Simplify the following text for a Grade 7 native English-speaking person.
Original: Early fossil tetrapods have been found in marine sediments , and because fossils of primitive tetrapods in general are found scattered all around the world , they must have spread by following the coastal lines â '' they could not have lived in freshwater only .
Simplified: Also , because fossils of early tetrapods are found widely in the Old Red Sandstone continent , they must have spread by following the coastal lines . This means they could not have lived only in freshwater .