Gondwanan Nautiloid Cephalopods from the Ordovician of Myanmar

Abstract

Abstract. Two species of Darriwilian (late Middle Ordovician) nautiloid cephalopods are described from the Wunbye Formation and its equivalent strata in the Shan Plateau of Myanmar (Sibumasu Block). They are the orthocerid Sibumasuoceras langkawiense (Kobayashi) and the discosorid Tasmanoceras sp. First, Sibumasuoceras is proposed for a new genus of the cayutoceratin pseudorthoceratids. Sibumasuoceras langkawiense [originally Ormoceras langkawiense, the type species of the genus] was previously assigned either to the Actinocerida or the Discosorida. However, the present investigations reveal that it possesses thin connecting rings and differentiated endosiphuncular deposits and lacks a detailed endosiphuncular canal system, all suggestive of a relationship to the Orthocerida. Sibumasuoceras is known to occur so far from Malaysia and Myanmar of the Sibumasu Block, which was part of northern Gondwana during the early Palaeozoic. Second, the rare genus Tasmanoceras, which was previously known only in Tasmani...