▁as ▁Fal lac ost eus ▁and ▁R olf ost eus , ▁both ▁of ▁the ▁G ogo ▁Form ation ▁of ▁Western ▁Australia , ▁had ▁stream lined , ▁bullet - sh aped ▁head ▁arm or , ▁strongly ▁supporting ▁the ▁idea ▁that ▁many , ▁if ▁not ▁most , ▁ar th rod ires ▁were ▁active ▁sw imm ers , ▁rather ▁than ▁pass ive ▁amb ush - h un ters ▁whose ▁arm or ▁pract ically ▁anch ored ▁them ▁to ▁the ▁sea ▁floor . ▁Some ▁pla cod erm s ▁were ▁her b iv orous , ▁such ▁as ▁the ▁Middle ▁to ▁Late ▁Dev onian ▁ar th rod ire ▁Hol on ema , ▁and ▁some ▁were ▁pl ank t iv ores , ▁such ▁as ▁the ▁gig antic , ▁ ▁long ▁ar th rod ire , ▁T itan ich th ys . ▁ ▁Ext ra ordinary ▁evidence ▁of ▁internal ▁fert il ization ▁in ▁a ▁pla cod erm ▁was ▁afford ed ▁by ▁the ▁discovery ▁in ▁the ▁G ogo ▁Form ation , ▁near ▁Fitz roy ▁Cross ing , ▁Kim ber ley , ▁Western ▁Australia , ▁of ▁a ▁small ▁female ▁pla cod erm , ▁about ▁ ▁in ▁length , ▁which ▁died ▁in ▁the ▁process ▁of ▁giving ▁birth ▁to ▁a ▁ 6   cm ▁(   in ) ▁off spring ▁and ▁was ▁foss il ized ▁with ▁the ▁um bil ical ▁cord ▁int act . ▁The ▁foss il , ▁named ▁M ater pis cis ▁at ten borough i ▁( after ▁scient ist ▁David ▁Att en borough ), ▁had ▁eggs ▁which ▁were ▁fert il ized ▁internally , ▁the