The Crocodilians (Crocodylia) are an order of reptiles belonging to the Archosauria clade. Alongside some of the other archosaurs (some of which become dinosaurs and pterosaurs of Jurassic and Cretaceous time periods), the first crocodilians appeared in the Late Cretaceous, around 80 MYA. Such reptiles already possessed long jaws, powerful neck muscles, short legs and a lengthy tail, and were adapted to life both on land and in fresh water, able to live for several months on fat stored during the times of plenty.
Postosuchus was another such archosaur reptile, featured in the first episode of Walking with Dinosaurs, "New Blood", which picked up where Walking with Monsters had stopped at. Postosuchus was a well-defended animal, and had a body plan more similar to some of the dinosaurs or other archosaur reptiles other than crocodilians: it was flat-footed but had long legs tucked underneath its body, which meant that it could run quickly and even stand up on both legs, though not for very long, unlike the dinosaurs. In the end of that WWD episode, a Postosuchus was eventually overpowered and killed by a pack of Coelophysis dinosaurs, which it had harassed earlier. Put otherwise, with the extinction of Postosuchus and other such reptiles cleared the path for the more advanced dinosaurs, such as Coelophysis and Plateosaurus.
Metriorhynchus. (Official image.)
Despite the extinction of Postosuchus and others, however, by the Jurassic period, relative of the crocodilians (crocodyliformes) were already well-established as aquatic predators, and even taken to life at seas. Metriorhynchus, featured in the third episode of Sea Monsters, was one such crocodyliform, belonging to the metriorhynchid branch of the family. Reaching 2.5-3 meters in length, this was a middle-sized predator, hunting fish, cephalopod mollusks, pterosaurs, and other denizens of the Jurassic seas, and was hunted by the giant pliosaurs, such as Liopleurodon, which swallowed them whole. By the time of the Cretaceous period, however, the sea crocodyliforms' ecological niche was taken over by various mosasaurs instead.
Sarcosuchus (photo by Collector1).
In fresh water bodies, however, the crocodyliforms flourished. Since 206 MYA, most of them belonged to the neosuchia branch of the family. The giant Sarcosuchus, featured in the first episode of Chased by Dinosaurs, was one such neosuchian - a killer of dinosaurs, this reptile easily reached up to 12 meters in length, making it almost equal in strength to the giant dinosaur Giganotosaurus, also featured in that episode. However, Sarcosuchus died out 93 MYA, a long time before the K/T mass extinction, during the middle Cretaceous, possibly due to the appearance of the eusuchians, the first modern crocodiles, alligators and caimans, who had evolved from such creatures as Anatosuchus (not featured by Impossible Pictures yet). Deinosuchus, a giant prehistoric alligator, featured in the last episode of Walking with Dinosaurs, was such an eusuchian, equal in size and power to the aforementioned Sarcosuchus.
The K/T extinction at the end of the Mesozoic affected the crocodilians as badly as it did the other reptiles:
most of the first Cenozoic crocodilians (eusuchians) were small; the one featured in the first episode of Walking with Beasts swam away, when it was approached by an Ambulocetus, one of the ancestors of the modern whales. (Still, at least one of them - Acherontisuchus - was as big as the biggest modern species of crocodilians.) There were other, terrestrial species of crocodilians, however, such as Pristichampsus, who for a while even challenged the mammals supremacy - until they died out in the Late Eocene. But the fully aquatic crocodilians, and now one of them, the 7 meter long saltwater crocodile, is one of the most dangerous reptiles and also THE biggest reptile on Earth today, implying that if the crocodilians get a chance, they will become giants again.