Akihiro Misaki

Belemnite extinction and the origin of modern cephalopods 35 m.y. prior to the Cretaceous−Paleogene event

Belemnites, a very successful group of Mesozoic cephalopods, flourished in Cretaceous oceans until the Cretaceous−Paleogene event, when they became globally extinct. Following this event the modern types of cephalopods (squids, cuttlefish, octopus) radiated in the Cenozoic in all oceans. In the North Pacific, however, a turnover from belemnites to the modern types of cephalopods about 35 m.y. before the Cretaceous−Paleogene event documents a more complex evolutionary history of cephalopods than previously thought. Here we show that the modern types of cephalopods originated and prospered throughout the Late Cretaceous in the North Pacific. The mid-Cretaceous cephalopod turnover was caused by cooling and the closure of the Bering Strait, which led to a subsequent faunal isolation of this area. In the Late Cretaceous the former niches of the fast-swimming belemnites were taken over by the modern types of cephalopods, which evolved endemically. The Cretaceous−Paleogene event only allowed the modern types of cephalopods to spread globally and to take over the niches previously held by belemnites.

First discovery of Pravitoceras sigmoidale Yabe from the Yezo Supergroup in Hokkaido, Japan

Abstract An almost intact specimen of an uppermost Cretaceous heteromorph ammonoid, Pravitoceras sigmoidale Yabe, was recently discovered from the Hakobuchi Formation of the Yezo Supergroup in the Hidaka area, Hokkaido. The specimen is preserved in a lenticular calcareous nodule. It retains two whorls of planispiral phragmocone and a succeeding retroversal hook, which is associated with a lower jaw apparatus near the aperture. P. sigmoidale occurs within the Inoceramus shikotanensis Zone at the Hidaka section. Patagiosites alaskensis, Gaudryceras sp., Inoceramus shikotanensis, and Gigantocapulus problematicus cooccur with P. sigmoidale. The megafossil assemblage resembles those of the Upper Campanian-Lower Maastrichtian in the Sakhalin area. P. sigmoidale has a very short range assignable to the Upper Campanian. However, it has been assumed to be an endemic species of the Izumi Group in southwest Japan, and has never before been found from northeast Japan. Scarcity of common zone-indexing taxa is a bottleneck for biostratigraphic research of the uppermost Cretaceous System in Japan. The first discovery of P. sigmoidale from the Hidaka area is a significant step for much more precise biostratigraphic correlation between the Yezo Supergroup and the Izumi Group.

Late Cretaceous Record of Large Soft-Bodied Coleoids Based on Lower Jaw Remains from Hokkaido, Japan

The origin and phylogenetic relationships of most modern coleoid groups have not yet been explained by reliable fossil evidence, in large part because of the reduction or disappearance of a calcified chambered shell during their evolutionary history. Herein we describe two exceptionally large coleoid lower jaws from the Upper Cretaceous strata in Hokkaido, Japan. On the basis of the comparison of gross morphology and morphometric data of the lower jaws of modern and fossil coleoids, we assigned the two lower jaws to the following new taxa: Nanaimoteuthis hikidai sp. nov. of the order Vampyromorpha (superorder Octobrachia) and Haboroteuthis poseidon gen. et sp. nov. of the order Teuthida (superorder Decabrachia). The lower jaw of N. hikidai is distinguished from other species of the same genus from the Upper Cretaceous of Vancouver Island (Canada) and Hokkaido by having a broader, more anteriorly curved hood of the outer lamella. The lower jaw of H. poseidon seemingly exhibits mosaic features like those of modern teuthids and sepiids but is assigned to Teuthida on the basis of the overall shape of the outer lamella and the development of a distinct fold on the lateral wall. Because of the unusually large lower jaws, these new taxa appear to be comparable in body size to modern giant squids (Architeuthis spp.) and the Humboldt squid (Dosidicus gigas). This and other discoveries of large jaws referable to octobrachian and decabrachian coleoids from the Upper Cretaceous strata of the North Pacific fill the gap in the relatively poor fossil record of mainly soft-bodied coleoids.

Discovery of mid-Cretaceous ammonoids from the Aridagawa area, Wakayama, southwest Japan

ABSTRACT Mid-Cretaceous ammonoid assemblages are newly identified from four localities in the Aridagawa area, Wakayama, southwest Japan. They consist of Middle to Upper Albian ammonoids such as Desmoceras (Pseudouhligella) dawsoni shikokuense, Puzosia subcorbarica, Mojsisoviczia sp., Oxytropidoceras sp., Mortoniceras sp., etc., and Mantelliceras japonicum, which indicates the Lower Cenomanian stage. The Albian ammonoids were found for the first time from the Chichibu Belt of Kii Peninsula. The discovery of these ammonoids indicates a considerably wide distribution of the mid-Cretaceous deposits in the Aridagawa area, and the necessity of a large revision of the previous stratigraphic division of the Cretaceous strata in the area. The mid-Cretaceous deposits in the Aridagawa area and their abundant molluscan fossils are important for reconstruction of the mid-Cretaceous paleoenvironment and tectonics in the eastern margin of Asia.

Gaudryceras tombetsense Matsumoto, a Maastrichtian ammonoid from the Aridagawa area, Wakayama, southwestern Japan

Abstract. Discovery of Gaudryceras tombetsense Matsumoto, 1984 in the Futakawa Formation of the Sotoizumi Group in the Aridagawa area, Wakayama, southwestern Japan establishes that this formation includes sediments of early Late Maastrichtian age, which is the youngest record of the Sotoizumi Group distributed in Shikoku and the Kii Peninsula. The occurrence of the youngest fossils from the easternmost area of the Sotoizumi Group may support the suggestion that the depocenter of the Sotoizumi basin migrated eastward due to movement along the Kurosegawa Tectonic Zone (KTZ), which was still active during Campanian to Maastrichtian time.

The jaw apparatus of the Late Cretaceous heteromorph ammonoid Pravitoceras

Abstract Well-preserved upper and lower jaws of the aptychus-type found inside the body chambers of two specimens of the heteromorph ammonoid Pravitoceras sigmoidale Yabe, 1902 (Nostoceratidae, Ancyloceratina) are described from the Upper Cretaceous Izumi Group in Southwest Japan. They are similar in overall morphology to those of other nostoceratid and diplomoceratid ammonoids currently known, suggesting the morphological stability of the jaw features among these taxa. The equal size of the upper and lower jaws with beak-like rostral projection suggests that the jaw apparatus of this species might function to bite and cut up prey.