Simplify the following text for a Grade 8 native English-speaking person.
Original: Ichthyosaurs , although common , are reduced in diversity ; while the top marine predators , the pliosaurs , grew to the size of killer whales and larger ( e.g. Pliosaurus , Liopleurodon ) .
Simplified: The top marine predators , the pliosaurs , grew to the size of killer whales and larger e.g. Pliosaurus , Liopleurodon .