Highlights d Dinosaurs and early birds had similar relative brain sizes d Major shifts in brain-body integration occur in the aftermath of the K-Pg extinction d Rates of brain-body evolution are highest in non-avian dinosaurs, early-diverging birds, parrots, and crows d Corvids, like hominins, evolved larger relative brains and bodies simultaneously

Plotopterids (Aves: Plotopteridae) are extinct flightless birds that were endemic to the North Pacific Ocean. As flightless, wing‐propelled diving birds they exhibit similar skeletal morphology to Sphenisciformes (penguins), especially in their wings. In contrast to the similarity, Plotopteridae have been placed in (traditional) Pelecaniformes in most palaeontological and phylogenetic studies, based on shared characters that are absent in penguins. The postcranial morphology of Plotopteridae has been well studied, but little is known about the cranial morphology, particularly the nervous system. The brain morphology of Plotopteridae, compared with other water birds, could prompt a reconsideration of those previous phylogenetic hypotheses, as the cranial morphology is conservative and could provide powerful signals for the phylogenetic reconstruction. In order to compare the brain morphology of Plotopteridae with that in other water birds (Ciconiiformes, Pelecaniformes, Suliformes, Procellariiformes, and Sphenisciformes), we generated virtual endocasts of Plotopteridae and extant water birds. We investigated the brain morphology of those birds using three‐dimensional geometric morphometric and linear measuring methods. The width of the cerebellum and the length of the floccular lobe varied considerably among water birds, and the relative lengths separate Procellariiformes + Sphenisciformes from Ciconiiformes + Pelecaniformes + Suliformes. The former group had a relatively wider cerebellum and longer floccular lobe, whereas the latter group had a relatively narrower cerebellum and shorter floccular lobe. The relative width of the cerebellum and length of the floccular lobe in Plotopteridae was comparable with that of the former group, in addition to many morphological similarities to the Sphenisciformes brain. On the basis of brain morphology alone, we dare not conclude that Plotopteridae forms a clade with, or belongs to, Sphenisciformes; however, the brain configuration of Plotopteridae is distinctly close to that of penguins, and could possibly reflect their phylogenetic relationship. © 2013 The Linnean Society of London

There is wide variation in brain shape among birds. Differences in brain dimensions reflect species-specific sensory capacities and behavioral repertoires that are shaped by environmental and biological factors during evolution. Most previous studies aimed at defining factors impacting brain shape have used volumetric or linear measurements. However, few have explored the quantitative indices of three-dimensional (3D) brain geometry that are absolutely imperative to understanding avian evolutionary history. This study aimed: (i) to explore the relationship between brain shape and overall brain size; and (ii) to assess the relationship between brain shape and orbital shape. Avian brain endocasts were reconstructed from computed tomography images and analyzed using 3D geometric morphometrics. Principal component analysis revealed dominant regional variations in avian brain shape and shape correlations between the telencephalon and cerebellum, between the cerebellum and myelencephalon, and between the diencephalon and optic tectum. Brain shape changes relative to total brain size were determined by multivariate regression analysis. Larger brain size was associated with a relatively slender telencephalon and differences in brain orientation. The correlation between brain shape and orbital shape was assessed by two-block partial least-squares analysis. Relatively round brains with a ventrally flexed brain base were associated with rounder orbits, while narrower brains with a flat brain base were associated with more elongated orbits. The shapes of functionally associated avian brain regions are correlated, and orbital size and shape are dominant factors influencing the overall shape of the avian brain.