Permian period:
So by the Permian, by about 250-260 million years ago, we finally have something that looks like a modern terrestrial ecosystem, with invertebrate detritivores, herbivores and carnivores, and tetrapod herbivores and carnivores. So what happened?
If we look at plant diversity, we observe that a major transition in life occurs across the Permo-Triassic boundary; it is such a major change that this is also recognized as the boundary between the Paleozoic and the Mesozoic. Hosts of water-requiring lineages go extinct, and there is a great diversification in the Mesozoic of conifers, cycads and related drought-tolerant plants. This extinction is not confined to plants on land, but also affects the synapsids - all the dicynodonts and most cynodonts go extinct. But the real extinction is in the oceans, where up to 95% of all species living in the Late Paleozoic go extinct - we lose such old standbys as trilobites. What is the cause of this change?
It appears to involve continental motions caused by plate tectonics. In the latest Paleozoic, all the continents drew together to form a giant continent, Pangea.
Plate tectonics is a contingent feature of Earth history. The continents move around because of the energy within the planet, it's sort of a dance in which the continents separate and come back together over hundreds of millions of years. As it chances, a great "separation" took place at the beginning of the Paleozoic, and a great "coming together" took place at the end of the Paleozoic. The result Pangea ("All Earth") had enormous effects on global environments, and thus on life.
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