In just about every description in popular literature crocodiles tend to be described as “ancient”, “primitive” or “unchanged”. Actually, this is far from the case, and the fossil history of crocodiles is as full of strange forms and unlikely lifestyles as any other group of animals.
The fossil record of crocodiles is indeed very long, the first known forms date back to the early Triassic. However, these were very different in appearance to modern forms, being small (about 1m), terrestrial, and in some species at least probably bipedal. They can be recognised as crocodiles however by details of the ankle joint and pelvis which are unique to the crocodiles and distinguish them from early dinosaurs.
During the age of dinosaurs most crocodiles became increasingly adapted to an aquatic lifestyle, producing among others the Metriorhynchids, fully marine forms with paddles for limbs and tail flukes, which however probably still laid eggs on land.
The modern crocodiles are divided into three groups, the Alligatoridae (Alligators and Caimans, 8 species), the Gavialidae (Gharials, 1 species) and the Crocodylidae (13 species). The number of species is likely to grow – the Spectacled Caiman for example is quite variable over its range and probably represents several separate species.
All the modern groups can trace their ancestry back to the Cretaceous period, the last part of the age of dinosaurs. Although crocodiles include the largest living reptiles, the Saltwater Crocodile of Australasia can grow to over 6m for example, they are dwarfed by fossil forms. The late Cretaceous Deinsosuchus (an alligator) grew to perhaps 15m for example, and even more recent forms could be similar in size – the giant Brazilian caiman Purrasaurus from perhaps 15 million years ago reached 12m (the size of a Tyrannosaurus rex!).
In the Cretaceous some crocodiles became terrestrial predators. The Sebecosuchids had a more upright gait and dinosaur-like teeth, and must have been significant predators in terrestrial ecosystems. Interestingly, these survived the end-Cretaceous mass extinction by at least 15 million years. Other forms became herbivorous, at least judging by their teeth.
Our own crocodiles here at Bristol are of course Dwarf Crocodiles, Osteolaemus. Thought until recently to be a single species, they have recently been split into three. Fortunately our own animals have been identified as belonging to the same species, the nominate form O.tetraspis. Crocodiles hybridize freely in captivity, which poses considerable problems for conservation breeding.
Although modern crocodiles are thought of as pure carnivores, some at least take some vegetable matter. It was thought until recently that any vegetation found in a crocodiles stomach was eaten by accident, but I have seen film of a (captive) alligator in Florida deliberately picking kumquats out of a bush in its enclosure, and it is possible that some wild caiman do much the same, at least in the rainy season when fish are harder to catch.