Figure 1. Genealogical relationships among the main groups of primitive birds. Segments in red represent the span of time for which each of these groups is recorded in the fossil record. The arrow in Ornithuromorpha indicates that the fossil record of these birds extends into the Cenozoic era: all living birds are classified within the Ornithuromorpha.

Figure 2. (a) The archaeopterygid Archaeopteryx lithographica from the Late Jurassic of Germany. (b) The confuciusornithid Confuciusornis sanctus from the Late Jurassic–earliest Cretaceous of China. (c) The alvarezsaurid Shuvuuia deserti from the Late Cretaceous of Mongolia. (d) The enantiornithomorph Cathayornis yandica from the Early Cretaceous of China.

Figure 3. Stratigraphic and geographic distribution of the main groups of primitive birds. Note the relative deficiency of the fossil record of primitive birds from the southern hemisphere as compared to that of the northern hemisphere.