Archigeisonoceras repplingense, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 66-67

Archigeisonoceras repplingense sp. nov.

Figs. 11E, F, G , 12D, E .

Geisonoceras sp. (aff. O. nilssoni sensu Angelin, 1880 ); Dzik 1984: 93, fig. 33, pl. 21: 6, 10.

Holotype / paratype: The specimen NRM−Mo 3285 (holotype), which possesses the four last chambers of the phragmocone and the adult chamber. The recrystallized external shell is partly preserved. The specimen NRM−Mo 3284 shows only subadult chambers. The external shell is partly preserved.

Type locality: Räpplinge of Öland ( Sweden).

Type horizon: Lower Grey Orthoceratite Limestone (Volkhovian).

Derivation of the name: From Repplingen, a village in Öland.

Material.— Fifty−eight specimens available in the NRM (termed O. repplingense , O. promontorii nomen nudum, and O. immutabile nomen nudum by an anonymous former student), 11 specimens from the NMB and one from the ES. Some specimens with preserved adult chambers and living chamber; sometimes the recrystallized shell is partly preserved.

Diagnosis.—Small Archigeisonoceras (adult diameter approx. 27 mm) with a medium apical angle.

Description.— Conch form: Conch slightly cyrtoconic. Cross section circular or very slightly compressed. Diameter of the adult living chamber: 26–29 mm. Apical angle of the conch 1.7–21.8°, mean e: 4.6° (n = 56).

Ornamentation: External shell smooth.

Phragmocone: Chamber height 0.3–0.6 (x = 0.3, n = 35) of the respective diameter. Sutures straight. Septa simple concave. Short orthochoanitic and in apical parts of the conch slightly cyrtochoanitic septal necks. Siphuncle subcentral to central. Siphuncular diameter ca. 0.13 of the respective diameter of the conch. In juvenile chambers hypo− and episeptal deposits ( Fig. 11E, F ). Small endosiphuncular annuli. Sometimes asymmetrical, thin lining as endosiphuncular deposits ( Fig. 11F ).

Living chamber: Living chamber cylindrical, aperture widens slightly.

Comparison.— Archigeisonoceras repplingense differs from Archigeisonoceras folkeslundense in its smaller adult size, slightly lower chamber height and the shape of the living chamber. It differs from Archigeisonoceras picus in the smaller adult size and in the general shape of the conch, from Archigeisonoceras robustum in its higher chambers and from Archigeisonoceras elegatum in its narrower siphuncle.

Distribution.— Archigeisonoceras repplingense occurs in the Lanna Limestone (Volkhovian), Holen Limestone (Kundan) of Öland, Östergotland, Västergotland and Närke. Dzik (1984) described Geisonoceras sp. (= Archigeisonoceras repplingense ) from erratics of the Volkhovian ( P. originalis Zone ) of northern Poland.

Figures

Fig. 11. Polished sections of the siphuncle. A. Ctenoceras schmidti (Noetling), NRM−Mo 160668b, Kandel (Estonia). Note the calcified connecting ring and the thin lining of a sphaerolitic−prismatic layer at the outer surface of the connecting ring, × 25. B. Archigeisonoceras folkeslundense sp. nov., NRM−Mo 3223, Källa (Öland). Note the porous structure of the calcified connecting ring and the sphaerolitic−prismatic layer, × 10. C. Orthoceras scabridum Angelin, NRM−Mo 154771, Södre Bäck (Öland), × 30. D. Plagiostomoceras fragile sp. nov., NRM−Mo 160559, Gardslösa (Öland), × 6.3. E. Archigeisonoceras repplingense sp. nov., NRM−Mo 3285 not numbered, Hälludden (Öland), note the asymmetrical development of the endosiphuncular annuli, × 18. F. Archigeisonoceras repplingense, NRM−Mo 3284, Hälludden (Öland), apical part, × 18. G. Archigeisonoceras repplingense sp. nov., NRM−Mo 3293, Torp (Öland), note the thin sphaerolithic−prismatic layer of the connecting ring, × 22.

Fig. 12. Polished sections of the siphuncle. A. Archigeisonoceras folkeslundense sp. nov., NRM−Mo 3223, Källa (Öland), showing the structure of the connectig ring and the layered structure of the endosiphuncular annuli, × 35. B. Archigeisonoceras picus sp. nov., NRM−Mo 14455, Resmo (Öland), apical part of the phragmocone, × 8. C. Nilssonoceras nilssoni (Boll), NRM−Mo 3328, Hulterstad Kanalen (Öland), middle part of the phragmocone, × 12. D. Kinnekulloceras kinnekullense (Foord), NRM−Mo 155245, Utby (Dalarna), × 12. E. Archigeisonoceras picus sp. nov., NRM−Mo 14455, Resmo (Öland), middle part of the phragmocone, × 8. F. Nilssonoceras nilssoni (Boll), NRM−Mo 3328, Hulterstad Kanalen (Öland), apical part of the phragmocone with irregular endosiphuncular annuli and lining and strong cameral deposits, × 12. G. Arionoceras lotskirkense sp. nov., NRM−Mo 3296, Hällar (Öland), × 25.