its ▁and ▁literal ▁ext inction . ▁ 3 8 0 - mill ion - year - old ▁foss ils ▁of ▁three ▁other ▁gener a , ▁In cis os cut um , ▁M ater pis cis ▁and ▁Aust ro pty ct od us , ▁represent ▁the ▁oldest ▁known ▁examples ▁of ▁live ▁birth . ▁ ▁In ▁contrast , ▁one ▁ 2 0 1 6 ▁analysis ▁concluded ▁that ▁pla cod erm i ▁are ▁likely ▁mon oph y let ic . ▁ ▁The ▁first ▁ident ifiable ▁pla cod erm s ▁appear ▁in ▁the ▁foss il ▁record ▁during ▁the ▁late ▁L land overy ▁epo ch ▁of ▁the ▁early ▁Sil ur ian . ▁The ▁various ▁groups ▁of ▁pla cod erm s ▁were ▁diverse ▁and ▁abund ant ▁during ▁the ▁Dev onian , ▁but ▁became ▁ext inct ▁at ▁the ▁end - Dev onian ▁H angen berg ▁event ▁ 3 5 8 . 9 ▁million ▁years ▁ago ▁ ▁Character istics ▁Many ▁pla cod erm s , ▁particularly ▁the ▁R hen an ida , ▁Pet al ich thy ida , ▁Ph yl lo le p ida , ▁and ▁Anti arch i , ▁were ▁bottom - d well ers . ▁In ▁particular , ▁the ▁anti arch s , ▁with ▁their ▁highly ▁modified , ▁joint ed ▁b ony ▁p ector al ▁fins , ▁were ▁highly ▁successful ▁inhabitants ▁of ▁Middle - L ate ▁Dev onian ▁fresh water ▁and ▁shall ow ▁marine ▁habit ats , ▁with ▁the ▁Middle ▁to ▁Late ▁Dev onian ▁genus , ▁Both ri ole pis , ▁known ▁from ▁over ▁ 1 0 0 ▁valid