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 morphological simplicity, numerous homeomorphies and iterative evolution. The Orthocerida, straight cephalopods that are characterised by a wide chamber spacing, a thin tubular siphuncle and a small spherical initial chamber, lacking a cicatrix (Kröger 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 Orthocerida are known from the Tremadocian. Here, I reassign the long known middle Tremadocian “Orthoceras attavus” to the new genus Slemmestadoceras belonging to a group of worldwide distributed orthoceratoids. The presence of Slemmestadoceras with a thin, probable tubular siphuncle and small initial chambers in the middle Tremadocian suggests that the Orthocerida may have originated already at that time. The comparison of Slemmestadoceras with following late Tremadocian 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; Kröger 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 deposits known from the Silurian Dissidoceratidae and several Early Ordovician families (Zhuravleva 1994; Evans 2005). The Orthocerida was emended by Kröger 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 Dissidocerida. Herein I always refer to the emended order Orthocerida sensu Kröger 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 either 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 central, 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, which occurred in the earliest Moridunian (earliest Floian). The latter two groups are orthocones with a wide marginal or eccentric siphuncle with characteristic endosiphuncular deposits. This may suggest an ancestry of the Orthocerida from vacuosiphonate Protocycloceratidae of the Ellesmerocerida by narrowing of the siphuncle and simultaneously widening of septal spacing in the Floian.

Fig. 1.

Orthoceratoid cephalopod Slemmestadoceras attavus (Brøgger, 1882) from Black nodule layer at base of Bjørkåsholmen Formation (Bienvillia angelini bed), Paltodus deltifer Conodont Zone, Timeslice lb of Webby et al. (2004), Tremadocian, Lower Ordovician; Oslo Region, Norway. A. PMO 1870b, lateral view of largest known specimen. B. PMO1219b, antisiphuncular? view. C. PMO 1219a, lateral view, note the long body chamber and the slightly oblique septa.

Fig. 2.

Details of the median section of detail of siphuncle and septa of Slemmestadoceras attavus (Brøgger, 1882), PMO 1219a, from Black nodule layer at base of Bjørkåsholmen Formation, Tremadocian, Lower Ordovician; Slemmestad, Oslo Region, Norway. Apex downward. Note the tubular siphuncle at left. Grey areas at right show traces of upward bent septa. A. Original photograph. B. Intepretation of the visible structures. Septa and septal necks dark grey, Connecting ring white line.

The monotypic new genus Slemmestadoceras from the middle Tremadocian (Stairsian equivalent) Bjørkåsholmen Formation of Norway possesses a narrow marginal siphuncle, orthochoanitic septal necks, and straight sutures, which are typical characters of the Orthocerida (Figs. 1, 2). In Slemmestadoceras, the connecting ring is poorly preserved and it is difficult to decide if the ring was slightly concave or tubular (Fig. 2). The slender, straight shell, and the nearly tubular, thin connecting ring are characters that occur in a number of cephalopods of similar and slightly younger age. In Kyminoceras forresti Teichert and Glenister, 1954 from the Emanuel Creek limestone, of Western Australia the rings also appear tubular or only slightly concave. K. forresti is considered of mid-Tremadocian (equivalent with the Rochdale Formation, compare Kröger and Landing 2008) by Glenister (1952), but Zhen et al. (2001) correlate the lowermost Emmanuel Limestone with earliest Floian. A third genus of this group of slender orthocones with thin tubular, or slightly concave connecting rings is Vassaroceras (Ulrich et al. 1944), from the Stairsian Rochdale Formation of New York, USA. In Vassaroceras the connecting ring is unusually thin and tubular, or slightly concave (Kröger and Landing 2008).

Therefore, together with Slemmestadoceas attavus, which is described herein, one or two other middle Tremadocian species are known, comprising slender orthoconic shells and thin nearly tubular connecting rings. These middle Tremadocian forms are transitional to the Orthocerida of the Floian. However, because the available data on the connecting ring shape are equivocal and data on apex morphology are lacking, a classification at the order level is impossible.

