Cranium
Irregular bone (complex structure) and part of the axial (core) skeleton.
Also known as: Skull, head bone, braincase
Classification
Irregular bone (complex structure) and part of the axial (core) skeleton.
Dimensions
Skulls vary immensely in size based on the species, but typically consist of a rounded hollow box in the back for the brain and an elongated snout or face in the front. In small mammals like rodents or opossums, the entire skull may only be 2 to 4 inches long.
Key Features
Look for the large openings for the eyes, the long rows of teeth on the bottom surface, and the two rounded knobs at the very back where the head attaches to the neck. Small mammals often have thin, arching cheekbones and a narrow bridge between the snout and the braincase.
Similar Bones
A fragment of a skull might be confused with a pelvic (hip) bone because both have large holes and hollow areas. However, the presence of teeth or tooth sockets is a giveaway that you are looking at a skull. Unlike the pelvis, a skull will also have a large internal cavity specifically designed to hold the brain.
Across Species
In the wild, skull shapes reveal an animal's lifestyle. Predators like cats have short snouts and large eye sockets for forward-facing vision, whereas prey animals like rabbits or deer have eyes on the sides of their heads to spot danger. Opossums are unique because they have a very small braincase relative to the size of their face and a high number of teeth—50 in total—which is more than most other North American mammals.
Evolutionary History
The skull evolved from early fish-like ancestors as a protective 'helmet' for the brain and sense organs. Over millions of years, the many individual plates of bone found in early vertebrates fused together into the sturdy, complex structure seen today. Mammals developed specialized teeth and a secondary palate, which allows them to eat and breathe at the same time, an essential trait for maintaining a high metabolism.
Photography Tips
To get the best identification, take photos from three main angles: looking straight down at the top, looking directly at the side, and looking at the underside to show the teeth. Place a common object like a coin or a ruler next to it so others can see how big it is. Clear, indirect light helps show the fine details of the teeth and bone texture.