Mandible (fragment)
Irregular bone; part of the axial skeleton (skull)
Also known as: Lower jawbone, jaw, chin bone
Classification
Irregular bone; part of the axial skeleton (skull)
Dimensions
In mammals, the size varies greatly from tiny shrews to massive whales. This specific piece shows a sturdy, curved horizontal section that houses teeth and a broad, flat upward-reaching plate at the back.
Key Features
Look for a thick, horizontal 'body' that contains tooth sockets and a tall, flat 'wing' at the back. A key identifying mark is the rounded hinge at the very back top corner, which sits in a socket at the base of the skull. You will also typically see a small, smooth hole on the outer side near the front, which serves as a passage for nerves.
Similar Bones
At a quick glance, a broken mandible plate might look like a shoulder blade (scapula) because both are flat and broad. However, the mandible is much denser and will almost always show evidence of tooth sockets or the distinctive rounded hinge at the back, which a shoulder blade lacks.
Across Species
The mandible is a masterclass in adaptation. In carnivores like cats or dogs, it is built for a powerful vertical snap, while in grazers like deer, it is long and slender to allow for side-to-side grinding. In rodents, the front section is specialized to support ever-growing orange incisors. This specific specimen features 'bunodont' teeth—rounded, bumpy crowns similar to those found in pigs or humans—which suggests an omnivorous diet capable of crushing varied foods.
Evolutionary History
The jaw is one of the most significant developments in vertebrate history, having evolved from the gill arches of ancient jawless fish. In early reptiles, the lower jaw was composed of several different bones. Over millions of years, as mammals specialized their hearing, most of these bones migrated to the middle ear to become tiny sound-amplifiers, leaving the single large bone we see today to handle the heavy work of chewing.
Photography Tips
To get a good identification, place a common object like a coin or a ruler next to the bone for scale. Take one photo directly from the side to show the profile and one looking down at the top of the teeth to show their unique chewing surface patterns.