A large bird from the Early Cretaceous of China: new information on the skull of enantiornithines

We describe the anatomy and bone histology of an enantiornithine specimen from the Early Cretaceous Jehol Group of northeastern China. CNUVB-0903 is larger than most Early Cretaceous enantiornithine birds and also different from all other named taxa. Thus, we erect the new species Zhouornis hani for the new specimen. CNUVB-0903 preserves a suite of new morphologies for the clade. Noteworthy are those of the braincase and occipital region of the skull, which was previously poorly known for enantiornithines. The morphology and placement of the large basipterygoid processes and the well-developed basisphenoid recesscomparable in morphology to those of non-avian dinosaurshighlights the evolutionary conservatism of the enantiornithine skull. The histological characterization of CNUVB-0903 indicates that it was not yet a full-grown individual at the time of death. This, combined with the comparatively large size of the skeleton, supports previous evidence indicating that early in their history, enantiornithines were able to achieve relatively large sizes.