Arthrodires

Arthrodires are an extinct order of armored jawed fishes known as placoderms. They were one of the most successful and widespread groups of early gnathostomes, flourishing mainly during the Devonian period in marine and freshwater environments and fossils found on multiple continents.

A defining feature of arthrodires is the mobile connection between the head shield and the trunk shield, a jointed neck that allowed a wider gape and more versatile jaw action. Their bodies were covered with articulated dermal plates, and their jaws consisted of specialized bony plates rather than the true teeth seen in later fishes. Size in arthrodires varied from small forms to giants, with some species like Dunkleosteus terrelli reaching several meters in length and serving as apex predators of their ecosystems.

Ecologically, arthrodires occupied a range of niches. They included large predatory forms that hunted other placoderms and jawed vertebrates, as well as smaller, possibly more specialized bottom-dwellers. Their fossil record reveals a broad geographic distribution, with rich assemblages in North America and Europe and helpful insights from other regions as well.

Taxonomically, arthrodires form a major subgroup within Placodermi, the armored early jawed fishes. They were especially diverse in the Devonian, a time often called the “Age of Fishes” for its remarkable vertebrate diversification. Most arthrodires, along with many other placoderms, disappeared by the end of the Devonian during mass extinction events, leaving a relatively short-lived but highly influential legacy in the evolution of early jawed vertebrates.