1. Among the two-legged theropod dinosaurs of the Cretaceous there are numerous mosaic forms with different features of birds and reptiles. Therefore they are considered to be the best candidates for bird ancestors.
2. The increasing similarity to birds of some species supports the evolutionary theoretical interpretation. All in all the features of the respective species are distributed in a mosaic way to such a high degree that in many cases convergences and reversions (=retrogression) have to be stipulated, even with regard to some key features.
3. The oldest fossil feathers occur in their finished form with Archaeopteryx. Other fossil findings of feather or hair-like structures are clearly younger and their interpretation is being controversially disputed. It is regarded as possible that some of the Cretaceous forms have lost their capability to fly.
4. It is controversial which selection pressures may have fostered the acquirement of feathers and the capability to fly.
5. During the Cretaceous as well as at the beginning of the Tertiary numerous groups of birds emerge out of a sudden and with remarkable discontinuities
The previous text is an abbreviated version of the chapter "The Origin of Birds" from "Evolution—a critical textbook" (http://www.wort-und-wissen.de/lehrbuch.html). Literature references can be found under the following link: http://evolutionslehrbuch.wort-und-wissen.de/anhang/literatur.html, provided in section VI.14.5.