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The placoderms are among the most ancient of fish, and, along with the Acanthodii, the only class of (gnathostomes) to become completely extinct. The name "placoderm" is from the Greek and means "tablet + skin", referring to the heavy armoured bony plates that completely covered the head and thorax of these curious prehistoric fish. Bothriolepis. Late Devonian - worldwide. Length about 20 to 30 cm With their armored head shield and trunk shield composed of overlapping bony plates, the placoderms appear at first glance extremely similar to the ostracoderms (armoured jawless fish). They are easily distinguished however by their paired fins and presence of jaws, an adaptation that gave them a tremendous advantage over their earlier jawless cousins, for it enabled them to bite solid food rather than simply suck up organic particles from the mud. Yet still these creatures were very primitive compared to other fish. The fourth pair of gill arches had not yet become supports for the jaws, and the backbone consisted of a notochord (an embryonic feature in higher vertebrates) that persisted throughout life and vertebra consisting only of Y-shaped spines above and below it. These were often cartilaginous, like the skeleton of sharks (Chondrichthyes), a group with which the Placoderms are considered to share a common ancestry. Placoderms also lacked teeth, but biting or grinding structures are often be found in the dermal bones lining their mouths. The head and trunk shields of most placoderms were articulated by bony joints, allowing the forward part of the skull to tilt up, increasing the gape and allowing them to take in larger food Although the head and thorax were heavily armored with dermal (skin) bone, the rest of their bodies were quite vulnerable, covered with small bony scales or lacking even that. It is not clear what purpose this armour may have served. It has been suggested that it was a defense against the giant sea-scorpions that inhabited brackish water environments around this time. Another explanation is that it may have served as an exoskeleton and (as with arthropods and mollusks) a support for the external organs (in view of the very weak cartilaginous skeleton this may not be unlikely) Placoderms evolved into a variety of ecomorphs (body types), including torpedo-shaped swimmers, flattened bottom-dwellers, and armoured box-like types. They included both carnivorous and detritivorous types. The pared fins and a heterocercal tail indicates they could swim quite efficiently when they wanted to, but the heavy armour would've weighed them down, and it is likely they spent much of their time sitting on the bottom.. Most placoderms were quite small, about 10 or 15 cm in length, but one specialized group of carnivorous types, Dinichthyidae included a few genera reached 4 to 9 meters in length, making them the largest animals of their time, and the most fearsome. Although they first appeared during the early Silurian period, the placoderms did not become common or widespread until the Early Devonian, when they underwent an extraordinary evolutionary radiation. They soon came to dominate most brackish and near-shore ecosystems, and also spread to marine and freshwater environments. More than 250 genera are known in all, making them the most diverse and important of early vertebrates. The placoderms were seemingly unaffected by the great Frasnian-Famennian mass extinction that rocked the late Devonian, yet all suddenly died out at the Devonian/Carboniferous boundary, without leaving any descendents