Size and Weight:
You might think a prehistoric reptile named after Dallas would be big and land-bound, like a buffalo, rather than small, sleek and semi-aquatic, like a seal. However, one of the ironies of the marine reptiles that lived alongside the dinosaurs during the Mesozoic Era is that their fossils are very common in the now-arid American west and midwest, which used to be covered with shallow seas during the Cretaceous period.
What makes Dallasaurus important is that it's the most "basal" mosasaur yet known, the distant ancestor of a fierce, sleek family of marine reptiles that preyed relentlessly on fish and other ocean life. In fact, Dallasaurus shows evidence of movable, limb-like flippers, a clue that this reptile occupied an intermediate niche between a terrestrial and an aquatic existence. In this way, Dallasaurus is the mirror image of the earliest tetrapods, which climbed from water onto land rather than vice-versa!