|There are 200 known genera of placoderms divided into
7 major orders. These orders are stensioelliformes, rhenaniformes, ptyctodontiformes,
petalichthyiformes, phyllolepidiformes, arthrodiriformes and antiarchiformes.
Of these orders the largest is Arthrodira which means jointed necks and includes more than 60% of all known placoderm species. The different orders are recognised by a combination of plate patterns making up the external armour and by the development of bony surficial ornament on the plates of some groups. The most primitive are the ptyctodontids and rhenanids. The most advanced are the antiarchs, arthrodires and phyllolepids.