古生物学：鳥類の脳の進化

A well-preserved new fossil bird species that lived 80 million years ago in what is now Brazil offers insights into the evolution of the bird skull and brain. The specimen, which is perched between Archaeopteryx (one of the earliest birds) and modern birds on the avian family tree, displays a combination of modern and ancient features. The findings are published in Nature this week.

Modern birds belong to a clade called Neornithes, but during the age of the dinosaurs another clade existed: the enantiornithines, a very diverse group of early birds that paralleled the adaptive radiation of modern birds and became extinct at the end of the Cretaceous. Few fossils of enantiornithines with complete preserved skulls have been found, leaving a large gap in the understanding of the evolution of bird neuroanatomy.

A complete and undistorted skull of a starling-sized enantiornithine, named Navaornis hestiae, from the Late Cretaceous (about 80 million years ago) of Brazil is described by Luis Chiappe, Guillermo Navalón and colleagues. The skull, which has a skeleton attached, is so well preserved that it allows the reconstruction of the brain, revealing a combination of archaic traits seen in Archaeopteryx and modern features. Primitive features of the brain include a small cerebellum (the part of the brain involved in movement and balance) and a weakly expanded cerebrum (or telencephalon, the largest part of the brain, responsible for higher-level cognition). Resemblances with modern birds include toothlessness, large eyes and a tall, globular skull. The findings help to clarify the timing and order in which distinctively avian neuroanatomical features evolved, the authors conclude.