Simplify the following text for a Grade 12 native English-speaking person.
Original: During the earliest Jurassic these diversified quickly into the long-necked small-headed plesiosaurs proper , and short-necked large-headed pliosaurs .
Simplified: During the Early Jurassic these diversified quickly into both long-necked small-headed plesiosaurs proper , and short-necked large-headed pliosaurs .