The origins of neural spine elongation in iguanodontian dinosaurs and the osteology of a new sail‐back styracosternan (Dinosauria, Ornithischia) from the Lower Cretaceous Wealden Group of England

The Wealden Group of southern England was deposited during the late Berriasian to early Aptian interval. It records a critical time in the development of iguanodontian dinosaur diversity, which increased from low levels during the Jurassic to higher levels in the Aptian and Albian. A new iguanodontian dinosaur, Istiorachis macarthurae gen. et sp. nov. from the Wessex Formation (Wealden Group) of the Isle of Wight, exhibits hyperelongation of the dorsal and caudal neural spines, suggesting that it possessed a possible sail structure. Ancestral state reconstruction for the relative height of dorsal neural spines in iguanodontians demonstrates that modest elongation began with Ankylopollexia in the Late Jurassic and elongation became established during the Berriasian stage of the Early Cretaceous, albeit with widely disparate values. Hyperelongation of neural spines occurred more sporadically throughout the Cretaceous, being recorded most often in the Barremian and early Aptian. Possible explanations for neural spine elongation in Ankylopollexia include biomechanical advantage, perhaps related to greater mass and a locomotory shift towards quadrupedalism, and visual signalling driven either by sexual selection or species recognition, or both. The function of elongate neural spines was probably pluralistic and differed in different taxa. No single explanation fully supports the variation seen throughout the Cretaceous.