Mandible (fragment)
Irregular bone; part of the skull and axial skeleton
Also known as: Lower jawbone, jaw, chin bone
Classification
Irregular bone; part of the skull and axial skeleton
Dimensions
In small rodents and insectivores, this bone is typically between half an inch to two inches long, characterized by a slender, curved horizontal section that holds the teeth and a broader, vertical section at the back for muscle attachment.
Key Features
Look for a distinct, upward-curving 'hook' or 'fin' (the coronoid process) at one end, which serves as an anchor for jaw muscles. You will often see a row of small pits or sockets where teeth were once seated, or a single long, curved channel at the very front designed for a large gnawing tooth.
Similar Bones
A jaw fragment can sometimes be confused with a hip bone (pelvis) because both have flat areas and rounded sockets. However, the jaw is usually thinner and will almost always show evidence of tooth rows or the specific hinge joint that looks like a small, smooth cylinder or knob.
Across Species
The mandible varies wildly across the animal kingdom based on diet. In rodents like squirrels or mice, it features a deep socket for a single, ever-growing front chisel tooth. In carnivores like cats, the back part of the jaw is tall to support powerful biting muscles, while in herbivores like deer, it is long and flat to allow for the side-to-side grinding of plants.
Evolutionary History
The jaw is a marvel of deep-time engineering, having evolved from the front-most gill arches of ancient jawless fish hundreds of millions of years ago. Over time, many of the smaller bones that once made up the reptilian lower jaw migrated to the ear, leaving mammals with a single, sturdy bone on each side that provides a strong platform for complex chewing.
Photography Tips
To help with identification, place the bone on a plain, neutral background and include a common object like a coin or a ruler for scale. Take one photo directly from the side to show the overall silhouette and another looking down at the top surface to reveal any tooth sockets or grinding surfaces.