The Jurassic period saw the continuation of the breakup of the primordial supercontinent, Pangea. A large southern continent, Gondwanaland, was balanced by a northern continental cluster, Laurasia. Here intense submarine volcanic activity caused the spreading of the ocean floor, in turn triggering subductions that would ultimately extrude the Andean, Tibetan and Rock Mountain Ranges. A narrow North Atlantic Ocean had formed between North America and Europe, and what would become the Gulf of Mexico. Climates were generally warm, with no polar landmass. Dinosaurs topped the food chain on land and in the sea and air. Mammals had appeared at the end of the Triassic, and while remaining small, evolved rapidly in the Jurassic to inhabit a range of ecological niches. The first birds evolved in the Jurassic, but were eclipsed in the skies by the pterosaurs. Gymnosperms were the signature plant variety, with cycads omnipresent.
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