Placoderms

Placoderms are an extinct group of armored prehistoric fish that represent some of the earliest jawed vertebrates (gnathostomes). They first appear in the Silurian and become highly diverse during the Devonian, a period sometimes called the age of fishes. The most distinctive feature of placoderms is their extensive dermal armor, consisting of large bony plates that shield the head (head shield) and the front part of the trunk. The presence of jawed mouths makes placoderms important for understanding early gnathostome evolution, as their jaws developed from the anterior gill arches.

Anatomical diversity within Placodermi is considerable. Some members, such as the antiarchs, wore heavy armor over the thorax and had unusual, often enclosed fins. The arthrodires included many predators and are known for a movable joint between the armored head and trunk that allowed powerful biting; Dunkleosteus is a famous example. Other orders, like the ptyctodontids, exhibited different armor configurations and growth patterns. Across the group, jaws were formed by gnathostome structures, and many platelike jaw elements bore tooth-like denticles rather than true teeth.

Ecology and distribution of placoderms were widespread in marine and freshwater environments during the Devonian. They ranged from small bottom-dwellers to some of the era’s largest predators and achieved a global fossil record, with particularly rich Devonian deposits preserving skulls and armor plates.

Placoderms are important for understanding the early evolution of jaws and paired fins in vertebrates. They declined and disappeared by the end of the Devonian, likely due to climate shifts and ecological changes that affected marine ecosystems. Their fossil record provides key insights into early vertebrate diversity and anatomy.