▁saw ▁between ▁Op ab ini as ▁lo be + g ill ▁arrangement ▁and ▁ar th rop ods ' ▁bir am ous ▁lim bs ▁led ▁them ▁to ▁conclude ▁that ▁Op ab in ia ▁was ▁very ▁closely ▁related ▁to ▁ar th rop ods . ▁In ▁fact ▁they ▁presented ▁a ▁family ▁tree ▁very ▁similar ▁to ▁Bud d ' s ▁except ▁that ▁their s ▁did ▁not ▁mention ▁tard igr ades . ▁ ▁The oret ical ▁significance ▁▁ ▁Op ab in ia ▁made ▁it ▁clear ▁how ▁little ▁was ▁known ▁about ▁soft - b od ied ▁animals , ▁which ▁do ▁not ▁usually ▁leave ▁foss ils . ▁When ▁Wh itt ington ▁described ▁it ▁in ▁the ▁mid - 1 9 7 0 s , ▁there ▁was ▁already ▁a ▁vig orous ▁debate ▁about ▁the ▁early ▁evolution ▁of ▁animals . ▁Pr eston ▁Cloud ▁argued ▁in ▁ 1 9 4 8 ▁and ▁ 1 9 6 8 ▁that ▁the ▁process ▁was ▁" expl os ive ", ▁and ▁in ▁the ▁early ▁ 1 9 7 0 s ▁N iles ▁E ld red ge ▁and ▁Stephen ▁Jay ▁G ould ▁developed ▁their ▁theory ▁of ▁pun ctu ated ▁equilibrium , ▁which ▁views ▁evolution ▁as ▁long ▁intervals ▁of ▁near - st asis ▁" p unct u ated " ▁by ▁short ▁periods ▁of ▁rapid ▁change . ▁On ▁the ▁other ▁hand , ▁around ▁the ▁same ▁time ▁Wy att ▁Dur ham ▁and ▁Martin ▁Gla ess ner ▁both ▁argued ▁that ▁the ▁animal ▁kingdom ▁had ▁a ▁long ▁Pro ter oz o ic ▁history ▁that ▁was ▁hidden ▁by ▁the ▁lack ▁of ▁foss ils . ▁Wh