Being at the top of the food chain, large mosasaurs "were capable of handling virtually anything that came in their way."
Whales emerged later, so they weren't a threat. Evidence suggests mosasaurs did battle with sharks, however.
Frank O'Keefe, a Marshall University biologist, has also studied prehistoric marine life.
O'Keefe told Discovery News that the mosasaur ecosystem, aside from sharks, included plesiosaurs (another large now-extinct marine reptile), fish, squid, ammonites and other invertebrates. Mosasaurs might have also encountered toothed birds and pterodactyls, which often hung out near water, hoping for vulnerable prey.
O'Keefe said both plesiosaurs and mosasaurs went extinct about 66 million years ago, when dinosaurs also died out.
It is possible that mosasaurs dined on dinosaurs, but the meal likely would not have been tasty, or fresh.
"Dinosaurs lived on land," Lindgren said, "so the only chance of a mosasaur ever getting a bite out of one of those reptiles would have been a bloated and floating carcass that somehow ended up at sea."