A hidden diversity of ceratopsian dinosaurs in Late Cretaceous Europe

Late Cretaceous Europe was an archipelago with a dinosaur fauna characterised by island effects such as low diversity, relictualism and insular dwarfism. Its dinosaur communities include a unique mix of groups with typical Laurasian or Gondwanan affinities and distinctive endemics. Chief among the latter are rhabdodontids, considered to be early-branching iguanodontians characterised by unusual dental and postcranial features and known from abundant but very incomplete fossil remains. By contrast, unequivocal evidence of horned dinosaurs (ceratopsians) is puzzlingly absent, despite their ubiquitous occurrence in contemporary ecosystems of Asia and North America. Ajkaceratops from the Late Cretaceous of Hungary was described as the first definite ceratopsian from Europe, but this identification has been strongly disputed. Here, we describe new material of Ajkaceratops and conduct phylogenetic analyses that support its ceratopsian affinities. Our results unexpectedly demonstrate that some ‘rhabdodontid’ taxa are not, in fact, iguanodontians but actually ceratopsians. This suggests a substantial but previously hidden diversity and evolutionary history of European horned dinosaurs, and co-occurrence of iguanodontians and ceratopsians indicates greater similarity than previously appreciated to other Laurasian ecosystems. Our results challenge conventional understanding of ornithischian dinosaur evolution and indicate the need for a fundamental re-evaluation of the Late Cretaceous herbivorous dinosaur assemblages of Europe.