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Oxygen isotope studies and biogeographic distributions indicate that the Jurassic Period was a time of only moderate latitudinal variation in temperature. Seasonal change was less well marked than at present. On land, vegetation varied geographically, but less so than at other times in earth history. Tetrapods from widely spaced land areas were quite similar. By contrast, marine invertebrates including forams, ammonoids, belemnoids, and pelecypods show distinct provincialism, and the boundaries of the marine invertebrate provinces shift through time. Physical controls that could have produced this provincialism include salinity and temperature differences in the sea, the distribution of shelf seas and deep water areas, and the geographic arrangement of sea and land, or a ombination of these. Consideration of likely geographic positions of the continents during the Jurassic (assuming that they have drifted since that time), and of the likely pattern of ocean currents of the time, explains many features of the pattern of biotic dispersal. It suggests that differences in water temperature were a prime physical control on the distribution of marine Jurassic life. In particular, it indicates that the Boreal realm was not a sea area of salinity lower than average as has been suggested previously. Analogy with other Mesozoic times, and certain evidences of bipolarity in biogeographic patterns in the Jurassic, further support the view that temperature difference was a prime physical controlling factor.
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