The Lias has a much more restricted extension than the later Jurassic stages. At the end of the Triassic had begun a transgression of the sea (the Rhaetic) which flooded many of the inland basins, and the same transgression continued into the Lias, proceeding northward from the Mediterranean, and covering large areas in central and southern Europe, as well as a belt across England, but not extending to Russia. By far the greater part of the Eurasian land mass was above the sea, and fresh-water lakes extended across Siberia, while in China widespread deposits of Liassic coal were accumulated.
Very early in the Upper Jura the transgression of the ocean was renewed, and this time on a vastly larger scale, cutting the continents by seas and straits, invading great areas that had long been land, and covering the larger part of Europe and Asia. This is one of the greatest transgressions of the sea in all recorded geological history, but it did not greatly affect North America. Central and northern Russia were submerged by an extension of the sea from the north, and in this Russian sea was developed a highly characteristic fauna. Strata distinguished by the Russian fauna extend into the northeast of England and through Siberia, Spitz-bergen, Nova Zembla, Alaska, the Black Hills of South Dakota, and the uppermost Jurassic of California and Mexico, and even penetrated to the Himalayan sea. In peninsular India the Jura is represented by the upper division of the Gondwdna system, which, as before, was laid down as continental deposits, containing much coal. The continental mass to which India then belonged was cut off from Asia by a strait or sea which covered the site of the Himalayas, and which was connected with the Mediterranean or Thetys by way of Persia and Asia Minor. The great Jurassic transgression submerged considerable areas of northern India, as it also covered narrow areas along the eastern and western coasts of Australia. Much of Madagascar was under water, but the southern portion is believed to have formed part of the narrow land which extended from South Africa to India. Some of eastern Africa was covered by a bay of the Indian Ocean, but no marine Jurassic has been found in the southern or western portions of that continent.
In South Africa, the uppermost part of the Karroo system, like the corresponding portion of the Indian Gondwana, is Jurassic.
The very striking faunal differences which obtain between different regions have led certain observers, especially the late Professor Neumayr of Vienna, to the conclusion that climatic zones had already been established in Jurassic times, - Boreal, central European, and Alpine or Equatorial zones, with corresponding ones in the southern hemisphere. This conclusion as to climatic belts is, however, a very doubtful one, and is in conflict with the distribution of the several geographical faunas, for the central European fauna is found in equatorial Africa, and the supposed equatorial fauna occurs in the Andes 200 of latitude south of its proper boundary. It is much more likely that some of the marked faunal differences are due to varying facies, depth of water, character of bottom, etc., and even more to the partly isolated sea-basins and the changing connections which were established between them. On the other hand, the Boreal fauna, both in its restrictions and in its migrations, seems definitely to lead to the conclusion that the Arctic Sea was distinctly colder than the other oceans, though there is no likelihood that the difference of temperature was nearly so great as at present, or that the Arctic contained ice, even in winter. "The suggestion of climatic influence on the dispersion of marine animals in the Upper Jurassic is very strong." (J. P. Smith).