Free Response

Explain why birds are thought to have evolved from theropod dinosaurs.


This is suggested by similarities observed between theropod fossils and birds, specifically in the design of the hip and wrist bones, as well as the presence of a furcula, or wishbone, formed by the fusing of the clavicles.

Describe three skeletal adaptations that allow for flight in birds.


The sternum of birds is larger than that of other vertebrates, which accommodates the force required for flapping. Another skeletal modification is the fusion of the clavicles, forming the furcula or wishbone. The furcula is flexible enough to bend during flapping and provides support to the shoulder girdle during flapping. Birds also have pneumatic bones that are hollow rather than filled with tissue.

How would the chest structure differ between ostriches, penguins, and terns?


Ostriches and penguins are flightless birds, but ostriches are entirely terrestrial, while penguins dive and swim in the ocean to find food. Therefore, penguins and flight birds like terns have similar chest structures with a keel sternum and relatively large pectoral muscles (penguins use their wings to “fly” through water). Conversely, since ostriches move by running, they do not have a keel to their sternum. They also have smaller pectoral muscles than would be predicted for a flying bird their size, but have larger thigh muscles.