Fossil basicranium clarifies the origin of the avian central nervous system and inner ear

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 southeastern 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 Ma, 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.

Automated summary

Birds and mammals have comparable relative brain sizes and behavioral complexity. The avian central nervous system and vestibular system are different from other reptiles. A recent discovery of an undistorted braincase from the Late Cretaceous in Brazil shows that early birds had similar endocranial traits to crown birds, suggesting these traits originated before the split between Enantiornithes and crown birds over 140 million years ago. This finding supports the idea that the distinct endocranial morphologies of crown birds and their Mesozoic relatives are influenced by spatial constraints during development.