Early Jurassic belemnites from the Gondwana margin of the Southern Hemisphere —Sinemurian record from South Tibet

Abstract

Belemnites (Order Belemnitida), a very successful group of Mesozoic coleoid cephalopods, dominated fossil coleoid assemblages throughout the Jurassic and Cretaceous. According to the current view, the phylogenetically earliest known belemnites have been reported from the lowermost Jurassic (Hettangian) of northern Europe. The earliest belemnites are characterized by low diversity and small-sized rostra. Their distribution has until recently been assumed to be restricted to northern Europe until the Pliensbachian. Early belemnites (the Sinobelemnitidae and the Belemnitina) are now also known from the Late Triassic of China and the Hettangian–Sinemurian of Japan, and therefore from the eastern Tethys and western Paleo-Pacific oceans. It has been suggested that the distribution of belemnites became more widespread and expanded to the Southern Hemisphere for the first time in the Toarcian. Here we re-describe a Sinemurian belemnite from South Tibet, which was located at the Gondwana margin at that time. The specimen is characterized by a long rostrum with one deep and long alveolar groove with a splitting surface, whose position cannot be determined as being either dorsal or ventral. This morphological feature allows an assignment of the re-described rostrum to either the Sinobelemnitidae or the Pachybelemnopseina. The Sinemurian belemnite from Tibet represents the earliest firm record of the Belemnitida from the Southern Hemisphere, suggesting that the wide distribution and diversification of this order was established much earlier than previously thought.