Anatomy of a New Specimen of <i>Piscivorenantiornis Inusitatus</i> (Aves: Enantiornithes) from the Lower Cretaceous Jehol Biota

We describe a new enantiornithine specimen from the Lower Cretaceous Jiufotang Formation of the Jehol Biota, northeastern China, which can be assigned to the species Piscivorenantiornis inusitatus. The new specimen confirms the presence of the unusual articulation of the cervical vertebrae that characterizes P. inusitatus. The disarticulated bony elements in the new specimen, some of which are preserved in different views or completely missing from the holotype, enable a more comprehensive morphological reconstruction and a refined phylogenetic assessment of P. inusitatus. The new fossil demonstrates that the occipitals are recessed by a pair of fossae lateral to the foramen magnum, a feature otherwise unknown among Mesozoic and extant birds. Our phylogenetic analysis tentatively resolves P. inusitatus as the sister taxon to the clade consisting of Pterygornis dapingfangensis and Mirusavis parvus, which is recovered deeply nested within the Enantiornithes. We hypothesize that the craniolateral processes of the sternum are a derived feature in enantiornithines.