how can fossils can be used as proof that the continents were once joined together as a supercontinent called pangaea .

Evidence From Fossils
Wegener also used fossils to support his argument for continental drift. A fossil is any trace of an ancient organism that has been preserved in rock. For example, Glossopteris (glaw sahp tuh ris), was a fernlike plant that lived 250 million years ago. Glossopteris fossils have been found in rocks in Africa, South America, Australia, India, and Antarctica. The occurrence of Glossopteris on these widely separated landmasses convinced Wegener that Pangaea had existed.

Other examples include fossils of the freshwater reptiles Mesosaurus and Lystrosaurus. These fossils have also been found in places now separated by oceans. Neither reptile could have swum great distances across salt water. Wegener inferred that these reptiles lived on a single landmass that has since split apart

The answer is constant.