Eryops and Xenacanthus by Doug Henderson: “The scene depicts the large amphibian Eryops lunging from a muddy lake bottom to capture a freshwater shark, Xenacanthus, during the Permian in Texas, some 275 million years ago. A smaller amphibian, a Trimerorhachis, swims in the foreground. ”
Tylosaurus may have ruled the Late Cretaceous seas because all other would-be competitors, such as ichthyosaurs, were already extinct. Though they evolved from terrestrial lizards, the paddle-like limbs of giant mosasaurs like Tylosaurus were useless on land.