Of the many giant crocodiles and alligators that lived in prehistoric times, the largest may have been Sarcosuchus, a distant relative of the modern crocodile that lived 112 million years ago. While a complete skeleton does not exist, estimates based on intact Sarcosuchus skulls suggest that individuals may have grown as long as 36 to 39 feet in total length and weighed as much as 18,000 pounds.
Crocodiles, along with dinosaurs and pterosaurs, were descendants of the archosaurs, a group of reptiles that dominated the Earth at the end of the Paleozoic Era. Among modern reptiles, alligators and crocodiles may be the least changed from their prehistoric ancestors. They were characterized by the shape and musculature of their jaws and by their relatively splayed limbs. However, the variety of prehistoric crocodilians far exceeds the modern types. The earliest true crocodiles were small, two-legged sprinters. Some early crocodiles had armor plates and some had horns. Others may have even been herbivores. By the beginning of the Jurassic period, crocodiles had mostly become aquatic animals with long, streamlined bodies, small splayed limbs and narrow, powerful jaws. Like their modern descendants, most of these ancient crocodiles probably fed by lying in wait for animals that ventured too close. The biggest difference between these Jurassic crocodiles and the modern variety is the extraordinary size of the prehistoric animals. The largest modern saltwater crocodiles are less than half the estimated length of Sarcosuchus. Along with the dinosaurs, the large prehistoric crocodiles perished in a mass extinction 65 million years ago.