m ▁itself ▁or ▁its ▁position ▁could ▁not ▁be ▁directly ▁observed . ▁Many ▁bird ▁li vers ▁are ▁large ▁too , ▁showing ▁that ▁such ▁a ▁trait ▁is ▁compatible ▁with ▁an ▁air - s ac ▁system . ▁The ▁small ▁body ▁cav ity ▁in ▁front ▁of ▁the ▁h alo ▁seemed ▁to ▁indicate ▁the ▁presence ▁of ▁small ▁st iff ▁bird - like ▁l ungs . ▁The ▁pres umed ▁M . ▁dia ph rag m atic us ▁was ▁shown ▁to ▁be ▁an ▁artifact ▁caused ▁by ▁the ▁pol ishing ▁and ▁eng ra ving ▁of ▁cal cite ▁nod ules ▁of ▁non - organ ic ▁origin ▁during ▁prepar ation , ▁creating ▁the ▁ill usion ▁of ▁mus cle ▁fib res . ▁ ▁The ▁empty ▁space ▁between ▁the ▁pub ic ▁b ones ▁and ▁the ▁int est ines ▁has ▁by ▁G . S . ▁Paul ▁and ▁David ▁Mart ill ▁been ▁hypoth es ised ▁to ▁have ▁been ▁the ▁location ▁of ▁a ▁large ▁air - s ac . ▁Dal ▁S asso ▁& ▁Mag anu co ▁however , ▁rejected ▁this ▁interpretation ▁because ▁with ▁living ▁birds ▁the ▁air - s ac ▁of ▁the ▁posterior ▁ab d omen ▁does ▁not ▁force ▁the ▁int est ines ▁for wards . ▁They ▁considered ▁the ▁space ▁more ▁likely ▁to ▁have ▁been ▁filled ▁by ▁a ▁large ▁y olk - s ac . ▁Air - s acs ▁were ▁never theless ▁probable ▁given ▁the ▁p neum at isation ▁of ▁the ▁verte bra e . ▁Ver te bra e ▁without ▁p neum at op ores ▁would ▁have ▁indicated ▁the ▁boundaries ▁between ▁three ▁air - s ac ▁systems :