The last two decades have seen a remarkable increase in the known diversity of basal
avialans and their paravian relatives. The lack of resolution in the relationships of
these groups combined with attributing the behavior of specialized taxa to the base of
Paraves has clouded interpretations of the origin of avialan flight. Here, we describe
Hesperornithoides miessleri gen. et sp. nov., a new paravian theropod from the
Morrison Formation (Late Jurassic) of Wyoming, USA, represented by a single adult
or subadult specimen comprising a partial, well-preserved skull and postcranial
skeleton. Limb proportions firmly establish Hesperornithoides as occupying a
terrestrial, non-volant lifestyle. Our phylogenetic analysis emphasizes extensive
taxonomic sampling and robust character construction, recovering the new taxon
most parsimoniously as a troodontid close to Daliansaurus, Xixiasaurus, and
Sinusonasus. Multiple alternative paravian topologies have similar degrees of
support, but proposals of basal paravian archaeopterygids, avialan microraptorians,
and Rahonavis being closer to Pygostylia than archaeopterygids or unenlagiines are
strongly rejected. All parsimonious results support the hypothesis that each early
paravian clade was plesiomorphically flightless, raising the possibility that avian
flight originated as late as the Late Jurassic or Early Cretaceous.