Halisaurus was the one of the smallest of Mosasaurs, reaching only 4 metres long (an even smaller Mosasaur still for example, being the 2-3 metre long Clidastes).
Halisaurus
It lived in the Late Cretaceous seas as did many other predatory sea creatures, like Xiphactinus and Tylosaurus - the latter being the second biggest known species and the top predator of the seas of that time and place.
Description[]
Halisaurus was carnivore, specialising on coastal shelf ambush, hunting smaller prey items such as the still relatively large 2 metre long Hesperornis. They were probably sneaky creatures, keeping away from potential predators such as large sharks and other Mosasaurs.
Chased by Sea Monsters: A Walking with Dinosaurs Trilogy[]
Halisaurus was seen in the last episode of Sea Monsters, in the Deadliest Sea of all time, but it wasn't considered a major threat though a 4 metre long Mosasaur is the match of most Sharks today besides ones like Great White Sharks.