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.