Coccyx
Irregular bone; part of the axial (core) skeleton
Also known as: Tailbone
Classification
Irregular bone; part of the axial (core) skeleton
Dimensions
This small, multi-segmented bone is generally triangular and tapers toward the bottom. It usually measures between one and four inches in length, depending on the species and individual.
Key Features
It has a distinct beak-like or triangular shape with a wider top that connects to the sacrum and a narrow, pointed bottom. It is made of three to five small, bead-like segments that often fuse together, creating a slightly curved, bumpy surface.
Similar Bones
It can be confused with the very lower tip of the sacrum. However, the tailbone is much smaller and lacks the large holes or broad 'wings' found on the sacrum just above it.
Across Species
While humans and great apes have a short, fused tailbone that remains internal, most other mammals possess an external tail used for balance or communication. In birds, a similar structure called the pygostyle supports tail feathers for flight. In many primates, the equivalent bones are numerous and separate to allow for a flexible, prehensile tail.
Evolutionary History
The tailbone is a classic example of a vestigial structure in lineage history. It is the remnant of a much longer tail found in our ancient ancestors. As certain primate lineages transitioned to upright movement or different styles of climbing, the long tail was no longer needed for balance and gradually shortened and tucked inward.
Photography Tips
Capture the bone from the front and back to show the curve and the segmentation. Placing a small coin or ruler next to it is helpful because its small size can make it hard to identify without a scale reference.
Notes
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