Past simple for actions happening over a period of time in the past

The past simple is used for actions that took place over a complete period of time in the past. This period is expressed with a time expression; however, the exact time may or may not be mentioned:

Pterodactyls existed between 1.5 billion and 70 million years ago.
From 1981 to 1984, he worked for a large multinational company.
Last week, I stayed with my sister for a few days. (the exact time is not mentioned)
Joe lived in Boston for ten years. (the exact time is not mentioned)