A New Genus of Middle Tremadocian Orthoceratoids and the Early Ordovician Origin of Orthoceratoid Cephalopods

Abstract

The cephalopods of the subclass Orthoceratoidea, which are termed “orthoceratoids” herein, are a group that remains “the last unexplored wilderness in the Cephalopoda” (Flower 1962: 23). After 45 years this statement still holds true because phylogeny reconstructions are hindered by their morphologi− cal simplicity, numerous homeomorphies and iterative evolu− tion. The Orthocerida, straight cephalopods that are charac− terised by a wide chamber spacing, a thin tubular siphuncle and a small spherical initial chamber, lacking a cicatrix (Kroger 2006) were the ancestors of bactritoids, ammonoids, and coleoids (Engeser 1996). The origin of the Orthocerida is poorly understood. The earliest unequivocal Orthocerida are known from the Floian (Early Ordovician). A number of poorly known possible Orthocerida and/or stem group Ortho− cerida are known from the Tremadocian. Here, I reassign the long known middle Tremadocian “Orthoceras attavus ”t o the new genus Slemmestadoceras belonging to a group of world− wide distributed orthoceratoids. The presence of Slemmesta− doceras with a thin, probable tubular siphuncle and small ini− tial chambers in the middle Tremadocian suggests that the Orthocerida may have originated already at that time. The comparison of Slemmestadoceras with following late Trema− docian and Floian orthoceratoids demonstrates that a higher level taxon comprising these forms, such as the subclass Orthoceratoidea may constitute a paraphylum. The Orthoceratoidae comprise a wide range of orthocones with short septal necks and tubular or expanded siphuncles of the orders Ascocerida, Dissidocerida, Lituitida, Orthocerida, and Pseudorthocerida (Sweet 1964; Dzik 1984; Zhuravleva 1994; Kroger et al. 2007). Two main groups are recognisable within the Orthoceratoidae, orthocones with a conical apex, having a cicatrix (Ascocerida, Pseudorthocerida) and orthocones with small spherical apex lacking a cicatrix (Lituitida, Orthocerida). The Ascocerida and Lituitida are very characteristic orders that appear in the Middle Ordovician (Furnish and Glenister 1964; Dzik 1984). The Dissidocerida include orthocones with slightly expanded tubular siphuncles and rod−like endosiphuncular de− posits known from the Silurian Dissidoceratidae and several Early Ordovician families (Zhuravleva 1994; Evans 2005). The Orthocerida was emended by Kroger and Isakar (2006: 143) and comprises now only orthoceratoids with a spherical apex and a simple siphuncle. By contrast, the Orthocerida sensu Sweet (1964: K223) comprise a wide range of orthoceratoids classified today within the Orthocerida, Pseudorthocerida, and Dissido− cerida. Herein I always refer to the emended order Orthocerida sensu Kroger and Isakar, 2006 as orthocerids. The origin of the Orthocerida and Pseudorthocerida is poorly understood. In the Early Ordovician a number of orthocones occurred that are ei− ther poorly known proper members of these latter orders or stem group members, respectively. Classically the origin of Orthocerida was sought within the middle Floian (Flower 1962; Hook and Flower 1977) when the first straight orthocerids with wide chamber spacing and a cen− tral, narrow, empty siphuncle appear. Recently, Evans (2005) described the new genus Semiannuloceras from the early Floian (Moridunian) of Wales and classified it within the orthocerid family Baltoceratidae. Furthermore, Evans (2005) formulated a scenario of the early evolution of orthoceratoid cephalopods and the origin of Orthocerida, in which he suggests an ancestry of the Orthocerida from the Troedssonellidae and/or Polymeridae,