Placoderm

Placoderms (from Ancient Greek πλάξ [plax, plakos] 'plate' and δέρμα [derma] 'skin') are vertebrateanimals of the classPlacodermi, an extinct group of prehistoric fish known from Paleozoicfossils during the Silurian and the Devonianperiods. While their endoskeletons are mainly cartilaginous, their head and thorax were covered by articulated armoured plates (hence the name), 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), as well as the first vertebrates to have true teeth. They were also the first fish clade to develop pelvic fins, the second set of paired fins and the homologous precursor to hindlimbs in tetrapods. 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.

Placoderms are thought to be paraphyletic, consisting of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. In contrast, one 2016 analysis concluded that Placodermi is likely monophyletic.

The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian. They eventually outcompeted the previously dominant marine arthropods (e.g. eurypterids) and cephalopodmolluscs (e.g. orthocones), producing some of the first and most infamous vertebrate apex predators such as Eastmanosteus, Dinichthys and the massive Dunkleosteus. Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at the end-Devonian Hangenberg event 358.9 million years ago, leaving the niches open for the osteichthyan and chondrichthyan survivors who subsequently radiated during the Carboniferous.