Of all the reptiles alive today, crocodiles may be the least changed from their prehistoric forebears of the late Cretaceous period, over 65 million years ago-although the even earlier crocodiles of the Triassic and Jurassic periods sported some distinctly un-crocodile-like features, such as bipedal postures and vegetarian diets.
Along with pterosaurs and dinosaurs, crocodiles were an offshoot of the archosaurs, the "ruling lizards" of the early to middle Triassic period; needless to say, the earliest dinosaurs and the earliest crocodiles resembled one another a lot more than either resembled the first pterosaurs, which also evolved from archosaurs. What distinguished the first crocodiles from the first dinosaurs was the shape and musculature of their jaws, which tended to be much more deadly, as well as their relatively splayed limbs-as opposed to the straight, "locked-in" legs of theropod dinosaurs. It was only well into the Mesozoic Era that crocodiles evolved the three main traits with which they're associated today: stubby legs, sleek, armored bodies, and marine lifestyles.
First Crocodiles of the Triassic Period
Before the first true crocodiles emerged on the prehistoric scene, there were the phytosaurs (plant lizards): archosaurs that looked very much like crocodiles, except that their nostrils were positioned on the tops of their heads rather than the tips of their snouts. You might guess from their name that phytosaurs were vegetarians, but in fact, these reptiles subsisted on fish and marine organisms in freshwater lakes and rivers worldwide. Among the most noteworthy phytosaurs were Rutiodon and Mystriosuchus.
Oddly enough, except for the characteristic location of their nostrils, phytosaurs looked more like modern crocodiles than the first true crocodiles did. The earliest crocodiles were small, terrestrial, two-legged sprinters and some of them were even vegetarians (presumably because their dinosaur cousins were better adapted to hunting for live prey). Erpetosuchus and Doswellia are two leading candidates for the honorific of "first crocodile," though the exact evolutionary relationships of these early archosaurs are still uncertain. Another likely choice is the reclassified Xilousuchus, from early Triassic Asia, a sailed archosaur with some distinct crocodilian characteristics.
Whatever the case, it's important to understand just how confusing the facts on the ground were during the middle to late Triassic period. The portion of the supercontinent Pangea corresponding to modern-day South America was crawling with dinosaur-like crocodiles, crocodile-like dinosaurs, and (presumably) early pterosaurs that looked like both crocodiles and dinosaurs. It wasn't until the start of the Jurassic period that dinosaurs began to evolve along a distinctive path from their crocodile cousins and slowly established their worldwide dominance. If you went back in time 220 million years ago and were swallowed whole, you probably couldn't tag your nemesis as a crocodile or a dinosaur.
Crocodiles of the Mesozoic and Cenozoic Eras
By the start of the Jurassic period (about 200 million years ago), crocodiles had mostly abandoned their terrestrial lifestyles, probably as a response to the terrestrial dominance achieved by dinosaurs. This is when we begin to see the marine adaptations that characterize modern crocodiles and alligators: long bodies, splayed limbs, and narrow, flat, tooth-studded snouts with powerful jaws (a necessary innovation, since crocodiles feasted on dinosaurs and other animals that ventured too close to the water). There was still room for innovation, though. For example, paleontologists believe that Stomatosuchus subsisted on plankton and krill, like a modern gray whale.
About 100 million years ago, toward the middle of the Cretaceous period, some South American crocodiles had begun to imitate their dinosaur cousins by evolving to enormous sizes. The king of the Cretaceous crocodiles was the enormous Sarcosuchus, dubbed "SuperCroc" by the media, which measured about 40 feet long from head to tail and weighed in the neighborhood of 10 tons. And let's not forget the slightly smaller Deinosuchus, the "deino" in its name connoting the same concept as the "dino" in dinosaurs: "terrible" or "fearsome." These giant crocodiles probably subsisted on equally giant snakes and turtles-the South American ecosystem, on the whole, bearing an uncanny resemblance to Skull Island from the film, "King Kong."
One way in which prehistoric crocodiles were indeed more impressive than their terrestrial relatives was their ability, as a group, to survive the K-T extinction event that wiped the dinosaurs off the face of the earth 65 million years ago. Why this is so, remains a mystery, though it may be an important clue that no plus-sized crocodiles survived the meteor impact. Today's crocodiles are little changed from their prehistoric ancestors, a telling clue that these reptiles were, and remain, extremely well adapted to their environment.