The origin of birds from theropod dinosaurs, by any measures, is the most eye-catching evolutionary transition in the history of life, which encompasses numerous extensive morphological and biological changes. Compared to postcranium, little progress has been made regarding the evolutionary assemblage of the birds’ skull, because of few detailed early records of cranial materials of stem lineages. Anchiornis is the oldest known record of the Paraves (~160 Ma), the most inclusive clade that contains all living birds but not Caudipteryx or Epidexipteryx. With hundreds of known specimens, Anchiornis constitutes an ideal taxon for investigating morphological modifications across the theropod-bird transition, but its cranial morphology remains enigmatic. Here we present in-depth description of the cranial morphology of Anchiornis based on three-dimensional reconstruction of a well-preserved specimen, including elements from the temporal and palatal regions that are poorly recognized previously. Our study shows that Anchiornis retains the plesiomorphic dinosaurian condition in having a diapsid akinetic skull. The mixture of cranial characters, shared with dromaeosaurids, troodontids, and stemward avialans, present in Anchiornis demonstrates the complex history of early avialan cranial evolution.
