Until 1990, there was a long gap in the fossil record of birds between Archaeopteryx, dated at 150 Myr ago, and Hesperornis and Ichthyornis from the Upper Cretaceous of North America, dated at 90-65 Myr ago. This time span represents the first half of the history of birds. New discoveries from the Lower Cretaceous of China, Mongolia, Spain and other parts of the world have helped to fill this gap, and they have revealed the existence of several unique Cretaceous bird lineages, and especially the radiation of a diverse group, the Enantiornithes. The new discoveries, and older materials, are arranged here according to the phylogenetic tree of Chiappe (2002b) (see Box 9.2).
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