Abstract
THE origin of birds from theropod dinosaurs1 is now broadly accepted, but there is little agreement as to which group of theropods is closest to avian ancestry2. Here we report the discovery of a fragmentary skull in a collection of Late Cretaceous verte-brates from Mongolia. The skull presents a novel combination of theropod and primitive avian characters, which suggests that it belongs to the closest of the known non-avian relatives of Archaeopteryx and other birds. The new fossil shows consistent similarities to the troodontid theropods, for whom close avian relationships have been proposed3, and to Baryonyx4 and Spinosaurus5, two unusual theropods from the Lower Cretaceous of England and the lowermost Upper Cretaceous of Africa, respec-tively.
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