MPM-334-1: Fossil basicranium and endocranial volumes and CT-stack (Enantiornithes, Avialae)

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.