The Triassic Period

The Triassic period (250-205 mya) during which the dinosaurs originated,was a time of comparative aridity, of xerophytic vegetation and abundant reptiles. About 200 mya, during the Late Triassic and Early Jurassic, the continental plates of Laurasia and Gondwanaland were united. Some portions of what are now central Asia and China may have remained as separate islands, but most of the landmass of the world was concentrated into the single supercontinent, Pangaea, as we have seen. Consequently,both plants and animals were remarkably similar everywhere. The main vegetation types consisted of seed ferns and conifers which were adapted to the dry climate, while massive horsetails dominated the damper regions. The terrestrial fauna comprised mainly earthworms, again in moister places, snails, arachnids, insects, and reptiles, which either captured earthworms and arthropods in the undergrowth, or else chewed the tough vegetation. Crustaceans, molluscs and fishes populated the lakes and rivers where they were preyed upon by amphibians and phytosaurs -distant relatives of the crocodilians (Sect. 4.6.2). Piscivorous ichthyosaurs and nothosaurs dominated the oceans, while early pterosaurs skimmed squids and fishes from the surface water. Numerous marine taxa became extinct at the end of the Triassic, including the primitive archosaurs - the phytosaurs, aetosaurs, and rauisuchians. These extraordinary animals will be discussed in the following chapters. The dinosaurs evolved from thecodont reptiles, and the mammals from theropsids about 230 mya.

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