During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were supplanted during the early Jurassic by various aquatic and marine forms. The later Jurassic, Cretaceous, and Tertiary saw a wide diversity of terrestrial and semi-aquatic lineages. Modern crocodilians do not appear until the Upper Cretaceous.
The crocodylian lineage (clade Crurotarsi) were a very diverse group of reptiles. Not only are they an ancient group of animals, at least as old as the dinosaurs, they also evolved into a great variety of forms.
The earliest forms, the sphenosuchians, evolved during the Upper Triassic, and were highly gracile terrestrial forms built like greyhounds. During the Jurassic and the Cretaceous, marine forms evolved forelimbs that were paddle-like and had a tail similar to modern fish.
Dakosaurus andiniensis, a species closely related to Metriorhynchus, had a skull that was adapted to eat large marine reptiles. Several terrestrial species during the Cretaceous evolved herbivory. A number of lineages during the Tertiary and Pleistocene became wholly terrestrial predators.
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Crocodylomorph Facts for Kids. Kiddle Encyclopedia.