A lot of people know that crocodiles come from an ancient lineage. Possibly almost as many people know that the first crocodiles appeared at about the same time as the dinosaurs, in the middle of the Triassic period. What most people don't know, however, is how different those early crocodiles were from the ones that are around today.

At that time - about 220 million years ago - there were a lot of crocodile-like animals coexisting with the early dinosaurs. The semi-aquatic phytosaurs were superficially very similar to today's gavials¹. The terrestrial aetosaurs, which were actually vegetarians, were similar in shape to modern alligators, and some of the rauisuchians (also terrestrial) looked rather like non-sprawling crocodiles.

But the early crocodiles looked very different from any of these creatures. They were lightly-built bipedal runners about a foot or so in length, living an entirely terrestrial existence. Some of them may even have climbed trees. Their gait was upright, like that of dinosaurs, with the legs held vertically beneath the body. In evolutionary terms they were sophisticated animals, and these were the ancestors of today's quadrupedal, sprawling, semi-aquatic crocodiles.

During their long history, therefore, crocodiles have re-evolved all these traits that make them seem closely related to those other sprawling reptiles: lizards. Whereas lizards sprawl on four legs simply because they always have and don't know any better, crocodiles sprawl because it suited their ancestors to adopt that gait.

The early, slender terrestrial crocodiles from the Triassic period were followed in the Jurassic by a mixture of larger terrestrial forms and some marine species that evolved paddle-shaped limbs and fish-like tail fins. These sea crocodiles, however, died out before the dinosaurs did.

In the Cretaceous period, modern semi-aquatic crocodiles appeared that were similar in appearance to today's crocs and alligators. One or two species grew enormous, reaching 50 feet in length and weighing up to 15 tonnes: in other words, ten feet longer than Tyrannosaurus Rex and three times the bulk.

Both terrestrial and semi-aquatic forms survived long after the reign of the dinosaurs, but eventually the terrestrial crocodiles died out mid-way through the Tertiary period. Now only the semi-aquatic crocodiles are around, having proved themselves best suited to the modern world.

Despite being superficially similar in body plan to certain lizards, crocodiles have had a vastly different history and are not closely related to lizards at all. In fact, they are closer relatives of the sparrows in your garden. Many reject the traditional form of classification which places crocodiles and lizards within the class reptilia, and birds within the class aves. Many prefer to group all three within the reptilia, and assign both crocodiles and birds to the reptilian group archosauria, which excludes lizards.

Crocodiles are often thought of as primitive, but this is a false impression. They are highly evolved creatures, and worthy of great respect. Maybe the title of 'King of the Beasts' has been assigned to the wrong animal.