Simplify the following text for a Grade 12 native English-speaking person.
Original: Similar gigantic birds of the Cenozoic were the South American terror birds ( phorusrhacids ) and the Australian mihirungs ( Dromornithidae ) .
Simplified: Similar gigantic birds of the Cenozoic were the South American terror birds ( phorusrhacids ) and the Australian mihirungs ( Dromornis ) .