I saw a fan theory video on YouTube a while back that provided a theory as to why the Spinosaurus was stalking them all through the movie that I can get behind. Basically how it goes was after the group landed on the island and the mercenaries went off into the jungle on their own to scout the area, they encountered the Spino nest with a baby. The baby then attacked them and they killed it, which would explain why that one guy who didn't make it back to the plane was injured when he was running away. The noise from that combined with Tea Leoni yelling on the megaphone attracted the adult, which then went into a rage once it saw what happened and that's why it stalked them across the island. It's slightly farfetched but I can buy it as headcanon. It's no less believable than the T-Rex in JPIII killed being an older version of the infant from The Lost World as far as fan theories go.