Furthermore , it appear that Iguanodon became more quadrupedal as it got older and heavier ; juvenile I. bernissartensis have shorter arms than adults ( 60 % of hindlimb length versus 70 % for adults ) . When walking as a quadruped , the animal 's hands would have been held so that the palms faced each other , as shown by iguanodontian trackways and the anatomy of this genus 's arms and hands . The three toed pes ( foot ) of Iguanodon was relatively long , and when walking , both the hand and the foot would have been used in a digitigrade fashion ( walking on the fingers and toes ) . The maximum speed of Iguanodon have been estimated at 24 km / h ( 15 mph ) , which would have been as a biped ; it would not have been able to gallop as a quadruped .