Studying marine paleobiogeographical conditions in the mid-latitudes of the Northwest Pacific around the Jurassic–Cretaceous boundary would be expected to contribute to a better understanding of the paleoclimatic and/or paleoenvironmental background of the evolution of the Late Mesozoic terrestrial ecosystem in East Asia. However, uncertainty about the paleogeography of the eastern margin of the Asian Continent has meant difficulties for paleobiogeographical discussion. In this paper, the strata containing Boreal faunal elements, Buchia, and cylindroteuthidid belemnites in East Asia (Far East Russia, Heilongjiang in northeastern China, and Japan) and their tectonic settings are reviewed.

The Uda and Torom (northern Sikhote-Alin), Suibin (Heilongjiang), and Tetori (northern Central Japan) regions were located from north to south in the eastern margin of the already amalgamated Asian Continent around the Jurassic–Cretaceous boundary and can be considered the “fixed points for paleobiogeographical reconstruction.” On the other hand, Buchia-bearing strata in the Komsomolsk (northern Sikhote-Alin) and Dong'an (Heilongjiang) regions can be considered to have been deposited in the fore-arc basin or trench slope basin on the accretionary complex along the East Asian continental margin. The strata around the Jurassic–Cretaceous boundary in the Partizansk Basin (southern Sikhote-Alin) contain both Buchia and Tethyan ammonoids and were deposited on the Paleozoic continental basement or block (Sergeevka Belt). The paleo-position of these three regions around the Jurassic–Cretaceous boundary is highly debated.

The Tethyan–Pacific ammonoids, Boreal belemnites, and Tetori bivalve fauna, showing some similarities with those in the Boreal Realm and Early Cretaceous strata in Heilongjiang, are present in the late Tithonian–Berriasian Mitarai Formation of the Tetori Group in the Tetori Region. This unit, deposited in the eastern margin of the North China Block, provides evidence that Boreal faunal elements reached the mid-latitudes of the Northwest Pacific. The position of the ecotone of the Boreal and Tethys realms in the Northwest Pacific can be discussed based on the comparison of the faunal elements among almost coeval strata in the Tetori Region (fixed point), the Sergeevka Belt, and the South Kitakami Belt (Pacific side of Northeast Japan), which is usually correlated with the Sergeevka Belt but contains only Tethyan faunal elements. Further studies of the records of Tethyan and Boreal taxa in the “fixed points” and other localities could provide clues to reveal the paleoclimatic and/or paleoenvironmental background of the evolution of the terrestrial and marine ecosystems in East Asia around the Jurassic–Cretaceous boundary.