Barremian–Albian (Early Cretaceous) ammonite faunas of the Katsuuragawa Basin, southwest Japan

• The Lower Cretaceous ammonite fauna of Japan is studied.
• Twenty-four species are described from the Barremian to Albian interval.
• Three ammonite associations are recognised: lower, middle, and upper.
• The composition of these associations was influenced by environmental factors.

The Lower Cretaceous ammonite fauna of Japan was influenced by the Tethyan, Boreal and North Pacific realms with their oceanic current patterns and ammonite distributions. The hypothesis of oceanic circulation can be utilized to interpret the existence of the “Bering Strait” and the changing position of the “Boreal front,” that is the contact region of warm and cold-water masses. To understand such a system fully, a comprehensive understanding of the geographical distribution of ammonite faunas is required. The occurrence of twenty-five ammonite species, belonging to twenty genera, is confirmed in the Barremian to Albian of Japan. Of these, 24 species are described in this paper, including Barremites macroumbilicus sp. nov. The fauna can be divided into three associations, lower, middle, and upper, indicating late Barremian, late Aptian, and late Albian of the European standard zonation. The faunal characters suggest that the habitats of these ammonite faunas may have changed during later Early Cretaceous, with faunas characterizing three different environments, i.e., nearshore, intermediate, and distal shelf to upper slope setting. Ammonites of the lower association (late Barremian) are related to those of the Tethys, Boreal European, and circum-Pacific regions, and suggest that the Early Cretaceous Katsuuragawa Basin was deposited under the influence of currents from both high latitude and equatorial areas. The occurrence of Crioceratites (Paracrioceras) suggests that the Boreal European elements, including Simbirskites and Crioceratites (Paracrioceras), transited between Northwest Europe and Japan through the Arctic Sea, indicating that the Pacific Ocean was connected with the Arctic Sea at that time. The second association (late Aptian) is composed of ammonites of the Tethyan and circum-Pacific regions. As European Boreal ammonite taxa are absent in this association, it is concluded that the Pacific Ocean probably was not connected with the Arctic Sea at the time. Consequently, the “Boreal Front,” marking the contact between warm and cold water masses, was located at mid-latitude in the “Bering Strait” region during Barremian and subsequently moved northward during Albian. The uppermost ammonite association (late Albian) also consists of Tethyan and circum-Pacific taxa. Desmoceras (Pseudouhligella) poronaicum expanded its range with northward and eastward circulation of oceanic currents, suggesting the current must have represented a warm water-mass from the equator. The species subsequently migrated from the southern Katsuuragawa Basin to the Hokkaido area during late to latest Albian.