The earliest crocodylians are known primarily from the Late Cretaceous of North America and Europe. The representatives of Gavialoidea and Alligatoroidea are known in the Late Cretaceous of both continents, yet the biogeographic origins of Crocodyloidea are poorly understood. Up to now, only one representative of this clade has been known from the Late Cretaceous, the basal crocodyloid Prodiplocynodon from the Maastrichtian of North America. The fossil studied is a skull collected from sandstones in the lower part of the Tremp Formation, in Chron C30n, dated at −67.6 to 65.5 Ma (late Maastrichtian), in Arén (Huesca, Spain). It is located in a continuous section that contains the K/P boundary, in which the dinosaur faunas closest to the K/P boundary in Europe have been described, including Arenysaurus ardevoli and Blasisaurus canudoi. Phylogenetic analysis places the new taxon, Arenysuchus gascabadiolorum, at the base of Crocodyloidea. The new taxon is the oldest crocodyloid representative in Eurasia. Crocodyloidea had previously only been known from the Palaeogene onwards in this part of Laurasia. Phylogenetically, Arenysuchus gascabadiolorum is situated at the base of the first radiation of crocodyloids that occurred in the late Maastrichtian, shedding light on this part of the cladogram. The presence of basal crocodyloids at the end of the Cretaceous both in North America and Europe provides new evidence of the faunal exchange via the Thulean Land Bridge during the Maastrichtian.

This work attempts at providing a revised framework for ornithischian phylogeny, based on an exhaustive data compilation of already published analyses, a critical re-evaluation of osteological characters and an in-depth checking of characters scoring to fix mistakes that have accumulated in previous analyses; we have also included recently described basal ornithischians, marginocephalians and ornithopods. ‘Heterodontosaurids’ are recovered as a paraphyletic group of basal Marginocephalia that progressively lead to the dome-headed ‘true’ pachycephalosaurs. ‘Heterodontosaurids’ consequently fall within Pachycephalosauria sensu Sereno, 1998. The reconfiguration of basal cerapodan relationships pulls the origins of ornithopods to the earliest stages of the Jurassic. Based on the present analysis, we also discuss ornithopod relationships, with a particular focus on basal Iguanodontia. Tenontosaurus is found as the basalmost iguanodontian. The monophyly of Rhabdodontomorpha in a position more derived than Tenontosaurus is supported by the present analysis.

Ornithischians form a large clade of globally distributed Mesozoic dinosaurs, and represent one of their three major radiations. Throughout their evolutionary history, exceeding 134 million years, ornithischians evolved considerable morphological disparity, expressed especially through the cranial and osteodermal features of their most distinguishable representatives. The nearly two-century-long research history on ornithischians has resulted in the recognition of numerous diverse lineages, many of which have been named. Following the formative publications establishing the theoretical foundation of phylogenetic nomenclature throughout the 1980s and 1990s, many of the proposed names of ornithischian clades were provided with phylogenetic definitions. Some of these definitions have proven useful and have not been changed, beyond the way they were formulated, since their introduction. Some names, however, have multiple definitions, making their application ambiguous. Recent implementation of theInternational Code of Phylogenetic Nomenclature(ICPN, orPhyloCode) offers the opportunity to explore the utility of previously proposed definitions of established taxon names. Since the Articles of theICPNare not to be applied retroactively, all phylogenetic definitions published prior to its implementation remain informal (and ineffective) in the light of the Code. Here, we revise the nomenclature of ornithischian dinosaur clades; we revisit 76 preexisting ornithischian clade names, review their recent and historical use, and formally establish their phylogenetic definitions. Additionally, we introduce five new clade names: two for robustly supported clades of later-diverging hadrosaurids and ceratopsians, one uniting heterodontosaurids and genasaurs, and two for clades of nodosaurids. Our study marks a key step towards a formal phylogenetic nomenclature of ornithischian dinosaurs.

Un nouveau dinosaure hadrosauridé, Arenysaurus ardevoli gen. et sp. nov., du Maastrichtien supérieur d’Arén (Huesca, Pyrénées méridionales) est décrit ici à partir d’un crâne partiel articulé, de restes mandibulaires et d’éléments postcrâniens, y compris des vertèbres et des os des ceintures et des membres. Arenysaurus est caractérisé par un dôme frontal très proéminent, par des processus postcotyloïde (squamosal) et jugal (postorbital) verticalisés et par une crête deltopectorale de l’humérus, orientée antérieurement. De plus, Arenysaurus montre une combinaison unique de caractères : frontal court (longueur/largeur environ 0,5) ; crête médiane du pariétal située au niveau de la barre postorbital-squamosal ; pariétal exclu de l’occiput ; squamosal bas au-dessus de la cavité cotyloïde. Une analyse phylogénétique montre qu’ Arenysaurus est un lambéosauriné plutôt basal et le groupe frère d’ Amurosaurus et du clade Corythosaurini-Parasaurolophini. Les relations phylogénétiques et biogéographiques d’ Arenysaurus et d’autres lambéosaurinés suggèrent une connexion paléogéographique entre l’Asie et l’Europe, au cours du Crétacé supérieur.

La Cantalera: an exceptional window onto the vertebrate biodiversity of the Hauterivian-Barremian transition in the Iberian Peninsula