Gnathostomata

… The oldest vertebrates were the jawless Agnatha Birkeniiformes (Anaspida), which lived from the Late Ordovician to the Devonian. The rest all belong to the Gnathostomata, which acquired jaws and paired fins (or limbs), and are presumed to have split off from the Agnatha in the Middle Ordovician. The oldest of this class are the Placodermi, which appeared in the Late Silurian and lived until the Permian, and the first to split off from them (in the Late Ordovician) is presumed to be the Chondrichthyes, a cartilaginous fish. However, their 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. …

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

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…　最古の脊椎動物はあごのない無顎綱Agnathaのバーケニア目(欠甲類)Birkeniiformes(＝Anaspida)で，オルドビス紀後期からデボン紀まで栄えた。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。…

※「Gnathostomata」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。

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