Slemmestadoceras attavus ( Brøgger, 1882 )

Kröger, Björn, 2008, Runter kommen sie immer: Kapi-Talfahrt, Acta Palaeontologica Polonica 53 (4), pp. 745-749 : 747-749

Slemmestadoceras attavus ( Brøgger, 1882)

Figs. 1 , 2 .

1882 Orthoceras attavus sp. nov.; Brøgger 1882: 53, 54, pl. 4: 9, 10, 17; pl. 10: 16.

1931 Orthoceras attavus Brøgger ; Foerste (1931): 280.

1953 Protocycloceras attavus (Brøgger) ; Balashov (1953): 212.

1999 Orthoceras attavus Brøgger ; Ebbestad (1999): 17.

Material.— Nine specimens from PMO. Impression of specimen PMO I 0001(the specimen itself is lost) in a nodule of black limestone from the Bjørkåsholmen Formation, labelled as paratype and originally figured in Brøgger (1882: pl. 4: 9), specimen PMO I 0004 (labelled as lectotype by unknown person) and figured in Brøgger (1882: pl. 10: 16) both from the base of the Bjørkåsholmen Formation, Vestfossen, Norway. PMO I 1875 – 1876, from Odden north of Slemmestad. Specimens PMO 1219a, 1220a from Bjørkåsholmen, Slemmestad, Norway, and PMO 58710a–c from a nodule of the Bjørkåsholmen Formation of unknown locality, collected by Lars Størmer in 1915 and 1936 respectively. Specimen PMO 1219a designated as lectotype herein.

Diagnosis.—Same as for genus.

Description.—Largest specimen PMO 1876 with conch cross section 9 mm. Angle of expansion approximately 0.11 (mean 0.11, n = 5). Cross section subcircular, very slightly compressed. Ornamented with regularly spaced wide transverse shallow undulations. Undulations form shallow ventral lobes, and at a distance equivalent to conch cross section diameter three rounded ridges occur. Depth of valley between ridges of undulation about 0.1 mm at conch cross section 6 mm. In specimen PMO I 0004 four ridges occur at distance similar to conch height. Conspicuous growth lines parallel to undulations, at specimen PMO 1876 approximately ten growth lines per millimetre. Shell thickness at conch cross section 9 mm 0.2 mm at PMO 1876. Septa nearly straight, sloping slightly adapically in direction of prosiphuncular conch side. Septal distance 2 mm at specimen PMO 1876. Conch straight or only very faintly bent. Siphuncle marginal. Long body chamber in juvenile growth stages. At specimen PMO 58710a conch diameter at base of body chamber 5.4 mm, body chamber length 16 mm, there apically five phragmocone chambers are preserved with septal distance 0.8 mm. Specimen very slightly bent with siphuncle at concave side of conch curvature.

Sutures straight and directly transverse. Siphuncle marginal, tubular with diameter 0.5 mm at conch cross section 5 mm. Septa in specimen PMO 1219a strongly recrystallized but orthochoanitic septal necks visible. Connecting ring preserved only as dark seam.

Remarks.—A small slender apex of Slemmestadoceras , similar to that of known Baltoceridae (e.g., Evans 2005: pl. 3: 15; pl. 4: 16) can be assumed by the slender conch fragments of the smallest preserved specimen, which are approximately 2 mm in diameter. However, it is not known if the apex is spherical or conical.

All known specimens from the Bjørkåsholmen Formation are from dark limestone nodules (“schwarze Kalksteinellipsoide” Brøgger 1882: 16) from a layer in the lowermost part of the formation. This level can be traced throughout the entire Bjørkåsholmen Formation in the Oslo Region; it contains abundantly the trilobite Bienvillia angelini (Linnarsson, 1869) as well as other trilobites and mollusks. Ebbestad (1999) supposed a depositional setting well below a normal storm wave base or even below a storm wave base. Beneath Slemmestadoceas attavus undescribed specimens of Ellesmeroceras sp. co−occur in the same horizon that are in the collection of the PMO. Lamansky (1905: 27) described “ Orthoceras attavus ” from the Billingen Regional Stage (B 1 beta) of Popovka, St. Petersburg Oblast, Russia. Lamansky’s original specimen at the Palaeontological museum of the VSEGEI at St. Petersburg (Nr. 122/222) shows surface characters similar to S. attavus . However, similar ornamented orthocerids are very common in the Early Ordovician, making a definite species and genus designation impossible and it remains questionable if this specimen is a Slemmestadoceras .

Stratigraphic and geographic range.—Same as for genus.

Acknowledgements.—The specimens described above where discovered at the Naturhistoriska Riksmuseet Stockholm, they are now returned to the Paleontologisk Museum Oslo. I am grateful for the support from Harry Mutvei and Elena Dunca (Naturhistoriska Riksmuseet Stockholm, Sweden), who helped to locate the specimens and supported various aspects of my work in Stockholm. The investigation was made possible by SYNTHESYS Grant SE−TAF 3189). The discussions with David Evans (Nature England, Petersborough, UK) increased the quality of the paper substantially, and Matilde Beresi (IANIGLA−Cricyt, Mendoza, Argentina) provided helpful comments. Ian Percival (Geological Survey of New South Wales, Lidcomb, Australia) provided information on the age of the Emmanuel Limestone cephalopods. Irene J. Blessing (Berlin, Germany) corrected the English. I am grateful to Thomas Servais (Université Lille I, Lille, France) for his general support during this study.

References

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Björn Kröger [[e-mail]], Université des Sciences et Technologies de Lille 1, Université Lille 1, Laboratoire Géosystèmes (UMR 8157 du CNRS), UFR des Sciences de la Terre – bâtiment SN5, 59655 Villeneuve d'Ascq cedex, France.

Figures

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 1b 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.