Simplify the following text for a Grade 12 native English-speaking person.
Original: Various theories had been proposed to explain the ancestry of the `` segnosaurids , '' with some scientists even suggesting they were descendants of the sauropodomorphs - but new , well-preserved finds such as Alxasaurus in 1993 and Beipiaosaurus in 1996 provided details about the bird-like pelvis , feet and skulls of primitive members , and helped confirm that segnosaurids belonged to the same group of theropod dinosaurs as Therizinosaurus ( and were therefore re-named therizinosaurids ) , and that therizinosaurs were , more specifically , advanced , herbivorous maniraptoran theropods .
Simplified: New , well-preserved finds such as Alxasaurus in 1993 and Beipiaosaurus in 1996 provided details about the bird-like pelvis , feet and skulls of primitive members . This helped confirm they belonged to the same group of theropod dinosaurs as Therizinosaurus and that therizinosaurs were , more specifically , advanced , herbivorous maniraptoran theropods .