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Jurassic Marine Faunal Differentiation in North America
Ralph W. Imlay
Journal of Paleontology
Vol. 39, No. 5 (Sep., 1965), pp. 1023-1038
Published by: SEPM Society for Sedimentary Geology
Stable URL: http://www.jstor.org/stable/3555321
Page Count: 16
You can always find the topics here!Topics: Ammonoidea, Fauna, Arctic regions, Genera, Fossils, Oceans, Seas, Climatic zones, Boreal age, Early Jurassic epoch
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Faunal differentiation of some Jurassic mollusks started on a worldwide scale near the middle of Bajocian time. Differentiation is shown mostly by ammonites and by the pelecypod genera Buchia and Inoceramus. Before differentiation began, the Jurassic molluscan faunas were cosmopolitan, or nearly so. After differentiation, the molluscan faunas, as reflected primarily in the ammonites, divided into three large but shifting geographic groupings which have been named by Arkell the Boreal, Tethyan and Pacific realms. The ammonite faunas of the Boreal and Tethyan realms intermingled in California and central Europe. The ammonite faunas of the Pacific realm intermingled with those of the Boreal realm in the Pacific Coast region of North America and in Japan. Elsewhere in the Pacific Ocean areas they intermingled with faunas of the Tethyan realm. Within the Pacific realm the characteristic ammonite genera at most places were outnumbered by ammonites of the other realms. Latitudinal shifting of Boreal and Tethyan molluscan faunas occurred several times in the Pacific Coast region of North America. During the Callovian and early Kimmeridgian, ammonites of Boreal affinities extended southward into central California. During the late Oxfordian and Portlandian, ammonites of Tethyan affinities dominated as far north as south-western Oregon. In Eurasia similar faunal shifts occurred at approximately the same times and in the same directions. Such shiftings were probably related to regional changes in the marine connections between the oceans, but possibly were related also to changes in solar radiation. The primary cause of faunal differentiation during Middle and Late Jurassic times probably was the general emergence of continents during Bathonian time, coupled with the development of physical barriers between the oceans near middle Bajocian time. These events probably resulted in partial isolation and cooling of the Arctic Ocean and in changes in ocean currents that influenced the dispersal of organisms. Although most of the faunal differentiation is related to latitude, cooling of the Arctic Ocean was not a primary cause of differentiation, as shown by the fact that some differentiation occurred throughout the Jurassic seas of the Pacific Ocean area.
Journal of Paleontology © 1965 SEPM Society for Sedimentary Geology