Fossil that fills a critical gap in avian evolution

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Abstract
Despite the discoveries of well-preserved Mesozoic birds1,2,3,4,5, a key part of avian evolution, close to the radiation of all living birds (Aves), remains poorly represented6. Here we report on a new taxon from the Late Cretaceous locality of Ukhaa Tolgod, Mongolia7, that offers insight into this critically unsampled period. Apsaravis and the controversial alvarezsaurids8 are the only avialan9 taxa known from the continental deposits at Ukhaa Tolgod, which have produced hundreds of fossil mammals, lizards and other small dinosaurs7. The new taxon, Apsaravis ukhaana, is the best-preserved specimen of a Mesozoic ornithurine bird discovered in over a century. It provides data important for assessing morphological evolution across Avialae, with implications for, first, the monophyly of Enantiornithes and Sauriurae; second, the proposition that the Mesozoic sister taxa of extant birds, as part of an ‘ecological bottleneck’, inhabited exclusively near-shore and marine environments2; and third, the evolution of flight after its origin.