"SuperCroc" is a nickname for Sarcosuchus, an ancient crocodile measuring 11-12 m (37-40 ft) and weighing up to 8 tonnes (8.75 tons). More broadly, the term "SuperCroc" refers to a common evolutionary motif that recurs throughout evolutionary history during several distinct periods, each separated by tens or even hundreds of millions of years. Crocodiles are archosaurs, related to dinosaurs and modern-day birds. The main feature that distinguishes them from dinosaurs is their sprawling, rather than upright limbs.
The ancestors of crocodiles originally evolved 230 million years ago from basal archosaurs just before the time of the dinosaurs, and the basic body plan has changed little since. Crocodiles hang out around streams and rivers, operating as ambush predators, crushing the jugular veins of vertebrate prey, dragging them into the water and drowning them. Crocodiles also feed on fish, birds, and essentially anything else that moves. Strictly speaking, "crocodiles" only include members of Crocodilia and their immediate ancestors, but throughout this article, the term "crocodile" is used interchangeably with crocodilian, which is a broader group.
The first SuperCroc evolved about 110 million years ago, during the early Cretaceous. Dinosaurs were huge animals, and it took huge predators to take them down. The SuperCroc motif had several things going for it: unlike dinosaurs, crocodilians could swim, lifting up their massive bulk. The armor scutes that cover crocodiles provide structural support, and they were protected from the largest dinosaur predators by living in the water.
Another cause behind the SuperCroc phenomenon was a phenomenon called Cope's rule, whereby the size of animals in a given lineage tends to increase over evolutionary time until it reaches some maximum. The ceiling is generally dictated by propensity to extinction, as large animals require more food and go extinct first when environmental conditions turn sour. Throughout the Cretaceous, the world was a very life-friendly place, as evidenced by climactic data from the time and the enormous size of the animals that lived then.
These animals also likely lived in near-perfect environments, vast swamplands with warm, shallow water and abundant prey. Like modern-day crocodiles, the ancient SuperCrocs had tremendous biting force, some as much as 120 kilonewtons (27,000 pounds), almost as great as a T. rex. The largest SuperCroc was Rhamphosuchus, which was as long as 18 m (60 ft), but its jaw structure actually indicates that it only ate fish.