Arionoceras lotskirkense, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 72-73

Arionoceras lotskirkense sp. nov.

Figs. 12G , 13L, M.

Holotype: The specimen NRM−Mo 154079. Adult chamber and nearly mature parts of the phragmocone, recrystallized shell well preserved.

Type locality: Löts Kyrka of Öland ( Sweden).

Type horizon: Folkeslunda Limestone (Lasnamägian).

Derivation of the name: From the locality of Löts Kyrka in Öland.

Material.— Fifteen specimens available in the NRM (termed O. schmalensei nomen nudum by an anonymous former student) . One specimen is housed at the MMW, and 3 specimens available at the NMB. Some specimens with preserved adult chambers and living chamber .

Diagnosis.— Small, smooth Orthoceratacea with a maximum diameter of no more than 12–13 mm and a length of approx.

140 mm. Very short suborthochoanitic septal necks and a thin tubular siphuncle.

Description.—Conch form: Slightly cyrtoconic conch. Cross section very slightly compressed. Mean apical angle of the conch 5.1°.

Ornamentation: Shell smooth. Very minute inaequidistant growth lines, which are ventrally slightly bent forward.

Phragmocone: Mean chamber height 0.5 of the respective diameter. Sutures straight. Septa simple, concave. Very short suborthochoanitic septal necks. Subcentral siphuncle. Siphuncular diameter ca. 0.08 of the respective diameter of the conch. No cameral and siphuncular deposits are known (only relatively adoral chambers are observed, Fig. 12G ).

Living chamber: Adult living chamber cylindrical and compressed. Aperture widens with a thickening of the shell in the most adoral part, forming a slight constriction on the cast.

Comparison.— Arionoceras ariel (Barrande, 1866) and Arionoceras valens (Barrande, 1866) are clearly larger than Arionoceras lotskirkense . Arionoceras gyratum ( Miagkowa, 1967) shows a larger adult diameter and a higher apical angle.

Distribution.— Arionoceras lotskirkense occurs in the Segerstad (Aserian), Seby and Folkeslunda Limestone (Lasnamägian) of Öland and Dalarna, and in the Upper Red and Grey Orthoceratite Limestone (Aserian–Lasnamägian) of the erratics of northern Germany.

Figures

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.