Late Cretaceous non-avian dinosaurs from the James Ross Basin,Antarctica:description of new material,updated synthesis,biostratigraphy,and paleobiogeography

Although the fossil record of non-avian dinosaurs from the Cretaceous of Antarctica is the poorest of any continent,fossils representing at least five major taxonomic groups(Ankylosauria,early-diverging Ornithopoda,Hadrosauridae,Titanosauria,and Theropoda)have been recovered.All come from Upper Cretaceous(Coniacian–Maastrichtian)marine and nearshore deposits belonging to the Gustav and Marambio groups of the James Ross Basin at the northern tip of the Antarctic Peninsula.The majority of these finds have come from the Campanian–Maastrichtian Snow Hill Island and López de Bertodano formations of James Ross and Vega islands.Given the rarity of Antarctic Cretaceous non-avian dinosaurs,discoveries of any fossils of these archosaurs,no matter how meager,are of significance.Here we describe fragmentary new ornithischian(ankylosaur and ornithopod)material from the upper Campanian–lower Maastrichtian Cape Lamb Member of the Snow Hill Island Formation and the Maastrichtian Sandwich Bluff Member of the López de Bertodano Formation.One of these specimens is considered to probably pertain to the holotypic individual of the early-diverging ornithopod Morrosaurus antarcticus.We also provide an up-to-date synthesis of the Late Cretaceous non-avian dinosaur record of the James Ross Basin and analyze the biostratigraphic occurrences of the various finds,demonstrating that most(including all named taxa and all reasonably complete skeletons discovered to date)occur within a relatively condensed temporal interval of the late Campanian to early Maastrichtian.Most or all James Ross Basin dinosaurs share close affinities with penecontemporaneous taxa from Patagonia,indicating that at least some continental vertebrates could disperse between southern South America and Antarctica during the final stages of the Mesozoic.