Placodermi

...the rest belong to the Gnathostomata, a genus of gnathostomata that acquired jaws and paired fins (or limbs) and are believed to have split off from the Agnathostomata in the Middle Ordovician. The oldest member of this class is the Placodermi, which appeared in the Late Silurian and continued into the Permian. The first to split off from this (in the Late Ordovician) is the Chondrichthyes, a genus of cartilaginous fish, but its fossils do not appear until the Middle Devonian, later than the Osteichthyes, a bony fish that also split off from the Placodermi and appeared in the Early Devonian. The amphibians Amphibia, which branched off from the Crossopterygii (a subclass of bony fishes) that acquired bones and gained limbs and lungs, appeared during the transition from the Devonian to the Carboniferous period; the reptiles Reptilia, which branched off from the Amphibians and gained an amniotic membrane, appeared in the Late Carboniferous period; the birds Aves, which branched off from the Archosauria subclass of reptiles and gained feathers, appeared in the Early Jurassic period; and the mammals Mammalia, which branched off from the Synapsida subclass of reptiles and gained split hairs, mammary glands, and three middle ear ossicles, appeared in the Late Triassic period.

*Some of the terminology that mentions "Placodermi" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…残りのものはすべてあごと対鰭(または四肢)を獲得した顎口類Gnathostomataに属し，オルドビス紀中期に無顎類から分かれたと推定されている。この類で最古のものはシルル紀後期に現れ二畳紀まで栄えた板皮類(綱)Placodermiで，これから最初に分かれた(オルドビス紀後期)のが軟骨魚類Chondrichthyesと推定されているが，これの化石は，やはり板皮綱から分かれデボン紀前期に現れた硬骨魚類Osteichthyesよりも後のデボン紀中期にならないと姿を見せない。硬骨を獲得した硬骨魚綱の中の総鰭類(亜綱)Crossopterygiiから分かれ，四肢と肺を獲得した両生類Amphibiaはデボン紀から石炭紀への移行期，両生綱から分かれ羊膜を獲得した爬虫類Reptiliaは石炭紀後期，爬虫類の祖竜亜綱Archosauriaから分かれ羽毛を獲得した鳥類Avesはジュラ紀前期，同じく爬虫綱の単弓亜綱Synapsidaから分かれ毛と乳腺および3個の中耳小骨を獲得した哺乳類Mammaliaは三畳紀後期に現れている。…

※「Placodermi」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報