1. The teeth of Mosasaurus species were conical and bone crushing. Some members of the family ate hard shelled organisms with very different teeth, but Mosasaurus species were purveyors of the larger, squishier, bodies of the ocean and preferred to crack down only on the shells of ammonites and sea turtles when looking for crunchy food items. These were, of course, much softer than giant clams, though a case could be made for the toughness of ammonite shells as extremely hard bodied. Either way, the conical teeth of mosasaurs were excellent in their presumably strong jaws at breaking into the softer shelled swimming organisms and certainly the soft bodies of plesiosaurs and whatever remaining ichthyosaurs that were in the oceans of the Cretaceous.
2. Swimming with paddles worked for the somewhat slower plesiosaurs; smaller necked plesiosaurs tended to be speedsters while their longer necked cousins took a more relaxed approach at life. Mosasaurus species, however, were built for speed all over. Their streamlined crocodile-esque shape is not a coincidence. That shape of body has been toyed with in many lines including mosasaurs and crocodiles and has proven effective for speed and maneuverability in aquatic landscapes. The evolution of paddles to stabilize and turn the body and a very powerful tail capable of motoring at high speed make for a deadly combination for slower animals and the ability to ambush, assuming mosasaurs followed the typical marine camouflage patterns of predators, would make them deadly for swifter prey as well, though with a lower success rate probably.