In Carnivores Cityscape, the Tylosaurus can be found swimming around in rivers and sewers. If they cannot travel well out of water, it is unknown how they got into the sewers from the rivers, or even out of the ship. Strangely, it is not deadly which is not in line with the actual animal. Since it is basically the prehistoric equivalent of a great white shark or a killer whale, it would be expected to lunge at agents and passing dinosaurs from under the water, although since it has been extinct for millions of years this cannot be known for sure.
A possible explanation for why they were able to travel off the ship is because they could have been there beforehand. Since the FMM UV-32 species is not a threat to humans, they could have been imported to Earth colonies from FMM UV-32 to add a little scenery to the rivers and sewers.
This however is just a possible inference and is not based on currently known information.