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2.19: Triassic Period (252 to 201 million years)

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    Triassic Period (252 to 201 million years)

    Following the great extinction event at the end of the Permian Period, life on Earth gradually reestablished itself both on land and in the oceans through succession. Scleractinians (modern corals) replaced earlier forms as dominant reef-forming organisms. On land, reptilian therapsids (an order related to the distant ancestors of mammals) and archosaurs (ancestors of dinosaurs and modern crocodillians) became the dominant vertebrates. New groups evolved in the middle to late Triassic Period including the first dinosaurs, primitive mammals, and flying vertebrates (pterosaurs) but these families did not flourish until after another global extinction event at the close of Triassic time. Current thought is that ancestral forms of both mammals and dinosaurs first appear in the fossil record in Late Triassic time, about 200 million years ago.

    During the middle Triassic, the supercontinent of Pangaea began to rift apart into separate landmasses, Laurasia to the north and Gondwanaland to the south. With the breakup of Pangaea, terrestrial climates gradually changed from being mostly hot and dry to more humid condition. Another mass extinction in the fossil record marks the end of the Triassic Period.

    The Chugwater Formation in Wyoming is a classic example of the Zuni Sequence Red beds are oxidized, iron-rich sedimentary deposits that occur extensively throughout western North America that were deposited in coastal terrestrial and nearshore environments during the Triassic Period. Red beds of Triassic age are well exposed in west Texas, throughout the Colorado Plateau and Rocky Mountain region, and in the Newark and Connecticut Basins on the East Coast. These are associated with the Absaroka Sequence that accumulated while Pangaea was still assembled and hot and dry climate conditions prevailed across most of North America.
    Figure 2.43. Red beds of the Chugwater Group of formations of Triassic age exposed near Lander, Wyoming.
    Tracks at Dinosaur State Park petrified wood Desmatosaur from West Texas Placerias
    Figure 2.41. Dinosaur tracks in Late Triassic sedimentary rocks, Dinosaur State Park, Connecticut. Figure 2.42. Extensive coniferous forests covered coastal regions at illustrated by the massive deposit of fossil wood preserved in Triassic-age sedimentary rocks in and around Petrified Forest National Park, Arizona. Figure 2.44. Desmatosuchus, an archosaur from the Triassic Period found in West Texas Figure 2.45. Placerias, a large mammal-like reptile from the Triassic Period from Petrified Forest National Park, Arizona

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