Image hosted for free at CtrlV.inThe smallest species of crocodilian is the Cuvier's Dwarf Caiman[2] and the largest is the Saltwater Crocodile.[3] The basic crocodilian body plan is a very successful one; modern species closely resemble their Cretaceous ancestors of 84 million years ago. Mammals, too, have adapted to this body plan at least once in history. One ancestral whale family, the Ambulocetidae, were aquatic predators living in rivers and lakes, and they filled an ecological niche similar to the crocodilians. Much earlier, before the age of reptiles, Amphibians like Prionosuchus and Platyoposaurus were the first animals to adopt this body plan and niche.
Locomotion.
Crocodilians have a flexible semi-erect (semi-sprawled) posture. They can walk in low, sprawled "belly walk", or hold their legs more directly underneath them to perform the "high walk".[4] Most other reptiles can only walk in a sprawled position, and chameleons are the only modern reptiles with a more erect posture than crocodilians.[citation needed] The semi-erect posture makes it possible for some species to gallop on land if necessary.[4] An Australian species can reach a speed of over 16 km/h while galloping on an irregular forest floor.[5] Crocodilian ancestors, fast-moving terrestrial predators like the rauisuchians, actually had a fully erect posture, indicating that the sprawling and semi-erect posture of crocodilians evolved after they adapted to life as semi-aquatic ambush predators. Their ankle bones, or tarsi are highly modified. Modern crocodilian locomotion is not a primitive trait, but a specialization for their semi-aquatic lifestyle.