An additional enigmatic form is “Ellesmeroceras elongatum Kobayashi”, described by Balashov (1955) from the Tremadocian Ustkutsk Regional Stage of Siberia, which is externally very similar to Ellesmeroceras but internally differs in having an eccentric tubular siphuncle.

In an attempt to resolve the early phylogeny of the early orthoceratoids I compiled a character matrix of several Tremadocian and Floian taxa (Table 1). This compilation includes key taxa of early orthoceratoids of the families Baltoceratidae, Polymeridae, Troedssonellidae, and of orthocone ellesmerocerids belonging to the Cyptendoceratidae, Ellesmeroceratidae, and Protocycloceratidae, and representatives of the Orthoceratidae. The genus Rioceras was included as representative of a group of slender ellesmerocerids, which are falsely assigned to the Baltoceratidae since Flower (1964) and are often referred to as “vacuosiphonate Baltoceratidae” (compare Evans 2005: text-fig. 10). Regrettably, with the current data no resolution is possible that would support the unequivocal placing of one of these families in a monophyletic clade supporting the Orthoceratoidea. Even the introduction of directionality of the siphuncle shape gives no satisfactory results as it produces more than hundred similar parsimonious trees and the consensus tree is not resolved. Several very different phylogenetical scenarios are possible and the paraphyly of the Dissidocerida cannot be ruled out with the current data (Fig. 3).

Table 1.

Character matrix for typical Tremadocian and Floian orthoceratoid cephalopods and straight annulated Ellesmerocerida. Cross section: 1, compressed, 2, circular, 3 depressed. Ornamentation: 1, annulated; 2, smooth. Siphuncle position: 1, marginal; 2, eccentric; 3, central-subcentral. Siphuncle diameter relative to conch cross section: 1, narrow (≤ 0.1); 2, medium (> 0.1 ≤0.3); 3, large (> 0.3). Connecting ring shape: 1, concave; 2, tubular; 3, convex. Septal distance given as number of chambers per distance similar to conch cross section: 1, narrow (≥ 8); 2, medium (> 8 ≥ 5); 3, wide (< 5). Suture shape: 1, lobate; 2, straight. No sufficient resolution results in a consensus tree of the parsimony analysis, consequently no equivocal grouping and phylogenetic scenario can be constructed.

Therefore, the new genus Slemmestadoceras cannot be unequivocally placed within any higher taxon. Moreover, the placing of the genus within the Orthoceratoidea is problematical because there is no cladistic support for this subclass. However, the presence of Slemmestadoceras shows that the earliest potential Orthoceratida appeared already in the middle Tremadocian, (Timeslice 1b; Webby et al. 2004; Stairsian and equivalent beds). The beginning of the Floian, roughly six million years later, when a large number of new orthoceratoid morphologies appeared almost simultaneously, marks the start of the Palaeozoic story of orthoceratoid success.

Systematic palaeontology

Fig. 3.

Two possible phylogenetical trees of the early evolution of Orthoceratoidea. Based on the cladistic analysis (see text) several alternative trees are possible. A. Tree with Dissidocerida and Orthocerida independently evolving and a paraphyletic Orthoceratoidea origin, suggesting Slemmestadoceras gen. nov. may be an ancestal orthocerid. B. Tree concurrent with Evans (2005: text-fig. 10), suggesting an ancestry of the Orthocerida from vacuosiphonate Protocycloceratidae of the Ellesmerocerida by narrowing of the siphuncle and simultaneously widening of septal spacing in the Floian. Stratigraphic occurrences corrected based on Ordovician stratigraphic chart of Webby et al. (2004: fig. 2.1–2). Grey intervals mark main radiation events in the Stairsian and equivalent beds and earliest Floian. Abbreviations: Ca., Cambrian; Darr., Darrivillian.