Ctenoceras schmidti Noetling, 1884

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 63

Ctenoceras schmidti Noetling, 1884

Fig. 11A .

Orthoceras undulatum Sowerby ; Schmidt 1858: 197.

Orthoceras verticillatum v. Hagenow; Krause 1877: 24–25. Cycloceras hisingeri Boll ; Schröder 1880: 63.

* Ctenoceras schmidti sp. nov.; Noetling 1884: 116–122, pl. 16: 7, 8, pl. 18: 3–5a.

Ctenoceras schmidti Noetling ; Troedsson 1931: 16.

Ctenoceras schmidti Noetling ; Balashov and Zhuravleva 1962: 11: 8. Ctenoceras schmidti Noetling ; Sweet 1964: 255, fig. 155: 1a–c. Ctenoceras schmidti Noetling ; Neben and Krüger 1971: 17, figs.14–15. Ctenoceras schmidti Noetling ; Neben and Krüger 1973: 81, figs.14–17. Ctenoceras schmidti Noetling ; Dzik 1984: 97, 105, fig. 39.66.

Material.— Four specimens available in the NRM, nine at the NMB and the GIH, most specimens with preserved adult chambers of the phragmocone and living chamber. The shell is generally partly preserved .

Description.—Conch form: Slightly cyrtocone. Cross section is slightly compressed. Maximum diameter: 17 mm at aperture; apical angle of conch very low (~ 1°).

Ornamentation: Shell annulated (distance of two ribs 2–3 mm) with fine, narrowly spaced transversal growth lines (<0.5 mm) parallel to the annulations, faint longitudinal costules. Annulation somewhat oblique, describing a lobe on the antisiphuncular side of the conch. Cast with normal line at ventral side of phragmocone.

Phragmocone: Chamber height approx. 0.5 (mean) of the respective diameter. Sutures straight. Septa strongly concave. Short orthochoanitic septal necks. Outer part of the connecting ring with a thin spherulitic−prismatic layer. Remarks.— The shell shape and ornamentation is nearly similar to some orthocones of younger beds of the Ordovician and Silurian of the Baltoscandic area (e.g., Tofangoceras , Dawsonoceras ). Especially parts of the phragmocone are indistinguishable from these taxa if only the surface characters are mentioned. This similarity led in the past to a repeated confusion (e.g., Schröder 1881). Possibly the name “schmidti ” is borrowed from Orthoceras schmidti Boll, 1857 which is a Silurian kionoceratid with very similar ornamentation but different siphuncular shape. The badly preserved Ctenoceras sp. B from Sweet (1958) may also be an undescribed kionoceratid (see Dzik 1984: 120).

Distribution.— Ctenoceras schmidti occurs in the Upper Red Orthoceratitic Limestone (Lasnamägian) of Öland. It is also found in the Upper Grey Limestone (Lasnamägian) of the erratics of northern Germany and 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.