Category:Placodermi

Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is a class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian period. 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 of their gill arches. Placoderms are paraphyletic, and consist of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil, Entelognathus, from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods.[1][2] The jaws in other placoderms were simplified and consisted of a single bone.[3] Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth.[4] Paraphyletic groupings are problematic, as one can not talk precisely about their phylogenic relationships, their characteristic traits and literal extinction. In contrast, one 2016 analysis concluded that placodermi are likely monophyletic.[8]

The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian.[9] The various groups of placoderms were diverse and abundant during the Devonian, but became extinct at the end-Devonian Hangenberg event 358.9 million years ago [10]

Characteristics
Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian genus, Bothriolepis, known from over 100 valid species.[11] The vast majority of placoderms were predators, many of which lived at or near the substrate. Many, primarily the Arthrodira, were active, nektonic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavities and were made of both bone and dentine. However, the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes.[4]