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| Heterocarinella japonica Aiba, 2026 |
Abstract
A new genus and species of the family Turrilitidae, Heterocarinella japonica gen. et sp. nov., is described from the Santonian (Upper Cretaceous) of the Haboro and Kotanbetsu areas in Hokkaido, northern Japan. This taxon is characterized by a persistently turriconic shell, a siphuncle positioned near the upper edge of the flank, and a vertically asymmetrical keel at or below the middle flank of the whorls. Notably, the keel differentiates Heterocarinella japonica gen. et sp. nov. from all other genera belonging to the Turrilitidae. The family Turrilitidae has previously been considered to range from the middle Albian to the late Cenomanian. However, the occurrence of Heterocarinella japonica gen. et sp. nov. demonstrates that this family survived until at least the Santonian. In addition, Tridenticeras from the Turonian to Coniacian of Europe, which is often assigned to the Nostoceratidae, is here re-evaluated as more appropriately belonging to the Turrilitidae. The previous misclassification likely resulted from limited knowledge of the position of the siphuncle, an internal structure, and from the tendency to prioritise stratigraphic consistency over morphological characters in taxonomic assignments. The discovery of Heterocarinella japonica gen. et sp. nov. suggests that the Turrilitidae survived the Cenomanian–Turonian Oceanic Anoxic Event 2 (OAE-2) while progressively narrowing their palaeogeographic range, ultimately contracting only in the northwestern Pacific realm.
KEYWORDS: ammonoid, Heterocarinella, Northwestern Pacific realm, Santonian, Turrilitidae, Yezo Group
Order Ammonitida Haeckel, 1866
Suborder Ancyloceratina Wiedmann, 1966
Superfamily Turrilitoidea Gill, 1871
Family Turrilitidae Gill, 1871
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| Heterocarinella japonica gen. et sp. nov. from the Haboro and Kotanbetsu areas, Hokkaido, northern Japan. A–J, MCM-A1322 (holotype): A, lateral view with the aperture facing the observer; B–D, lateral views after successive 90° rotations; E, apical view; F, basal view; G, tomographic image; H, whorl section; I, suture line; J, 3D model, viewed at approximately 45° posteriorly from the orientation of D. |
Genus Heterocarinella gen. nov.
Type species.—Heterocarinella japonica Aiba gen. et sp. nov.
Diagnosis.—This genus is characterised by turriconic whorls in contact with each other, each bearing a keel at or just below the middle flank, and a siphuncle located near the upper edge of the flank. The whorl cross-section is vertically asymmetrical with respect to the keel, with a gently curved upper part above the keel apex and a somewhat steeper, sharper lower part. The keel is continuous and is not interrupted by ribs, except for a double collar rib associated with a constriction that appears in the later growth stage.
Discussion.—Heterocarinella gen. nov. exhibits several features consistent with those of other turrilitid genera, including the presence of a siphuncle near the upper edge of the flank, turriconic whorls throughout ontogeny, and a double collar rib associated with a constriction appearing in the later growth stage.
Heterocarinella japonica sp. nov.
Type locality.—Float in a tributary of Machiyoizawa Creek in the Haboro area of northwestern Hokkaido, northern Japan.
Daisuke Aiba. 2026. Heterocarinella, a New Ammonoid Genus: Turrilitidae Survived into the Santonian (Late Cretaceous) in the Northwestern Pacific Realm. Paleontological Research. 30(1); 164-173. DOI: doi.org/10.2517/prpsj.260014 (29 August 2026)
Daisuke AIBA (相場 大佑；深田地質研究所). 2026. Heterocarinella, a new ammonoid genus: Turrilitidae survived into the Santonian (Late Cretaceous) in the north-western Pacific realm [新属アンモナイト類Heterocarinella：北西太平洋地域ではツリリテス科はサントニアン期（白亜紀後期）まで生き残っていた]. Paleontological Research. vol.30, p.164-173. DOI: doi.org/10.2517/prpsj.260014 [2026年8月29日]
【プレスリリース】北海道で中生代白亜紀の新属新種アンモナイトを発見 Heterocarinella japonica (ヘテロカリネラ・ジャポニカ) と命名（10/14更新 https://fukadaken.or.jp/?p=9866
