Archigeisonoceras picus, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 67-68

Archigeisonoceras picus sp. nov.

Figs. 10A , 12B, E , 13I.

Holotype: The adult specimen NRM−Mo 14455a–c, which has juvenile as well as adult chambers and parts of the living chamber. The recrystallized external shell is partly preserved.

Type locality: Resmo of Öland ( Sweden).

Type horizon: Limbata Limestone (Volkhovian).

Derivation of the name: From Latin picus , woodpecker or bird of prey, referring to the shape of the shell.

Material.— One specimen available in the NRM (termed O. picus nomen nudum by Holm) .

Diagnosis.— Archigeisonoceras relative cyrtocone, of medium size (maximum diameter: 38 mm) and with high apical angle.

Description.—Conch form: Slightly cyrtoconic in cross section, slightly compressed. Apical angle of the conch 6.3°.

Ornamentation: External shell smooth with minute, irregular, spaced growthlines.

Phragmocone: Chamber height 0.2of the respective diam − eter. Sutures straight. Septa simple concave. Short orthochoanitic septal necks in adult parts of the phragmocone, suborthochoanitic in apicad parts. Siphuncle subcentral to central. Siphuncular diameter 0.1 of the respective diameter of the conch. In middle parts of the phragmocone episeptal deposits ( Fig. 12E ) and well developed endosiphuncular annuli and asymmetric lining in the apical parts of the siphuncle ( Fig. 12B ).

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

Comparison.— Archigeisonoceras picus differs from Archigeisonoceras repplingense in its greater adult size, and the lower height of chamber, from Archigeisonoceras folkeslundense in its lower chamber height and smaller adult size and from Archigeisonoceras elegatum in its narrower siphuncle.

Distribution.— Archigeisonoceras picus occurs only in the Limbata Limestone (Volkhov) of Öland.

Figures

Fig. 10. Longitudinal section of Archigeisonoceras picus sp. nov., NRM−Mo 14455, Limbata−Limestone, Resmo (Öland) (A) and Nilssonoceras nilssoni (Boll, 1857), NRM−Mo 3328, Segerstad−Limestone, Hulsterstadkanalen (Öland) (B) showing the development of cameral and endosiphuncular deposits (× 0.55).

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.