Fish Pelvic Girdle or Cleithrum
Flat and irregular bone, part of the appendicular and gill-arch support systems of a teleost fish
Also known as: Fish shoulder bone or pelvic bracket
Classification
Flat and irregular bone, part of the appendicular and gill-arch support systems of a teleost fish
Dimensions
Varies widely from a few centimeters in small panfish to over thirty centimeters in large predatory marine species; characteristically thin, blade-like, and elongated.
Key Features
Thin, blade-like profile with delicate curved hooks, pointed tips, and serrated or comb-like edges along one side, resembling a miniature anchor or an ornate leaf shape.
Similar Bones
Can be easily confused with flattened mammalian skull fragments, turtle shell pieces, or large bird sternum pieces. It is distinguished by its paper-thin curved walls, lack of spongy interior sandwich layers typical of mammalian flat bones, and asymmetrical hook-like prongs.
Across Species
Unlike the rigid shoulder bones of land-dwelling mammals, fish shoulder and pelvic elements are delicate, highly varied architectural brackets that anchor the fins directly to the skull region or body wall. In fast-swimming open-water fishes, these bones are streamlined, whereas bottom-dwelling species often develop broader, heavier plates to support bracing against currents.
Evolutionary History
These bony elements evolved from ancient ancestral fish armor plating and gill-arch supports, gradually shifting over hundreds of millions of years into internal struts that stabilize the paired fins for steering and braking in water.
Photography Tips
Place the bone flat against a solid, contrasting dark background. Use soft overhead lighting to highlight the delicate ridges and serrated edges, and include a coin or ruler nearby for scale.