Locomotor reassessment of Iguanodon, Ouranosaurus and Lurdusaurus (Dinosauria, Ornithopoda) using osteological correlates

Large ornithopod dinosaurs have been successively described as quadrupeds, bipeds and facultative bipeds. Here we study the case of three ankylopollexians ( Iguanodon bernissartensis , Ouranosaurus nigeriensis and Lurdusaurus arenatus ) for which locomotion remains debated. We examine in detail their appendicular skeleton, focusing on osteological features that were previously associated with either bipedality or quadrupedality in dinosaurs. We compare our observations with data obtained for 12 saurischians and 32 ornithischians whose bipedal or quadrupedal locomotion is unambiguous. Iguanodon and Ouranosaurus score as quadrupeds for nine of the eleven correlates used in this study and are therefore best interpreted as obligate quadrupeds—contrasting with previous hypotheses of facultative bipedalism in Ankylopollexia—although this is sensitive to how hadrosaur locomotion is assessed. Lurdusaurus is found among quadrupeds for all the osteological correlates employed and is thus considered as an obligate quadruped, implying that obligate quadrupedality in ornithopods arose long before the hadrosaurs. Moreover, its extreme humeral and femoral eccentricity, along with its very short metatarsus, indicate graviportality, making Lurdusaurus the first graviportal ornithopod. Further biomechanical investigations are definitely needed to fully appreciate the locomotion of this unique taxon. Osteological correlates appear to be useful tools for investigating the transition to quadrupedalism in ornithopods, and may provide a basis for more comprehensive reconstructions of locomotor and postural evolution across diverse dinosaur lineages.