Placoderms are members of the class Placodermi, a group of prehistoric fish known from Paleozoic fossils which lived from the Silurian to the end of the Devonian period. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates, and the rest of the body was scaled or naked depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first pair of gill arches.
Placoderm
Dunkleosteus, among the first of the vertebrate apex predators, was a giant armoured placoderm predator.
Amazichthys, a pelagic arthrodire from the Middle Famennian of the Late Devonian.
Fin spine of Eczematolepis, from the Middle Devonian of Wisconsin.
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.
Ordovician-Silurian boundary on Hovedøya, Norway, showing brownish late Ordovician mudstone and later dark deep-water Silurian shale. The layers have been overturned by the Caledonian orogeny.
Fossils of the late Silurian sea bed
Silurian sea bed fossils collected from Wren's Nest Nature Reserve, Dudley UK
Crinoid fragments in a Silurian (Pridoli) limestone (Saaremaa, Estonia)