During the Jurassic, marginal seas occupied considerable areas in southern and northern Alaska and in the western part of the Kuskokwim region of southwestern Alaska. They appear to have been less extensive during Bathonian and Tithonian times and absent during late Callovian time. They appear to have been absent from large areas in the interior of Alaska. Connection of the northern with the southern seas may have occurred through Siberia, through Yukon territory or, possibly, through westernmost or central Alaska.

The ammonite succession in Alaska is similar in general to that in central and northern Europe; in the Lower Jurassic, it is essentially identical. In Bajocian and in Callovian to Kimeridgian beds, the ammonite succession in Alaska differs mainly by the presence of some genera that have been found only in areas bordering the Pacific Ocean. In contrast, the Bathonian rocks of Alaska contain ammonites, such as Arcticoceras, Arctocephalites, and Cranocephalites that are widespread in the Arctic region but are unknown in central Europe. Comparisons with the Tithonian of Europe are not possible because ammonites of that age are not yet known from Alaska. The presence of Tithonian strata, however, is shown by the occurrences of Buchia piochii (Gabb) at a few places.

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