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Abstract
Non-avian dinosaurs are mostly medium to large-sized animals, and to date all known
mature specimens are larger than the most primitive bird, Archaeopteryx. Here we report
on a new dromaeosaurid dinosaur, Microraptor zhaoianus gen. et sp. nov., from the
Early Cretaceous Jiufotang Formation of Liaoning, China. This is the first mature
non-avian dinosaur to be found that is smaller than Archaeopteryx, and it eliminates
the size disparity between the earliest birds and their closest non-avian theropod
relatives. The more bird-like teeth, the Rahonavis-like ischium and the small number
of caudal vertebrae of Microraptor are unique among dromaeosaurids and improve our
understanding of the morphological transition to birds. The nearly completely articulated
foot shows features, such as distally positioned digit I, slender and recurved pedal
claws, and elongated penultimate phalanges, that are comparable to those of arboreal
birds. The discovery of these in non-avian theropods provides new insights for studying
the palaeoecology of some bird-like theropod dinosaurs.