Scapula
Flat bone, part of the appendicular skeleton
Also known as: Shoulder Blade
Classification
Flat bone, part of the appendicular skeleton
Dimensions
Varies widely by animal size, generally forming a broad, flat, triangular shape with curved ridges and a prominent socket area at one corner for the upper arm connection.
Key Features
Look for a broad, paper-thin flat plate crossed by a prominent raised ridge on one side, ending in a hook-like projection and a smooth, cup-shaped socket where the arm bone attaches.
Similar Bones
Can sometimes be confused with large pelvic bones due to the flat plate-like sections, but the shoulder blade is distinguished by its distinct triangular outline, prominent spine ridge, and the specific socket shape designed for the upper arm rather than the leg.
Across Species
In mammals, the shoulder blade rests freely against the ribcage, providing great flexibility for running and climbing. In running specialists like horses, the collarbone disappears entirely and the shoulder blade becomes long and narrow to extend the stride. In swimming mammals like whales, the shoulder blade is greatly reduced. In birds, it fuses with other elements to form a sturdy brace for flight muscles.
Evolutionary History
The shoulder blade originates from the pectoral girdle of early tetrapods, evolving from ancestral fish fin supports into a stable anchor point that allowed land vertebrates to lift their bodies off the ground and support active locomotion.
Photography Tips
Place the bone on a neutral, solid-colored surface with good overhead lighting to highlight the shadows cast by the raised ridges. Take one photo of the broad flat side showing the main ridge, and a second photo focusing on the socket and hook projections from the side.