化石：鳥類の進化を見直すきっかけになるかもしれない白亜紀の鳥の化石

An ancient, toothed bird from the Cretaceous period shares unexpected features with modern birds, such as chickens and ducks, a Nature paper reveals. The discovery challenges a long-standing assumption about the origins of birds, and may prompt a rethink of bird evolution and classification.

All living birds fall into one of two basic groups. Palaeognaths, such as cassowaries, ostriches and tinamous, have rigid skulls and no flexible joint in the palate. Neognaths (comprising all other living birds, including chickens and ducks), have much looser skulls and a joint in the palate. Because bird ancestors, such as theropod dinosaurs, had a fused palate it was previously assumed that birds with a palaeognathous skull gave rise to neognaths. However, this assumption has been difficult to verify because delicate palate elements rarely preserve well in the fossil record. Daniel Field and colleagues describe the partial skeleton of a 67-million-year-old bird, Janavis finalidens, and find that its palate is almost indistinguishable from that of living landfowl and waterfowl. This suggests that birds with a neognathous skull evolved first, and that palaeognaths are derived from them.

Janavis finalidens is the closest known relative of the iconic toothed seabird Ichthyornis, although it would have weighed over twice as much as the largest-known Ichthyornis specimen. The study also demonstrates that the Ichthyornithes clade survived into the terminal stages of the Cretaceous period, and was eventually a victim of the mass extinction of bird-like forms at the end of this time.