Among terrestrial vertebrates, only crown birds (Neornithes) rival mammals in terms of relative brain size and behavioural complexity. Relatedly, the anatomy of the avian central nervous system and associated sensory structures, such as the vestibular system of the inner ear, are highly modified with respect to those of other extant reptile lineages. However, a dearth of three-dimensional Mesozoic fossils has limited our knowledge of the origins of the distinctive endocranial structures of crown birds. Traits such as an expanded, flexed brain, a ventral connection between the brain and spinal column, and a modified vestibular system have been regarded as exclusive to Neornithes. Here, we demonstrate all of these ‘advanced’ traits in an undistorted braincase from an Upper Cretaceous enantiornithine bonebed in south-eastern Brazil. Our discovery suggests that these crown bird-like endocranial traits may have originated prior to the split between Enantiornithes and the more crownward portion of avian phylogeny over 140 million years ago, while coexisting with a remarkably plesiomorphic cranial base and posterior palate region. Altogether, our results support the interpretation that the distinctive endocranial morphologies of crown birds and their Mesozoic relatives are affected by complex trade-offs between spatial constraints during development.

MPM-334-1 was scanned using a GE Phoenix Nanotom M at the Molecular Imaging Center of the University of Southern California (USC). MPM-334-1 was scanned at 9.99 μm voxel size, 125 kV, 200 mA, exposure time 750.36 ms, averaging two frames and skipping one frame, 360 degrees rotation 1440 frames and 0.1 mm Cu + Cu filter. The scans were initially reconstructed using GE Phoenix datos|×2 2.3.3.160. 

The three-dimensional reconstruction of the skull was generated in Avizo Lite (9.2). Digital mesh cleaning was conducted using Geomagic (2013). Final imaging of the volumes was conducted using Blender and Avizo Lite (9.2).