Earth's Triassic Period

The Triassic Period followed the Permian Period and lasted from around 251 to 199 million years ago. The Triassic begins the three-stage Mesozoic Era which is most known for the evolution and proliferation of the dinosaurs. The beginning of the Triassic was marked by a series of major extinctions as have been previously described in the discussion of the Permian Period.

Continental drift was continuing into the Triassic and the major continents were still fused into one giant landmass called Pangaea. However, the westward encroachment of a large body of water called the Tethys Sea began to widen the coastline of Pangaea and reduce some of the desert conditions at its interior. The rest of the Earth continued to be covered by the vast Panthalassa Ocean. Although the deserts began to shrink during the Triassic, the climate was still relatively hot and dry due to the distance between shorelines and continental interiors. The continental climate became more seasonal, as it had during the early Permian Period, with hot summers alternating with cold winters. There is little evidence for glaciation at the poles during the Triassic Period, which meant that sea levels would have been higher than they were during the Permian Period.

As in the Permian Period, tree-ferns and other plants that reproduced through spores and required moist habitats did not fare well in the hot and dry Triassic climate. As in the late Permian, forests were now dominated by conifers and other gymnosperms: plants that use seeds in order to reproduce. Regional diversity was able to progress much more than in the Permian due to the encroachment of the Tethys Sea to the interior of Pangaea as well as a more active storm cycle. With the comeback of widespread vegetation and forest over once desert areas came the resurgence of large herbivores.

Marine fossils from the Triassic are rare compared to other periods in which the continents were broken up. The small shoreline meant that marine mineral deposits did not form as easily or extensively. Most of the marine fossil deposits that scientists have been able to study come from inland lagoons or hypersaline lakes in continental interiors. It is clear that bivalves (clams and their relatives), ammonoids (nautilus-like creatures), and the brachiopods survived the mass-extinction and continued to dominate the water in the Triassic Period. Fish fossils reveal a remarkable uniformity, which suggests that few fish species survived the mass extinction. It is also during the Triassic that modern forms of marine invertebrates evolved such as the starfish and sea urchins. As on land, the reptiles were beginning to dominate the marine ecosystems. The dolphin-like ichthyosaurs were evolving as well as seal-like nothosaurs, and the first plesiosaurs.

Following the Permian-Triassic mass extinction, the Earth was slowly repopulated by survivors of the extinction, new groups that would play out brief success stories, and new groups that would go on to flourish in the Mesozoic world. New groups of reptiles known as the archosaurs began to replace synapsids (mammal-like reptiles) as the dominant reptiles on Earth. Archosaurs include present-day crocodiles and birds, as well as their ancestors and dinosaurs. The reason for their success might be due to the inability of warm-blooded therapsid predators to adapt to the increasingly arid environment. Reptiles continued to diversify into many shapes and forms including proto-crocodiles, flying pterosaurs, and the first dinosaurs. Although the mammal-like reptiles largely died out as a result of the Permian-Triassic mass extinction, some survived and were very successful in the Triassic. One species of synapsid known as lystrosaurus at one point accounted for nearly one half of all animal life on land. It is also during the Triassic that the first true mammals evolved. Although these tiny relatives of humans were to remain in the evolutionary shadows for millions of years, their importance would emerge after the dinosaurs' rule was over on Earth.

The Triassic Period came to a close when the Earth experienced another mass extinction event. Although not as devastating as the mass extinction that opened the Triassic Period, this particular episode was particularly destructive to marine ecosystems. It is unclear what caused this Late Triassic Extinction, but many scientists have attributed the disappearance of a number of species to asteroid impacts on the Earth's surface. The extinction of many of the large land animals allowed the newly evolved dinosaurs to fill the ecological niches and begin one of the most successful living dynasties ever to play out on the planet.