and sensory organs as vertebrates transitioned to land.
Photography Tips
Take pictures from directly above to capture the pattern of the suture lines, and from the bottom to show the openings, using natural indirect sunlight and placing a coin or ruler next to the specimen for scale.