High-density 3-D coordinate data of avian and non-avian dinosaur endocasts

How do large and unique brains evolve? Historically, comparative
neuroanatomical studies have attributed the evolutionary genesis of highly
encephalized brains to deviations along, as well as from, conserved
scaling relationships among brain regions. However, the relative
contributions of these concerted (integrated) and mosaic (modular)
processes as drivers of brain evolution remain unclear, especially in
non-mammalian groups. While proportional brain sizes have been the
predominant metric used to characterize brain morphology to date, we
perform a high-density geometric morphometric analysis on the encephalized
brains of crown birds (Neornithes or Aves) compared to their stem taxa—the
non-avialan coelurosaurian dinosaurs. When analyzed together with
developmental neuroanatomical data of model archosaurs
(Gallus, Alligator), crown birds exhibit a distinct allometric
relationship that dictates their brain evolution and development.
Furthermore, analyses by neuroanatomical regions reveal that the
acquisition of this derived shape-to-size scaling relationship occurred in
a mosaic pattern, where the ‘avian’-grade optic lobe and cerebellum
evolved first among non-avialan dinosaurs, followed by major changes to
the evolutionary and developmental dynamics of cerebrum shape after the
origin of Avialae. Notably, the brain of crown birds is a more integrated
structure than non-avialan archosaurs, implying that diversification of
brain morphologies within Neornithes proceeded in a more coordinated
manner, perhaps due to spatial constraints and abbreviated growth period.
Collectively, these patterns demonstrate a plurality in evolutionary
processes that generate encephalized brains in archosaurs and across
vertebrates.