erm s ' ▁seem ingly ▁instant aneous ▁appearance ▁and ▁divers ity ▁at ▁the ▁very ▁beginning ▁of ▁the ▁Dev onian . ▁ ▁During ▁the ▁Dev onian , ▁pla cod erm s ▁went ▁on ▁to ▁inhab it ▁and ▁domin ate ▁almost ▁all ▁known ▁aqu atic ▁e cos ystem s , ▁both ▁fresh water ▁and ▁salt water . ▁But ▁this ▁divers ity ▁ultimately ▁suffered ▁many ▁cas ual ties ▁during ▁the ▁ext inction ▁event ▁at ▁the ▁F ras n ian – F am enn ian ▁boundary , ▁the ▁Late ▁Dev onian ▁ext in ctions . ▁The ▁remaining ▁species ▁then ▁died ▁out ▁during ▁the ▁Dev onian / Car bon ifer ous ▁ext inction ▁event ; ▁not ▁a ▁single ▁pla cod erm ▁species ▁has ▁been ▁confirmed ▁to ▁have ▁surv ived ▁into ▁the ▁Car bon ifer ous . ▁ ▁History ▁of ▁study ▁The ▁earliest ▁studies ▁of ▁pla cod erm s ▁were ▁published ▁by ▁Louis ▁Ag ass iz , ▁in ▁his ▁five ▁volumes ▁on ▁foss il ▁fish es , ▁ 1 8 3 3 – 1 8 4 3 . ▁In ▁those ▁days , ▁pla cod erm s ▁were ▁thought ▁to ▁be ▁sh elled ▁j aw less ▁fish ▁a kin ▁to ▁ost rac od erm s . ▁Some ▁natural ists ▁even ▁suggested ▁that ▁they ▁were ▁sh elled ▁in verte br ates ▁or ▁even ▁t urt le - like ▁verte br ates . ▁ ▁In ▁the ▁late ▁ 1 9 2 0 s , ▁Dr . ▁Erik ▁St ens i ö , ▁at ▁the ▁Swedish ▁Museum ▁of ▁Natural ▁History ▁in ▁Stockholm ,