Phalanx
Long bone (though small in size), part of the appendicular skeleton
Also known as: Toe bone, finger bone, knuckle bone
Classification
Long bone (though small in size), part of the appendicular skeleton
Dimensions
These bones are generally much longer than they are wide, characterized by an hourglass-like waist with flared ends. In small mammals, they may be less than an inch long, while in larger ungulates like deer, they are several inches long and more robust.
Key Features
Look for a smooth, spool-shaped hinge at the bottom end and a concave or indented surface at the top end where it meets a neighbor. The middle section, or shaft, is typically flattened on the side that would face the palm or sole of the foot and rounded on the outer side.
Similar Bones
They are most often confused with the bones of the hand or foot (metapodials). You can tell them apart because metapodials are usually much longer and have a more pronounced ball-shaped joint at one end, whereas these bones have more rectangular or 'squared-off' hinge joints.
Across Species
The variety of these bones across species reflects different lifestyles. In birds, they are found inside the wing and support flight feathers. In whales and dolphins, these bones are flattened and numerous, forming the structural support for a paddle-like flipper. In grazing animals like horses, the third toe bone has evolved into a large, sturdy coffin bone to support the weight of the entire animal on a single hoof.
Evolutionary History
These structures trace back to the early lobe-finned fish that first transitioned to life on land. Over millions of years, the sprawling five-digit limb of early tetrapods specialized. For example, in the lineage of cats and dogs, the foot became elongated for sprinting on the tips of the toes, while in primates, these bones remained flexible to allow for grasping branches.
Photography Tips
Lay the bone against a flat, neutral background with a common item like a coin or a ruler for scale. Take one photo looking straight down at the flat side, and another from the side to show the characteristic curve and the shape of the joint ends.