Fil: Dieudonné, Paul E. Universidad de Zaragoza; EspañaFil: Tortosa, Thierry. Réserve Naturelle Nationale Sainte-Victoire; FranciaFil: Fernández Baldor, Fidel T. Municipio de Salas de los Infantes. Museo de Dinosaurios; EspañaFil: Canudo, José I. Universidad de Zaragoza; EspañaFil: Díaz Martínez, Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Díaz Martínez, Ignacio. Universidad Nacional de Río Negro; ArgentinaFil: Díaz Martínez, Ignacio. Instituto de Investigación en Paleobiología y Geología; ArgentinatrueDisarticulated and incomplete remains from a new diminutive ornithopod are described. They come from the Cameros Basin in the north of Spain and were collected from the red clays of the Castrillo de la Reina Formation, ranging from Upper Barremian to Lower Aptian. The new ornithopod described here is slender and one of the smallest ever reported. An up-to-date phylogenetic analysis recovers this taxon as a basal iguanodontian. Its unique combination of characters makes itmore derived than slender ornithopods like Hyphilophodon and Gasparinisaura, and bring very interesting insights into the basal iguanodontian phylogeny. Though possessing a minimum of three premaxillary teeth, this taxon also bears an extensor ilio-tibialis groove on the distal part of its femur. Moreover, its dentary and maxillary teeth are unique, remarkably similar to those regarded as having a "rhabdomorphan" affinity. This unknown taxon is suggested to be a stemtaxon within Rhabdodontidae, a successful clade of basal iguanodonts from the Late Cretaceous of Europe. The Gondwanan ornithopods share the strongest affinities with this family, and we confirm Muttaburrasaurus as a sister taxon of the Rhabdodontidae within a newly defined clade, the Rhabdodontomorpha.