The Mosasaurus was the first fully aquatic creature added to Beasts of Bermuda. It is an oceanic apex predator, but it is also capable of swimming up waterways and underwater cave systems to access waterholes that terrestrial creatures frequent. It is currently the largest (length-wise) aquatic creature in the game, beaten in weight by the Kronosaurus. It has a host of abilities it can employ to be a successful ambush predator and the true terror of high tide.
Primary Attack: Mosasaurus' primary attack is a bite. At 1.0 growth, it costs a base 8.5 ability power and does 140 health damage.
Secondary Attack: Mosasaurus' secondary attack is a dart, similar to those possessed by the Ichthyovenator and Elasmosaurus. At 1.0 growth, it costs a base 48 ability power and 32 stamina. Additionally, if one comes into contact with a player during the duration of the dash, they will inflict 140 health damage.
Special Ability: Mosasaurus' special ability is a grab. When pressing the ability key within a short distance of a creature within the Mosasaurus' grab threshold (default ability keybind is Z), the Mosasaurus will snatch the other player in its jaws at the cost of 15 ability power. Upon pressing the primary attack key, the Mosasaurus will thrash, dealing 163.1 health damage (in 7 bursts of 23.3.) The Mosasaurus can continue to hold and thrash prey for as long as it has ability power, which will consistently decrease as the enemy creature is held. Larger prey items will cause the Mosasaurus' ability power to decrease faster.
For more information on the Mosasaurus, watch this video.