synsacrum

The synsacrum is a fused series of vertebrae found in the pelvic region of birds, dinosaurs, and pterosaurs. It typically includes the last rib-bearing thoracic vertebra, the two sacral vertebrae, and some caudal vertebrae. This fused structure provides a solid base for the pelvic girdle. In birds, the synsacrum is part of a larger fused vertebral column that contributes to their skeletal strength and lightness, which are crucial for flight. Unlike the sacrum in humans, which is usually composed of five fused vertebrae, the synsacrum in birds is a more extensive fusion that anchors the pelvic girdle.123