The Devonian Period, roughly 419 to 359 million years ago, saw major diversification among fishes. Placoderms carried bony plates over the head and front of the body, while the rest of the body could be covered by scales or remain less heavily armoured. Their fossils occur in marine and freshwater deposits, so the group did not inhabit one uniform ocean. The same rocks also preserve invertebrates, including eurypterids, trilobites, molluscs and reef-building organisms.
What placoderm armour tells us
Placoderm head and trunk plates formed a jointed protective covering. In arthrodires, a movable joint between the head shield and trunk armour allowed the head to lift as the jaws opened. Some species had cutting tooth plates rather than teeth like those of modern sharks. Dunkleosteus is famous for its large skull and powerful jaw apparatus, but the best-preserved fossils are often the hard head and thoracic armour. Complete body length is therefore estimated from proportions and comparisons, and older reconstructions may overstate size.
Placoderms were diverse in shape and habitat. Some were broad, bottom-associated forms; others had bodies suited to swimming in open water. Fossil localities such as the Gogo deposits of Western Australia preserve exceptional anatomical detail, including soft tissues in some specimens. These sites show that a fish group once thought known mainly from armour can reveal reproduction, sensory anatomy and life history when preservation is unusually good.
Eurypterids and other arthropods
Eurypterids, often called sea scorpions, were chelicerate arthropods. Their fossils occur in marine, brackish and freshwater deposits, and the group's members varied greatly in size and ecology. Some had paddle-like appendages associated with swimming; others were adapted to life near the bottom. A few eurypterids became large, but most should not be pictured as giant predators. Fragmentary fossils also make it difficult to reconstruct body size from isolated segments.
These arthropods shared Devonian waterways with trilobites, brachiopods, corals and many fishes. The fossil record captures local assemblages rather than one global food web. For example, a reef, lagoon, lake and river channel each preserve different organisms and different parts of their lives. A fossil appearing in a marine rock does not automatically mean that every member of its group lived in open seawater.
How to read the “age of fishes”
The phrase describes the diversification and abundance of fish fossils, not a time when other animals were absent. Placoderms coexisted with jawless fishes, early sharks and bony fishes. On land, plants and arthropods were also changing, while the first forests reshaped soils and river systems. Fossils from the period include evidence of interaction, but a predator's large jaws alone do not establish its exact prey.
Armour can reflect several functions: protection, structural support or other biological roles. Researchers test these possibilities by comparing anatomy, wear, injuries and the setting in which specimens were buried. The Devonian was not simply a contest between the largest animals. It was a patchwork of marine and freshwater ecosystems with many different ways to feed, move and avoid being eaten.
For a closer look at one widespread armoured fish, see our profile of Bothriolepis. The broader placoderm story is one part of a much larger transition in vertebrate diversity.
Frequently asked questions
Were all placoderms giant predators?
No. Placoderms included many sizes and body forms. Some large arthrodires were predators, while other species had different diets and habitats.
Did eurypterids live only in the sea?
No. Fossil eurypterids come from marine, brackish and freshwater deposits. Their ecology varied across the group.
Can fossils tell us exactly how long Dunkleosteus was?
Not from the best-known armour alone. Many estimates rely on proportions from related animals because complete skeletons are rare, and later studies have revised some high estimates.
Why is the Devonian called the Age of Fishes?
Fish groups diversified and became abundant in the fossil record during the Devonian. The phrase does not mean that invertebrates or terrestrial life were unimportant.