1.   Dinosaurs became extinct millions of years ago.

2.   There are several theories as to why the dinosaurs became extinct.

3.   There are several theories as to why the dinosaurs become extinct?

4.   Dinosaur partisans argue that when some dinosaurs became warmblooded they developed down as insulation and this led to feathers, which then gave them the ability to fly.

5.   He has found there remains of dinosaurs in layers of earth laid down long after it had been commonly believed that dinosaurs had become extinct.

6.   Nobody knows precisely how dinosaurs became dominant.

7.   They believe dinosaurs did not become extinct.

8.   But then something happened to the earth, and the dinosaurs became extinct.