1. ice sheets
The sea levels are directly connected with the global climate. When the global climate rises, the sea levels increase, and when the global temperature lowers down, the sea levels decrease. The reason for this is that with the rise of the global temperature, the ice sheets and the glaciers start to melt, thus lot of water that was frozen is released into the sea, resulting in increasing sea levels. When the temperature is lower though, the water from the sea starts to freeze, so it ends up trapped in the ice sheets, thus decreasing the sea levels.
2. Triassic
The first mammals appeared in the late Triassic period, around 200 million years ago, though some clues suggest that it might have been few million earlier. The mammals actually appeared on the global scene almost at the same time as the dinosaurs, being only 10 million years apart. The ancestor of the mammals are the synapsids (also referred to as stem-mammals), which were the dominant animals before the Permian mass extinction. The smaller synapsids that managed to survive the mass extinction later gave rise to the mammals.
3. tetrapods
The first four-legged animals that reached the land are classified as tetrapods. The term tetrapods actually means that these animals have four limbs. The animal groups that fall into this category and are still in existence are the mammals, amphibians, reptiles, and birds. In the past, the synapsids and the dinosaurs also were part of this classification. The first tetrapods were amphibians, and they were able to live both on land and in water, having an appearance like a cross between a small reptile and fish.
4. Permian
Pangaea is the latest of the numerous super-continents that have formed during the geological past of Earth. This super-continent formed at the end of the Permian period, and lasted until the Jurassic period when it started to break up after 100 million years of existence. The parts that started to break up from Pangaea gradually gave rise to multiple new continents, which are actually the continents that we know nowadays, with India being the exception as it has merged with Eurasia in meantime.
5. The glaciations occur because of the periodic changes in the orbit of the Earth. The Earth doesn't have a fixed orbiting pattern, and over longer periods of time, its orbit is changing, causing greater or lesser exposure to solar radiation on the hemispheres. When the Northern Hemisphere is the one that has less sunlight, than the glaciation starts, and the global temperatures drop, resulting in the formation of enormous ice sheets and glaciers. The reason why the glaciers grow during a glaciations is simple - the temperatures are much lower, thus the water freezes and forms glaciers, and since the ice has the property to increase its volume, they spread even more.
6. The sea levels are constantly changing, mostly it is a slow process, but occasionally it has been a very quick process, depending on the factors that have led to it. Because the sea levels have been increasing and decreasing, lot of areas that were underwater in the past, now are part of the landmasses. This brings to an interesting situation where marine fossils are constantly found on dry land, even at places such as deserts or the Himalayas. This is actually one of the best proofs that the continental drift exists, the climate has been changing in the past, and that the sea levels have had lot of variations.
