Placoderms are an extinct class of armored fishes, the first known animals of any type to evolve true jaws. Placoderms evolved from agnathan (jawless) fishes in the Silurian period, about 425 million years ago. This is around the same time as the first terrestrial animals, such as millipedes, are known to have ventured on the land.
Placoderms (class Placodermi) diversified in the mid-Silurian or possibly earlier, an early pioneer in vertebrate evolution that would continue with the Devonian period immediately afterwards, which featured so much fish diversification that is has been dubbed “The Age of Fishes.” Unfortunately, no complete placoderm fossils are known — the species have been described from fragments of their body armor that are preserved when the fishes’ body breaks apart after death.
The characteristic feature of placoderms is a layer of segmented armor across the head and thorax. Depending on species, the remainder of the body would be scaled or naked. Instead of true teeth, placoderms grinded their food using sharpened points of bone which protrude from the head. This method of feeding is a clear evolutionary intermediate between jawless fish and the highly adapted toothed jaws of modern fish groups.
Placoderms ranged in size from a few inches to the huge 6 m (20 ft) Dunkleosteus telleri, which lived about 360 – 415 million years ago and weighed a ton. Dunkleosteus is considered the first vertebrate superpredator, and probably took over some segments of the evolutionary niche previously occupied only by large predatory invertebrates such as sea scorpions. These animals demonstrated the ability of members of phylum Chordata (vertebrates) to occupy the apex predator niche, which they would continue to dominate for the rest of history. Dunkleosteus is thought to have had a bite force similar to that of the strongest biters in history, like Tyrannosaurus rex modern crocodiles.
Placoderms were also the first animals to internalize egg fertilization, leading to the first live births in evolutionary history. They perished in the late Devonian extinction, which also wiped out practically all jawless fishes as well as numerous other species.