Placoderm
Class of fishes (fossil) / From Wikipedia, the free encyclopedia
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Placoderms (from Greek πλάξ (plax, plakos) 'plate' and δέρμα (derma) 'skin')[1] 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 (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.
Placoderm | |
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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 | |
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Synonyms | |
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Placoderms are thought to be paraphyletic, consisting of several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks.[2] In contrast, one 2016 analysis concluded that placodermi are likely monophyletic.[3]
Placoderms were also the first fish clade to develop pelvic fins, the second pair of paired fins and the precursor to hindlimbs in tetrapods, as well as true teeth.[4] 380-million-year-old fossils of three other genera, Incisoscutum, Materpiscis and Austroptyctodus, represent the oldest known examples of live birth.[5]
The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian.[6] 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.[7]