Tylosaurus

Biology
Mosasaurs were serpentine marine lizards and ferocious predators. Mosasaurs were not dinosaurs but evolved from semi-aquatic squamates known as the "aigialosaurs", close relatives of modern-day monitor lizards. Mosasaurs breathed air and were powerful swimmers that were well-adapted to living in the warm, shallow epicontinental seas prevalent during the Late Cretaceous Period. They grow up to 17 meters long but the longest ever recorded was 17.5 meters longA fully grown mosasaur attacks a swimming pool lifeguard and kills him. It then returned through the Anomaly that brought it their when the clorine in the water began irritating its skin. It then appeared at a reservoir in west London and regurgitates a bolus of the undigested remains of the lifeguard. Connor and Abby who had waded out into the reservoir to take water samples where then stalked by the creature. They then rush onto land but are stopped by a high metal fence. Another smaller mosasur comes ashore after them and Connor fights it with an oar. It backs off into the water returns through the anomaly which then closes.