Simplify the following text for a Grade 12 native English-speaking person.
Original: Through the skies over late Miocene South America ( 6 Ma ago ) soared the largest flying bird known , the teratorn Argentavis , with a wing span of 6 m or more , which may have subsisted in part on the leftovers of Thylacosmilus kills .
Simplified: Some of South America 's aquatic crocodilians , such as Gryposuchus , Mourasuchus and Purussaurus , reached monstrous sizes , with lengths up to 12 m. Through the skies over late Miocene South America ( 6 Ma ago ) soared the largest flying bird known , the teratorn Argentavis , with a wing span of 6 m or more , which may have subsisted in part on the leftovers of Thylacosmilus kills .