Reptiles in the Ocean

The ocean going reptiles of the Mesozoic were not a small faction of the population of those oceans at that time. Since different groups were considered the ruling reptiles of the ocean at different times it can be safely assumed that the various groups of pelagic reptiles were the apex predators of their day. The families that held those titles became increasing larger to usurp the title from previous groups but, because of the needs of aquatic locomotion, did not appear to be strangely exaggerated in shape or form (think of the reactions of most people the first time they see the arms of Tyrannosaurus or Carnotaurus). From ichthyosaurs to plesiosaurs there was not an enormous swing in terms of what family was considered the apex predator group, though both had higher members in specific niches. Mosasaurs, on the other hand, evolved forms that were capable of being not only the apex predators in a wide variety of niches, but also dominated the previous acme of icthyosaur and plesiosaur domination. In fact, they ate quite a few of the largest ichthyosaurs and plesiosaurs. There were copious reasons for this changing of the guard.

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.

©Nobu Tamura

There are other obvious, and less obvious reasons for their domination of the oceans, but these two are highly visible to all and rather obvious to infer, therefore lending themselves to quick observations. I know there are a lot of people that can add to these reasons, and their input is definitely welcome in the comments!