The Late Triassic period is an epoch which lasted from roughly 230 to 200 million years ago. It is also referred to as the Upper Triassic, because in paleontology, more recent eras tend to be represented by fossils in higher layers of sediment, while the fossils of older periods are found in deeper layers. The Late Triassic is best known for the emergence of the first species of dinosaurs, which is believed to have occurred during this period.
The Triassic as a whole spans 50 million years from 200 million years ago to 250 million years ago. It followed a massive extinction event dated to roughly 251.4 million years ago (which ended the Permian period), during which over two-thirds of land animals became extinct and virtually all sea life died.
During the Late Triassic, the Earth's land was still predominantly on a single continent, known to geologists as Pengaea, from which the current continents all broke off. The rest of the planet was, for the most part, covered by an ocean known as Panthalassa. The first rifts in Pangaea probably appeared during this period, but the continent did not actually begin to break up until during the next period, the Jurassic, before being completed during the period after that, the Cretaceous. The climate was relatively dry and hot, even at the poles (where there appears to have been dry land or ocean rather than permanent ice cover).
Because the continents were all locked together during the Upper Triassic, the spread of plant and animal life was relatively uniform, with successful species distributed across enormous spans of territory. (Contrast this with today, where distinctively different species exist on all continents, and particularly on the oldest continent, Australia.) Emerging steadily during this period were a new class of animals, the dinosaurs.
Most of the first dinosaurs were small, not the giant predators and sauropods of later periods, but their predecessors can still be seen in the Late Triassic. The first dinosaurs were small bipedal carnivores which emerged from an earlier class, the archosaurs, in a relatively gradual process (so that the division between the last archosaur and the first dinosaur in dinosaurs' genetic lineage is somewhat arbitrary). These early dinosaurs included Staurikosaurus, Eoraptor, Saturnalia, and Herrerasaurus. Herbivores like Thecodontosaurus also emerged, although the first of these were still bipedal, too. These sauropods were the ancestors of the later, massive herbivores of the Cretaceous, while the mostly carnivorous early theropods were the ancestors of the most recognized predatory species, like Tyrannosaurus.
The Late Triassic period ended 199.6 million years ago, in another mass extinction event known as the Triassic-Jurassic extinction event. This was not nearly as massive an extinction event as the mass deaths which occurred at the end of the Permian period (mentioned above), but it still destroyed about half of existing species. It was also immensely significant. Just as mammals had been relatively insignificant prior to the Cretaceous extinction event of 65 million years ago, it was dinosaurs who emerged victorious from the Triassic-Jurassic extinction event. Why this extinction event happened is still unknown, although an asteroid impact, climate change, and huge volcanic eruptions are all possibilities. Whatever the reason, large numbers of species were wiped out, and it was dinosaurs which filled the gaps. They would remain the predominant class of life forms throughout the next two periods, the Jurassic and the Cretaceous.