Pelvis
Irregular bone, part of the core (axial) skeleton
Also known as: Hip bone, hip girdle, pelvis
Classification
Irregular bone, part of the core (axial) skeleton
Dimensions
Varies immensely depending on the species, ranging from a few centimeters in small rodents to over a meter across in large ungulates and cetaceans. It forms a symmetrical ring or basin shape in most land mammals.
Key Features
Characterized by a butterfly or bowl-like symmetry, featuring large distinct eye-like openings called obturator foramina on either side of the center line, and deep socket joints where the leg bones attach.
Similar Bones
Can sometimes be confused with the shoulder blades or skull fragments due to its flat, curved plates, but the presence of symmetrical matching halves and deep ball-and-socket attachment cups clearly distinguishes the hip girdle.
Across Species
In humans and other bipedal walkers, the pelvis is wide and bowl-shaped to support internal organs and balance upright movement. In four-legged animals like dogs and cats, it is elongated and narrow to accommodate a powerful running stride. In birds, the pelvis is fused with the spine into a long, lightweight structure to withstand the heavy forces of landing and taking off. In swimming mammals like whales, the pelvis is heavily reduced and disconnected from the spine, existing only as tiny vestigial remnants.
Evolutionary History
The pelvis originated in early fish as a small set of supportive cartilages for pelvic fins. As vertebrates crawled onto land, this structure expanded and connected directly to the spine to bear body weight against gravity, evolving into the robust hip girdle seen in modern tetrapods.
Photography Tips
Place the bone flat against a neutral background and photograph it directly from above to show its full symmetry. Use soft, even lighting to highlight the curved edges and central openings, and always include a coin or ruler for scale.