Iba Yasuhiro, Sano Shin-ichi, Mutterlose Jörg
Department of Natural History Sciences, Hokkaido University, Sapporo, Japan.
Fukui Prefectural Dinosaur Museum, Katsuyama, Japan.
PLoS One. 2014 May 2;9(5):e95632. doi: 10.1371/journal.pone.0095632. eCollection 2014.
Belemnites (Order Belemnitida), a very successful group of Mesozoic coleoid cephalopods, dominated the world's oceans throughout the Jurassic and Cretaceous. According to the current view, the phylogenetically earliest belemnites are known from the lowermost Jurassic (Hettangian, 201-199 Ma) of northern Europe. They are of low diversity and have small sized rostra without clear grooves. Their distribution is restricted to this area until the Pliensbachian (191-183 Ma). Here we describe two new belemnite taxa of the Suborder Belemnitina from the Sinemurian (199-191 Ma) of Japan: Nipponoteuthis katana gen et sp. nov., which represents the new family Nipponoteuthidae, and Eocylindroteuthis (?) yokoyamai sp. nov. This is the first reliable report of Sinemurian belemnites outside of Europe and the earliest record of typical forms of Belemnitina in the world. The Sinemurian belemnites from Japan have small to large rostra with one deep and long apical groove. Morphologically these forms are completely different from coeval European genera of Hettangian-Sinemurian age. These new findings suggest that three groups of Belemnitina existed in the Hettangian-Sinemurian: 1) European small forms, 2) Japanese very large forms, and 3) the typical forms with a distinctive apical groove, reported here. The Suborder Belemnitina therefore did not necessarily originate in the Hettangian of northern Europe. The new material from Japan documents that the suborder Belemnitina had a much higher diversity in the early Jurassic than previously thought, and it also shows strong endemisms from the Sinemurian onwards.
箭石（箭石目）是中生代非常成功的一类头足纲软舌螺动物，在整个侏罗纪和白垩纪时期主导着世界海洋。根据目前的观点，系统发育上最早的箭石发现于北欧最底层的侏罗纪（赫唐阶，2.01 - 1.99亿年前）。它们的多样性较低，吻部较小且没有明显的沟槽。在普连斯巴奇阶（1.91 - 1.83亿年前）之前，它们的分布仅限于该地区。在此，我们描述了来自日本辛涅缪尔阶（1.99 - 1.91亿年前）的箭石亚目两个新的箭石分类单元：新属新种日本剑箭石（Nipponoteuthis katana gen et sp. nov.），它代表了新科日本箭石科；以及新种横山野氏始柱箭石（Eocylindroteuthis (?) yokoyamai sp. nov.）。这是欧洲以外地区关于辛涅缪尔阶箭石的首次可靠报道，也是世界上典型箭石亚目形态的最早记录。来自日本的辛涅缪尔阶箭石吻部大小不一，有一条深且长的顶端沟槽。在形态上，这些形态与同时期欧洲赫唐阶 - 辛涅缪尔阶的属完全不同。这些新发现表明，在赫唐阶 - 辛涅缪尔阶存在三类箭石亚目：1）欧洲的小型形态；2）日本的非常大型形态；3）此处报道的具有独特顶端沟槽的典型形态。因此，箭石亚目不一定起源于北欧的赫唐阶。来自日本的新材料证明，箭石亚目在早侏罗世的多样性比之前认为的要高得多，并且从辛涅缪尔阶开始就表现出强烈的地方特有性。
