They lived in the Late-Triassic and the Cretaceous, but disappeared through the Jurassic!
An Aigialosaurus specimen was found with eggs developing inside of it. This is proof that mosasaurs did give birth to live young, which developed in eggs that the mother held in her body. Pliosaurs, also known as short necked plesiosaurs, also gave birth to live young.
They include Ankylosaurs (the armored dinosaurs), Mosasaurs, and Plesiosaurs (both marine reptilian groups).
Mosasaurs
It is probally the mosasaurs. During their times, it was the most dominant and succesful predator ever. Also, it was unkillable.
Animal life was then dominated, by large archosaurian reptiles: dinosaurs, pterosaurs, and aquatic reptiles such as ichthyosaurs, plesiosaurs, and mosasaurs. the first birds and placental mammals also appeared
Yes, crocodiles, caimans, alligators, gavials, saltwater crocodiles, sea turtles, terapins, hellbenders, and marine iguanas, are some that live today. During the mesozoic era, many reptiles lived in the water, like plesiosaurs, placodonts, mosasaurs, sea turtles, nothosaurs, thalattosuchians ichthyosaurs, and more.
Tylosaurus was a type of mosasaur (mosasaurs were large, carnivorous, marine lizards). Tylosaurus existed about 83 million years ago. That was during the late Cretaceous time period of the Mesozoic era.
Giant Mosasaurs ate and killed them.
Certain kinds of sharks and maybe killer whales do, and in the past, mosasaurs and plesiosaurs did.
It would be difficult to find a specific answer to this question. The animals which were known to have lived 91 million years ago in the area now known as North Dakota are: Mosasaurs Hesperorni Xiphactinus Tyrannosaurus rex Anklosaur Triceratops
First of all there is no proof that mosasaurs existed so if they did exist, then no they would be dead due to change of water temperatures