Synsacrum
Irregular bone; core (axial) skeleton
Also known as: Avian pelvic bone / Bird hip-back plate
Classification
Irregular bone; core (axial) skeleton
Dimensions
The size varies significantly by species, but it is typically a wide, flat, and elongated shield-like structure that is longer than it is wide. Its proportions are defined by the fusion of many segments into one rigid unit.
Key Features
It looks like a delicate, hollow cage or a ridged shield. You will see a central row of bumps that look like fused backbones, flanked by thin, wing-like plates. It is often full of small holes which make it very light. One end usually has a larger flat area for the hip attachment, while the other narrows down.
Similar Bones
It is often confused with a skull because of its complex shape and holes, but a skull will have a brain case and eye sockets, whereas the synsacrum has a clear central ridge where the backbones joined. It might also be mistaken for a standard mammal pelvis, but it lacks the wide, circular opening seen in mammals.
Across Species
Unlike the flexible lower backs of mammals, birds have a synsacrum where many backbones and the hip bones fuse together into a light, stiff plate. In flightless birds like ostriches, this bone is massive and heavy for support, while in songbirds it is paper-thin and delicate to save weight for flight. Whales and horses lack this specific structure, as they require different types of pelvic and spinal movement.
Evolutionary History
This structure evolved as dinosaurs transitioned into birds. Early ancestors had separate backbones and hip bones, but as flight became a primary way of moving, the skeleton adapted by fusing these pieces together. This created a solid foundation for the wings to push against and a sturdy landing gear for the legs, representing a major milestone in avian engineering over millions of years.
Photography Tips
Place the bone on a plain, dark background to show the white edges clearly. Take one photo from directly above to show the overall shield shape, and another from the side to show the height of the ridges. Including a coin or a ruler in the frame helps show how small or large the specimen is.