The Placodermi were a class of armored prehistoric fish, known from fossils, which lived from the late Silurian to the end of the devonian Period.

Their head and thorax were covered by articulated armored plates and the rest of the body was scaled or naked, depending on the species.

The Silurian fossil record of the placoderms is both literally and figuratively fragmented. All known Silurian placoderms exist today only as fragments, either scraps of armor, or isolated scales, of which some have been tentatively identified as either antiarch or arthrodire due to histological similarities.

During the Devonian, in stark contrast to the Silurian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater. But this diversity ultimately suffered many casualties during the extinction event at the Frasnian–Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the Devonian/Carboniferous extinction event; not a single species survived into the Carboniferous. During the Devonian, in stark contrast to the Silurian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater. But this diversity ultimately suffered many casualties during the extinction event at the Frasnian–Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the Devonian/Carboniferous extinction event; not a single species survived into the Carboniferous.

The vast majority of placoderms were predators, many of which lived at or near the bottom. Many, primarily the Arthrodira, were mid- to upper-water dwellers, and were active predators. The vast majority of placoderms were predators, many of which lived at or near the bottom. Many, primarily the Arthrodira, were mid- to upper-water dwellers, and were active predators.

regarded as the world's first vertebrate "super-predator". largest known arthrodire, Dunkleosteus telleri, was 8–11 m (26–36 ft) long,

The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833–1843. In those days, the placoderms were thought to be shelled jawless fish akin to ostracoderms. Some naturalists even suggested that they were shelled invertebrates, or even turtle- like vertebrates. The work of Dr. Erik Stensiö, at the Swedish Museum of Natural History, Stockholm, from the late 1920s established the details of placoderms anatomy, and identified them as true jawed fishes related to sharks.