Marine reptiles evolve, such as ichthyosaurs and plesiosaurs. Ferns, cycads, ginkgoes, and conifers flourish. Mass extinctions occur at the end of the Triassic Period, reducing some marine and terrestrial groups, such as the ammonites, therapsids, early reptiles, and primitive amphibians, by as much as 75 percent. Jurassic Period 206-144 million years ago The Atlantic Ocean begins to form as North America separates from Africa and South America. Tectonic plate subduction along western North America causes the Earth's crust to fold, and mountains form in the western part of the continent.
Reptiles adapt to life in the sea, in the air, and on land.
Dinosaurs are the dominant reptile on land. Archaeopteryx , the first bird, appears. Early amphibians, extinct by the Late Triassic, are succeeded by the first frogs, toads, and salamanders. Mammals are small, shrew-like animals. Plant forms are dominated by the cycads and cycadeoides.