Cretaceous bird from Brazil informs the evolution of the avian skull and brain

Abstract

A dearth of Mesozoic-aged, three-dimensional fossils hinders understanding of theorigin of the distinctive skull and brain of modern (crown) birds. Here we report Navaornis hestiae gen. et sp. nov., an exquisitely preserved fossil species from the Late Cretaceous of Brazil. The skull of Navaornis is toothless and large-eyed, with a vaulted cranium closely resembling the condition in crown birds; however, phylogenetic analyses recover Navaornis in Enantiornithes, a highly diverse clade of Mesozoic stem birds. Despite an overall geometry quantitatively indistinguishable from crown birds, the skull of Navaornis retains numerous plesiomorphies including a maxilla-dominatedrostrum, an akinetic palate, a diapsid temporal coniguration, a small cerebellum anda weakly expanded telencephalon. These archaic neurocranial traits are combinedwith a crown bird-like degree of brain lexion and a bony labyrinth comparable inshape to those of many crown birds but substantially larger. Altogether, the emergentcranial geometry of Navaornis shows an unprecedented degree of similarity betweencrown birds and enantiornithines, groups last sharing a common ancestor more than 130 million years ago. Navaornis provides long-sought insight into the detailedcranial and endocranial morphology of stem birds phylogenetically crownward of Archaeopteryx, clarifying the pattern and timing by which the distinctiveneuroanatomy of living birds was assembled.