Placoderm

Placoderm Temporal range: 439–358.9 Ma PreꞒ Ꞓ O S D C P T J K Pg N Late Llandovery – Late Devonian[a]
Fossil of Bothriolepis panderi showing its caliper-like pectoral fins
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Class:	†Placodermi McCoy, 1848
Orders
†Antiarchi †Arthrodira (includes Phyllolepida) †Petalichthyida †Ptyctodontida †Rhenanida †Acanthothoraci (paraphyletic) "Maxillate placoderms" †Qilinyu †Entelognathus †Silurolepis †Bianchengichthys †Minjinia ?Eugnathostomata †Xiushanosteus †Brindabellaspis ?†Pseudopetalichthyida ?†Stensioella
Synonyms
Placodermata

Placoderms (from Greek πλάξ (plax, plakos) 'plate' and δέρμα (derma) 'skin')^[1] are vertebrate animals of the class Placodermi, an extinct group of prehistoric fish known from Paleozoic fossils during the Silurian and the Devonian periods. 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.^[2] 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.^[3]

Placoderms are thought to be paraphyletic, consisting of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks.^[4] In contrast, one 2016 analysis concluded that placodermi are likely monophyletic.^[5]

The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian.^[6] They eventually outcompeted the previously dominant marine arthropods (e.g. eurypterids) and cephalopod molluscs (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,^[7] leaving the niches open for the osteichthyan and chondrichthyan survivors who subsequently radiated during the Carboniferous.

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