Evolution: The dawn of modern birds

A newly discovered fossil bird that may represent the oldest ‘modern’ bird reported to date is described in Nature this week, offering insights into the timing of bird diversification. The fossil skull, dated to between 66.8 and 66.7 million years ago, has features seen in present-day ducks and chickens. This finding indicates that modern birds emerged just before the end-Cretaceous mass-extinction event, which was caused by the impact of a large asteroid or comet.

The early evolution of crown birds (the group of the common ancestor of all living birds, and all its descendants, whether living or extinct) is poorly understood, owing to gaps in the fossil record. In particular, the Mesozoic era (around 250 - 66 million years ago) has only one well-supported example, and several fragmented fossils that have ambiguous links to the crown group. A new Mesozoic fossil bird named Asteriornis maastrichtensis, found in the Maastricht Formation of Belgium, may help to fill this gap.

Asteriornis is a well-preserved fossil that includes a nearly complete three-dimensional skull, report Daniel Field and colleagues. The fossil exhibits a combination of landfowl-like and waterfowl-like features, such as a beak that resembles that of modern landfowl. Its name, Asteriornis, is derived from Asteria - a Greek goddess of falling stars who turns into a quail - reflecting both the impending end-Cretaceous asteroid impact and the affinity of Asteriornis to fowl birds. The authors estimate that the bird weighed just under 400 grams. This relatively small size, together with its provenance from marine sediments, indicates that Asteriornis may have been a shore-dwelling bird, which supports a hypothesis of shorebird-like origins for much of the diversity of crown birds