Simplify the following text for a Grade 7 native English-speaking person.
Original: Two species of Pteranodon are traditionally recognized as valid : Pteranodon longiceps , the type species , and Pteranodon sternbergi .
Simplified: Pteranodon had a big crest at the back of its head . Pteranodon longiceps had a straight crest that went backwards , and Pteranodon sternbergi had a bigger , flatter crest that went upward .