New anatomical information on the bohaiornithid Longusunguis and the presence of a plesiomorphic diapsid skull in Enantiornithes

Bohaiornithidae is currently the most diverse recognized family of Early Cretaceous enantiornithines, with unique morphology of the rostrum and pedal digits. Here we describe a second specimen of the bohaiornithid Longusunguis kurochkini from the Jiufotang Formation. This specimen provides new anatomical information regarding this taxon, in particular clarifying uncertain aspects of enantiornithine cranial morphology. The rarely preserved postorbital is completely preserved on both sides of the skull, confirming the presence of a complete postorbital bar in some enantiornithines. This suggests that the plesiomorphic diapsid skull was retained by at least some basal enantiornithines and the infratemporal fenestra in Ornithothoraces may have been lost independently multiple times, providing a better understanding of cranial evolution from non-avian dinosaurs to modern birds.