▁Car bon ifer ous ▁( Ste phan ian ) ▁age , ▁about ▁ 3 0 5 – 2 9 9 ▁million ▁years ▁old . ▁Mont ce au ▁foss ils ▁are ▁generally ▁preserved ▁in ▁such ▁a ▁way ▁that ▁fine ▁details ▁can ▁be ▁observed ▁and ▁three - dimensional ▁analysis ▁is ▁possible . ▁In ▁the ▁case ▁of ▁Id mon ar ach ne , ▁computer ized ▁tom ography ▁was ▁used ▁to ▁construct ▁a ▁" virtual ▁foss il ". ▁ ▁Description ▁The ▁total ▁body ▁length ▁of ▁the ▁foss il ▁is ▁around ▁ 1 0 . 5   mm , ▁with ▁the ▁preserved ▁part ▁of ▁the ▁car ap ace ▁of ▁the ▁ce phal oth or ax ▁about ▁ 5   mm ▁long ▁and ▁the ▁op is th os oma ▁( ab d omen ) ▁about ▁ 6   mm ▁long . ▁The ▁eight ▁walking ▁legs ▁are ▁more - or - less ▁uniform ▁in ▁appearance , ▁with ▁the ▁fourth ▁leg ▁longest ▁at ▁about ▁ 8 . 5   mm ▁and ▁the ▁first ▁short est ▁at ▁about ▁ 6 . 5   mm . ▁The ▁legs ▁terminate ▁in ▁at ▁least ▁two ▁cla ws . ▁The ▁two ▁ped ip al ps ▁are ▁slightly ▁shorter ▁than ▁the ▁legs , ▁at ▁just ▁under ▁ 4   mm ▁long . ▁The ▁ch el ic era e ▁are ▁sh aped ▁like ▁a ▁cl asp - kn ife , ▁i . e . ▁with ▁the ▁f ang ▁at ▁the ▁end ▁fol ding ▁back ▁into ▁the ▁part ▁to ▁which ▁it ▁is ▁attached . ▁The ▁op is th os