The continents of South America and Africa were once joined together as one landmass. Evidence for this claim can be found in the fact that fossils of mesosaurs, extinct reptiles, have been unearthed in both western Africa and eastern South America. Which one of the following statements, if true, most weakens the argument? ['The fossils of mesosaurs are the only ones unearthed in both western Africa and eastern South America.', 'The fossils of mesosaurs found in western Africa are of the same geologic period as those unearthed in eastern South America.', 'Mesosaurs in Africa migrated from Europe and mesosaurs in South America migrated from Asia.', 'The fossils of mesosaurs were found very near the west African and east South American coasts.']