Final answer:
The prehistoric pit houses in the American Southwest were primarily oval and circular in shape, providing insulation and suiting the environment.
Step-by-step explanation:
Prehistoric pit houses at sites in the American Southwest, such as those of the Anasazi or Ancestral Puebloans, were primarily in the shapes of oval and circular. These pit houses were partly underground and constructed with wooden poles covered in dried mud, well-suited for the environment to provide insulation from extreme temperatures. The early settlements consisted of such pit houses before the development of more complex structures like kivas and cliff dwellings. Kivas, used as ceremonial centers, were also circular in shape and dug into the earth, reflecting the architectural and cultural evolution of these ancient peoples.