Halisaurus was a species of Mosasaur from the Late Cretaceous.
Halisaurus was a mosasaur. But it was much smaller than Platecarpus, Hainosaurus and Tylosaurus. It loitered in submarine caves and cracks. It may have waited around ledges above the water where Hesperornis gathered.
When Hesperornis leave their rocky ledges to dive for fish, Halisaurus are down formations. And below, waiting for an opportunity to ambush them. Mosasaur teeth are good at piercing the skin of their prey but bad at slicing flesh. So once Halisaurus has caught its prey, it swallows it. Its jaw has flexible joints within it and can open incredibly wide.
Halisaurus made a brief appearance in the Cretaceous segment of this episode waiting to ambush a diving Hesperornis.