This chapter explored the evolution of birds from dinosaurs and provided a framework for understanding their origins in the overall context of the world of the dinosaurs.
1. Birds are the surviving descendants of dinosaurs and retain many anatomical traits that first evolved in their non-flying ancestors.
2. Archaeopteryx, the first true bird, retained many archaic anatomical features—such as teeth, a long and bony tail, a cartilaginous breastbone, and clawed, underdeveloped wings— that are not seen in modern birds.
3. Traits shared by birds include lightweight skeletons with hollow bones; a furcula (or wishbone); a short, bony tail tipped with a pygostyle; feathers; wings; bipedal posture; and four clawed toes.
4. The first birds evolved from non-avian maniraptoran dinosaurs.
5. Feathers first appeared on non-avian theropods, probably as a way to insulate the body.
6. Two main hypotheses of the evolution of flight include the "trees-down" and "ground-up" concepts. The "trees-down" concept is most widely accepted by paleontologists.
7. The basal Avialae is divided into four principal groups, from least to most like modern birds; Archaeopterygidae, Confu-ciusornithidae, Enantiornithes, and Ornithuromorpha, the latter including three main subgroups: Hesperornithes, Ich-thyornithiformes, and Neornithes.
Ornithiscian Dinosaurs of the Cretaceous Period
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