Maniraptora includes birds and their closest relatives among theropod dinosaurs. During the Cretaceous period, several maniraptoran lineages diverged from the ancestral coelurosaurian bauplan and evolved novel ecomorphologies, including active flight, gigantism, cursoriality and herbivory. Propagation X-ray phase-contrast synchrotron microtomography of a well-preserved maniraptoran from Mongolia, still partially embedded in the rock matrix, revealed a mosaic of features, most of them absent among non-avian maniraptorans but shared by reptilian and avian groups with aquatic or semiaquatic ecologies. This new theropod, Halszkaraptor escuilliei gen. et sp. nov., is related to other enigmatic Late Cretaceous maniraptorans from Mongolia in a novel clade at the root of Dromaeosauridae. This lineage adds an amphibious ecomorphology to those evolved by maniraptorans: it acquired a predatory mode that relied mainly on neck hyperelongation for food procurement, it coupled the obligatory bipedalism of theropods with forelimb proportions that may support a swimming function, and it developed postural adaptations convergent with short-tailed birds.

Department of Biological Sciences University of Alberta Edmonton Alberta T6G 2E9 Canada

Department of Zoology and Laboratory of Ornithology Palacký University CS 40220 Olomouc Czech Republic

Directorate Earth and History of Life Royal Belgian Institute of Natural Sciences B 1000 Brussels Belgium

Earth System Science AMGC Vrije Universiteit Brussel B 1050 Brussels Belgium

European Synchrotron Radiation Facility F 38043 Grenoble France

Geological and Palaeontological Museum 'Giovanni Capellini' 1 40126 Bologna Italy

Institute of Palaeontology and Geology Mongolian Academy of Sciences Ulaanbaatar 210 351 Mongolia

Palaeontological Center Mongolian Academy of Sciences Ulaanbaatar 201 351 Mongolia

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