Mosasaurus are gigantic marine reptiles that live in the seas and use flippers for swimming. However, in Dinotopia, they are depicted to look like large crocodiles (similar to Deinosuchus, Sarcosuchus, or Kaprosuchus) with legs and living in fresh water.
After David and Karl got into a fight, they accidentally fell off the balcony and to the waterfall. Karl survived and his brother, he lost consciousness. While he was carrying his brother to shore, a sleeping Mosasaurus wakes up to see them in its territory. While traveling in the forests, Karl went to take a drink in the river and not noticing the Mosasaurus family are awake. The Messanger Bird (which is a Dimorphodon, but looks more like a parrot) arrives and asks if he is Karl or David. Karl said that it is him and the Messanger Bird went to tell Zippo and Marion, who were looking for them. When they found Karl and David, the Mosasaurus notice them. While Zippo and Karl were in a sacred temple, they spotted a wall picture of the Great King of the World Beneath. Zippo realized that the carnivores are thinking the travlers are violating their sacred space and Karl said "he hasn't seen any carnivores lately". However, when Karl comes out, he notices two Mosasaurus swimming. Marion also noticed them. He told Marion that everyone needs to get out and needs to help David. Marion went to get Zippo. While Karl was carrying his brother, he crosses the bridge and a large Mosasaurus rise out of the water to attack. When it knocked the side of the
bridge, the Scott brothers fall and it nearly bit David's arm off, but Karl managed to pull him away and got to the other side, but Marion and Zippo are still on the other side. When Marion and Zippo began crossing, one of the Mosasaurus came out of the water and on to the bridge. When she and Zippo were walking slowly, the Mosasaurus approaches them. However, another Mosasaurus distracts the one on top of the bridge which gives Marion and Zippo to run and the Mosasaurus nearly bit Zippo. However, the Mosasaurus have made a hole in the bridge. Marion jumps, but Zippo falls when one of the Mosasaurus knocks the bridge. Karl went to help Zippo. When Zippo tried to escape, one of the Mosasaurus grabs him on the tail with its jaws and tries eating him. It then twirls around to finish off Zippo. Karl, however, managed to grab Zippo out of the Mosasaurus' jaws.
When Karl, David, and Marion went to the Pteranodon nest, the Mosasaurus family once again have awaked. After Karl used a flare gun, the flare light wakes up all of the Pteranodons. When the Scott brothers and Marion leave, a Pteranodon was blocking their path on the bridge with the awaken Mosasaurus. However, a Mosasaurus jumps out and hits the bridge and scares the Pteranodon away. They protagonists then cross the bridge and the Mosasaurus nearly ate them.
Soon, when Marion, Zippo, and the Scott brothers went to an abandoned barge (where Cyruss Crabb told them to meet them at), they were shocked (especially Zippo) when they saw that one of the Mosasaurus was killed by Crabb and he still has its head in a glass tank.
After the Scott Brothers found their father and made it to the swamp, they crossed the bridge and Karl warned his father to not put his foot in the water and he asked why and Karl said he doesn't want to know (implying that he might get scared by the Mosasaurus). However, the Mosasaurus family don't seem to be awake, once they crossed the bridge.
Dinotopia TV Series
The Mosasaurus appear in some episodes where they are seen attacking Freefall.