Archigeisonoceras folkeslundense, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 68

Archigeisonoceras folkeslundense sp. nov.

Figs. 7 , 11B , 12A , 13J, K.

Holotype: The adult specimen (two fragments) NRM−Mo 155476, 155477, which preserves juvenile as well as adult chambers and parts of the living chamber.The recrystallized external shell is partly preserved.

Type locality: Folkeslunda of Öland ( Sweden).

Type horizon: Seby Limestone (Lasnamägian).

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

Material.— 35 specimens available in the NRM (termed O. rabrum nomen nudum and O. tallbergi nomen nudum by an anonymous former student), one specimen known 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.—Large Archigeisonoceras (maximum diameter: 46 mm) with a medium apical angle and very well developed endosiphuncular annuli.

Description.— Conch form: Conch slightly cyrtoconic. Cross section slightly compressed in juvenile specimens and circular in adult ones. Apical angle of the conch 1.7–5.7°, mean e: 4.6° (n = 32).

Ornamentation: Shell smooth with slight irregular growth lines.

Phragmocone: Chamber height 0.3–0.6 (x = 0.5, n = 24) of the respective diameter. Sutures straight. Septa simple concave. Short orthochoanitic septal necks in adult suborthochoanitic necks in juvenile parts of the phragmocone. Siphuncle subcentral to central. Siphuncular diameter 0.1–0.12of the respective diameter of the conch. In juvenile chambers hypo− and episeptal deposits (see Fig. 7 ) and in the apical parts of the siphuncular tube well developed endosiphuncular deposits forming annuli at the septal foramen which fuse apically. Sometimes the episeptal deposits form a horseshoe−shaped thin lining on the concave side of the septa surrounding the area of the siphuncular neck and a tongue toward the ventral mural area (see Fig. 13J).

Living chamber: Adult living chamber cylindrical. Aperture not known.

Comparison.— Archigeisonoceras folkeslundense differs from Archigeisonoceras repplingense in its greater adult size and in the living chamber shape, from Archigeisonoceras robustum in its higher chambers, and from Archigeisonoceras elegatum in its narrower siphuncle.

Distribution.— Archigeisonoceras folkeslundense occurs in the Segerstadt, Seby and Folkeslunda Limestone (Aserian – Lasnamägian) of Östergotland, Västergötland, Öland, and Dalarna and from the Upper Grey Orthoceratite Limestone of the erratics of northern Germany.

Figures

Fig. 7. Longitudinal section of the apical part of the phragmocone of Archeisonoceras folkeslundense sp. nov. (NRM−Mo 155476, holotype, diameter of the shell 10 mm).The cameral and endosiphuncular deposits are dotted. Note the short cyrtochoaniticseptal necks.

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.