▁those ▁of ▁the ▁neck ▁base , ▁the ▁l ungs , ▁and ▁the ▁ab d omen . ▁The ▁double ▁rib ▁heads ▁would ▁indicate ▁a ▁rather ▁st iff ▁thor ax , ▁vent il ated ▁by ▁the ▁g astr alia . ▁A ▁system ▁of ▁hook - like ▁un cin ate ▁processes ▁on ▁the ▁rib s ▁as ▁with ▁the ▁Man ira pt or iformes , ▁allowing ▁the ▁rib c age ▁to ▁move ▁flex ibly , ▁in ▁art ic ulation ▁with ▁an ▁o ss ified ▁stern um , ▁was ▁absent ▁in ▁Sci p iony x . ▁ ▁G row th ▁The ▁hol ot ype ▁of ▁Sci p iony x ▁is ▁a ▁rare ▁example ▁of ▁a ▁non - av ian ▁ther op od ▁h atch ling ; ▁the ▁most ▁important ▁other ▁very ▁young ▁spec im ens ▁are ▁the ▁ch icks ▁of ▁By ron osa urus ▁that ▁however ▁are ▁much ▁less ▁complete . ▁The ▁young ▁age ▁is ▁reflected ▁by ▁the ▁proport ions ▁and ▁the ▁low ▁degree ▁of ▁o ss ification ▁and ▁fusion ▁of ▁several ▁ske let al ▁elements . ▁The ▁most ▁obvious ▁youth ful ▁trait ▁is ▁the ▁relatively ▁large ▁and ▁short ▁head . ▁ ▁Dal ▁S asso ▁& ▁Mag anu co ▁have ▁tried ▁to ▁determine ▁the ▁absolute ▁age ▁of ▁the ▁h atch ling . ▁The ▁fact ▁that ▁the ▁font an elle ▁had ▁not ▁closed ▁yet , ▁pos es ▁an ▁upper ▁age ▁limit ▁of ▁about ▁five ▁weeks . ▁An ▁even ▁lower ▁limit ▁is ▁indicated ▁by ▁the ▁lack ▁of ▁any ▁to oth ▁replacement , ▁which ▁with ▁Arch osa uria ▁begins ▁after ▁a