Chapter objectives

• Understand birds as theropod dinosaurs

• Learn about the origin of flight

• Refine our cladograms as a tool for investigating evolution

Birds

Birds are dinosaurs

Birds are dinosaurs. We don't mean that they are related to dinosaurs – although, if they are dinosaurs, they must be related them. We don't mean that they come from dinosaurs – although they obviously evolved from something that was itself a dinosaur. We mean that birds are dinosaurs, a statement that, as this chapter unfolds, will be no more radical than saying that humans are mammals.

So how do we figure out who birds are related to? The same way that we explored in Chapter 3: using diagnostic characters. Here we choose those features that might be easily observed in a fossil.

Diagnostic features of living birds

Among living vertebrates, birds possess a remarkable and largely unique suite of diagnostic features (Figure 10.1 and Table 10.1).

Feathers. All living birds have feathers – complex, distinctive structures that consist of a hollow, central shaft that decreases in diameter toward the tip. Radiating from the shaft are barbs, feather material that, when linked together along the length of the shaft by small hooks called barbules, form the sheet of feather material called the vane (Figure 10.1a). Feathers with well-developed, asymmetrical vanes are usually used for flight and are therefore called flight feathers.