0 , 0 0 0 ▁species . ▁F oss il ▁divers ity ▁rates ▁make ▁up ▁a ▁larger ▁proportion ▁than ▁ext ant ▁divers ity ▁would ▁suggest ▁with ▁ 1 , 5 9 3 ▁ara chn id ▁species ▁described ▁out ▁of ▁ 1 , 9 5 2 ▁recognized ▁ch el ic er ates . ▁Both ▁ext ant ▁and ▁foss il ▁species ▁are ▁described ▁year ly ▁by ▁research ers ▁in ▁the ▁field ▁( see ▁External ▁links ▁for ▁most ▁recent ▁list ▁of ▁foss il ▁species ). ▁Major ▁develop ments ▁in ▁sp ider ▁evolution ▁include ▁the ▁development ▁of ▁sp inner ets ▁and ▁sil k ▁secret ion . ▁ ▁Early ▁sp ider - like ▁ara chn ids ▁Among ▁the ▁oldest ▁known ▁land ▁ar th rop ods ▁are ▁Tr ig on ot ar b ids , ▁members ▁of ▁an ▁ext inct ▁order ▁of ▁sp ider - like ▁ara chn ids . ▁ ▁Tr ig on ot ar b ids ▁share ▁many ▁super f icial ▁characteristics ▁with ▁sp iders , ▁including ▁a ▁terrest rial ▁l ifest yle , ▁resp iration ▁through ▁book ▁l ungs , ▁and ▁walking ▁on ▁eight ▁legs , ▁with ▁a ▁pair ▁of ▁leg - like ▁ped ip al ps ▁near ▁the ▁mouth ▁and ▁mouth ▁parts . ▁Arg uments ▁still ▁remain ▁open ▁as ▁to ▁whether ▁they ▁possessed ▁the ▁ability ▁to ▁create ▁sil k . ▁This ▁had ▁been ▁a ▁popular ▁thought ▁for ▁quite ▁some ▁time , ▁until ▁an ▁un pub lished ▁foss il ▁was ▁described ▁with ▁distinct ▁micro t uber cles ▁on ▁its ▁h ind ▁legs , ▁a kin ▁to