Middle and Late Jurassic

Although not many sedimentary rocks are preserved from the terrestrial Middle Jurassic, we can still obtain a relatively good idea about Middle and Late Jurassic climates because of a wealth of data available from oceanic sediments. Most importantly, the latter two-thirds of the Jurassic, as well as the entire Cretaceous, are thought to have been without polar ice or glaciers on the northern parts of the the continents. This is quite beyond our own experience; now, glaciers occur at high latitudes at both poles, and the poles themselves are covered in ice. The conclusion that there was no polar ice or glaciers in the Middle and Late Jurassic is based largely upon the presence of warm climate indicators at high latitudes, and upon the absence of any evidence of continental glaciation from that time. Biologically, warm climate indicators include plants and certain fish, whose distributions are thought to have been as high as 75° N and 63° S. This would put them beyond the polar fronts, which in turn suggests that the poles must not have been as cold then as they are today.

The absence of polar ice had an important consequence for climates: water that would have been bound up in ice and glaciers was located in ocean basins. This in turn means higher eustatic sea levels than now, which led to extensive epeiric seas. The increased abundance of water on the continents as well as in the ocean basins had a stabilizing effect on temperatures (because it decreased continental effects), and decreased the amount of seasonality experienced on the continents.

Continental climates are enormously variable and, as we have seen, short distances can encompass huge climatic differences. Indeed, there

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