• Archaeopteryx;
  • computed tomography;
  • Neuroanatomy;
  • Odontopteryx;
  • Prophaethon

Investigation of how the avian brain evolved to its present state is informative for studies of the theropod–bird transition, and as a parallel to mammalian brain evolution. Neurological anatomy in fossil bird species can be inferred from endocranial casts, but such endocasts are rare. Here, we use computed tomographic analysis to determine the state of brain anatomy in two marine birds from the Lower Eocene London Clay Formation of England. The brains of Odontopteryx (Odontopterygiformes) and Prophaethon (Pelecaniformes) are remarkably similar to those of extant seabirds, and probably possessed similar somatosensory and motor capabilities. Each virtual endocast exhibits a degree of telencephalic expansion comparable to living avian species. However, the eminentia sagittalis (wulst), a feature characteristic of all living birds, is poorly developed. Our findings support the conclusion that much of the telencephalic expansion of modern birds was complete by the end of the Mesozoic, but that overall telencephalic volume has increased throughout the Cenozoic through dorsal expansion of the eminentia sagittalis. We suggest that improvements in cognition relating to telencephalic expansion may have provided neornithine avian clades with an advantage over archaic lineages at the Cretaceous–Tertiary boundary, explaining their survival and rapid diversification in the Cenozoic. © 2009 The Natural History Museum. Journal compilation © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155, 198–219.