The early evolutionary history of belemnites: new data from Japan.

Abstract

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.