Sarcosuchus ('flesh crocodile')

Sarcosuchus was ridiculous. Growing up to 39 feet long and weighing 8 tonnes, sarcosuchus make modern crocodiles look positively Toys 'R' Us by comparison.  For a fun juxtaposition, here's what a sarcosuchus head would look like stood next to a modern man in an Hawaiian shirt:

Our understanding of this true giant started in 1964, when a nearly complete skull was found in Niger, Africa. To make room for its 132 teeth, the head of the sarcosuchus was 6 foot alone!

Sarcosuchus lived 112 million years ago, in the mid-Cretaceous period of the Mesozoic era, and displayed a lot of the features and characteristics of their crocodilian descendants today. The sarcosuchus could not 'death roll' like modern crocodiles do, but had a bite that was so powerful that it didn't need to perform such acrobatics to prepare its meal. Such was its size that its only potential for competition came from the 49-foot dinosaur Spinosaurus, one of the fiercest and largest predators this planet has ever seen (Spinosaurus has a star turn in Jurassic Park III, if you remember.)

Without knowing who would win in a sarcosuchus vs spinosaurus fight, what can be said with certainty is that both of these creatures were unable to use their awesome statures to survive the changing climate towards the end of the Cretaceous period. As the Earth's climate became drier, neither could adapt from their reliance of an aquatic habitat. Both went extinct roughly 90 million years ago, a good deal of time before the Cretaceous-Paleogene mass extinction came for pretty much every other living thing 25 million years later.

PS: Here's Nigel Marvin re-enacting what a human-sarcosuchus encounter might play out like. I love you, Nigel Marvin.