The creature in the lagoon is indeed a Mosasaur (albeit stupid large) and not a dinosaur (marine reptiles and pterosaurs were not dinosaurs by definition). I thought it looked great up until RAPTOR ASSAULT COMBAT TEAM GO GO! I will see it anyhow as I am a huge paleontology nerd. Considering some of the original scripts had like laser beam military trained dinosaurs as the bad guy, I will take their snake/raptor/trex/cuttlefish hybrid that the design docs said they are going for. I just hope they don't try to do the same thing like they did with the Spinosaurus and have it fight and kill a Rex just to show how badass it is.
That fight was retarded, and if it happened like it did in the movie, the Rex won that fight in the first five seconds. T-Rex teeth are serrated and if it got its jaws around the relatively thin Spinosaurus neck, it wasn't coming back off without ripping out its throat at the least. Also, the entire strength of a Rex is its head musculature (obviously). The Rex would have crushed the backbone and/or windpipe of the Spinosaurus and that shit was done. Otoh, Spinosaurus teeth were not serrated to hold onto the T-Rex's neck, and it has no where near the strength in its jaws to snap the neck of such a heavily muscled creature like a T-Rex. Spinosaurus was not built to be an alpha predator in the sense it was depicted, it was an extremely large specialist dinosaur that while physically large than a Rex, was no where near as heavily built. /dinosaurnerdrantoff