Dentary (Fish Mandible)
Irregular bone belonging to the facial skeleton and head
Also known as: Fish jaw, lower jaw bone
Classification
Irregular bone belonging to the facial skeleton and head
Dimensions
In many teleost fish, this bone is relatively flat and wide, often possessing a toothed edge on the upper margin and a hollowed-out back section. Proportions vary significantly depending on the species' diet.
Key Features
Look for a flat, plate-like bone with a row of small, repetitive sockets or serrated points along one edge where teeth once sat. One end usually features a distinct point or 'hooked' appearance, while the larger body often has a smooth, curved surface with a hollow inner channel.
Similar Bones
This can be confused with a bird's sternum or even a large scale or piece of shell. To differentiate, look for the 'alveoli' or tooth-attachment sites; shells and scales will be much thinner and lack the complex three-dimensional socket structure found in jaw bones.
Across Species
Across the animal kingdom, the lower jaw has undergone radical transformations. In predatory fish like snapper or barracuda, the dentary is elongated and packed with sharp teeth for grasping. In contrast, in baleen whales, this bone becomes a massive, toothless curved beam used to scoop massive amounts of seawater. Birds have a lower jaw known as the mandible which is covered in a keratinous beak, while mammals have a single fused dentary on each side that connects directly to the skull.
Evolutionary History
The jaw is one of the most significant developments in vertebrate history, having evolved from the front-most gill arches of early fish over 400 million years ago. Originally used just for breathing, these structures adapted into grasping tools, allowing early fish to transition from passive filter-feeding to active hunting, which fundamentally changed the course of aquatic life.
Photography Tips
Photograph this item flat against a high-contrast background with the toothed edge facing upward. Include a second photo of the 'inner' side to show the hollow channel, and use a common object like a coin or a ruler to give a clear sense of scale.