▁still ▁debate ▁about ▁whether ▁lo b op ods ▁are ▁mon oph y let ic , ▁i . e . ▁whether ▁they ▁include ▁all ▁and ▁only ▁the ▁descend ants ▁of ▁a ▁single ▁common ▁ancest or , ▁and ▁about ▁whether ▁ar th rop ods ▁are ▁a ▁sub - group ▁of ▁lo b op ods ▁or ▁a ▁sister - group . ▁ ▁While ▁this ▁discussion ▁about ▁specific ▁foss ils ▁such ▁as ▁Op ab in ia ▁and ▁An om al oc aris ▁was ▁going ▁on , ▁the ▁concept ▁of ▁stem ▁groups ▁was ▁introduced ▁to ▁cover ▁evolution ary ▁" a un ts " ▁and ▁" c ous ins ". ▁A ▁crown ▁group ▁is ▁a ▁group ▁of ▁closely ▁related ▁living ▁animals ▁plus ▁their ▁last ▁common ▁ancest or ▁plus ▁all ▁its ▁descend ants . ▁A ▁stem ▁group ▁contains ▁off sh o ots ▁from ▁members ▁of ▁the ▁line age ▁earlier ▁than ▁the ▁last ▁common ▁ancest or ▁of ▁the ▁crown ▁group ; ▁it ▁is ▁a ▁relative ▁concept , ▁for ▁example ▁tard igr ades ▁are ▁living ▁animals ▁that ▁form ▁a ▁crown ▁group ▁in ▁their ▁own ▁right , ▁but ▁Bud d ▁( 1 9 9 6 ) ▁regarded ▁them ▁also ▁as ▁being ▁a ▁stem ▁group ▁relative ▁to ▁the ▁ar th rop ods . ▁View ing ▁strange - looking ▁organ isms ▁like ▁Op ab in ia ▁in ▁this ▁way ▁makes ▁it ▁possible ▁to ▁see ▁that , ▁while ▁the ▁Camb rian ▁explos ion ▁was ▁unusual , ▁it ▁can ▁be ▁understood ▁in ▁terms ▁of ▁normal ▁evolution ary ▁processes . ▁ ▁See ▁also ▁ ▁References ▁ ▁Further ▁reading