Kronosaurus vs Platypterygius
Kronosaurus was the top predator at that time. It’s jaws were huge. At ten metres long it dwarfed 6-7 metre Platypterygius. When Kronosaurus attacked, it ATTACKED! Platypterygius didn’t stand a chance. The attacker must have been smaller. Pliosaur vs Platypterygius
How about a smaller pliosaur? It had a smaller head and was about the same size. But Pliosaurs want to kill! If it wants to have Platypterygius for dinner it has to kill it first! Compared to some of the things big Al went through this is nothing.
Platypterygius vs Platypterygius
The most likely explanation is that the Platypterygius was injured by another Platypterygius in a dispute about teritory or a mate. The injured Platypterygius swam away before any terrible damage was done.