Dinosaurs evolved when much of the Earth’s landmass was concentrated into the supercontinent Pangea. They roamed throughout Pangea, which began to break apart during the early Triassic, caused by a seafloor-spreading rift. Pangea’s partitioning resulted in isolated populations, engendering speciation as climates diverged into different biomes, with distinct plant and animal life.
Dinosaurs rapidly radiated to fill available ecological niches. Their bodies burgeoned in size 220 MYA, as a way to dominate competitors. Upsizing slowed by 200 MYA, and even went into reverse in some lineages, most impressively toward the eventuation of birds.
Dinosaurs proliferated during the early Jurassic, becoming the dominate land animal, fitting into most every ecological niche: as scavengers, hunters, and herbivores. 1,850 dinosaur genera are estimated to have existed. Most genera had only 1 to a very few species, so there may have been just ~2,000 different types of dinosaurs.
The Jurassic (201–145 MYA) was the great age of dinosaurs, though the biggest and best-known dinosaurs did not emerge until the Cretaceous (145–66 MYA). By that time there were thousands of species. Some supersized to become the largest terrestrial animals of all time.
The supercontinent Pangea was breaking up throughout the Jurassic and Cretaceous periods. The process became especially marked during the Cretaceous. Sea level rose so much that there were shallow seas on the continents. About 1/3rd of the land was covered with water.
In the initial stages of Pangea becoming piecemeal the large landmasses had extreme continental climates, with marked seasons and little rain. These arid conditions returned at the end of the Cretaceous, when sea level fell.
The tectonic exertions enlivened volcanic activity. As such, these periods were punctuated with extinction events. At the end of the Cretaceous, there was a great outpouring of basaltic lava in India, known as the Deccan Traps.
Global temperature was high in the early- to mid-Cretaceous. Swamp forests flourished along the borders of the Tethys Ocean.
As the leaves and trees of the forest fell, peat accumulated. Upon geological pressure and heat, the land deposits eventuated into coal, while organic marine decay produced petroleum: the fossil fuels.