Size and Weight:
Dubbed "SuperCroc" by the media, the prehistoric crocodile Sarcosuchus looked and behaved like its modern descendants, but it was much, much bigger--about the length of a city bus and the weight of a small whale (or about twice as long and 10 times as heavy as the largest crocodiles alive today). Like modern crocodiles, it's believed that Sarcosuchus lay half-submerged in rivers and streams, tilting its eyes upward to catch sight of its middle Cretaceous prey--which may well have included mid-sized sauropods and titanosaurs, or even the occasional tyrannosaur or other large theropod. (See 10 Facts About Sarcosuchus and a gallery of Sarcosuchus pictures.)
Aside from its enormous length and girth, the most notable feature of Sarcosuchus was the strange, bulging protuberance on the end of its snout, called a "bulla." This Jimmy Durante-like schnozz may have helped the SuperCroc to communicate (via grunts, squeaks, and other vocalizations), it may have enhanced its sense of smell, or it may simply have been a sexually selected characteristic (meaning a Sarcosuchus with an especially big bulla was more likely to mate and have SuperCroc kids).
As big as it was, Sarcosuchus wasn't the only plus-sized prehistoric crocodile of the Mesozoic Era. Its closest competitor was the aptly named and slightly later Deinosuchus ("terrible crocodile"), adults of which attained lengths of over 30 feet and weights in the neighborhood of 5 to 10 tons (compared to 40 feet long and 10 to 15 tons for Sarcosuchus). Part of the problem with comparing these two genera is that crocodiles grow throughout their life spans (about a foot per year for Deinosuchus), and some individuals were longer-lived than others.
By the way, we now know that Sarcosuchus shared its territory with Spinosaurus, the largest carnivorous dinosaur of the Mesozoic Era. What would an encounter between these two gigantic reptiles have been like? See Spinosaurus vs. Sarcosuchus - Who Wins? for a blow-by-blow description!