Orthoceras bifoveatum Noetling, 1884

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 60-61

Orthoceras bifoveatum Noetling, 1884

Fig. 3 .

Orthoceras trochleare Hisinger ; Karsten 1869: 50, pl. 17: 7b.

* Orthoceras bifoveatum sp. nov.; Noetling 1884: 111–115, pl. 16: 5, 6, pl. 17: 3, 4a, pl. 18: 1, 2.

pars Orthoceras scabridum Angelin ; Rüdiger 1889: 5–8, pl.1: 1, 2a, b.

Orthoceras bifoveatum Noetling ; Troedsson 1931: 16, 29–31, pl. 1: 3–5, pl. 4: 6–10, text−fig. 2.

Bifoveoceras bifoveatum (Noetling) ; Balashov and Zhuravleva 1962: pl. 10: 9–11.

Bifoveoceras (“ Orthoceras ”) bifoveatum (Noetling) ; Hucke and Voigt 1969: 57, pl. 16: 1.

Orthoceras bifoveatum Noetling ; Dzik 1984: 95, 105, pl. 22: 8–10, text−figs. 34a, 39.4.

Material.— Seventeen specimens available in the NRM, 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: Cross section is slightly compressed or circular. Maximum diameter: 30 mm in the aperture (mean diameter of the aperture 25 mm); mean diameter of last chamber of the adult phragmocone: 21 mm (n = 6). Apical angle of the conch is 2.3–2.9°.

Ornamentation: Shell with fine, narrowly spaced transversal striae (<1 mm). The raised spaces between the striae appear imbricated (see Fig. 6A ) in nearly mature and mature growth stages and sharp and acute in juvenile ones. Striae turn apical in the flanks. Striae of the ventral side fall back in comparison to that of the opposite side. Inner shell smooth, sometimes slightly irregularly punctured, as in Orthoceras regulare . The cast shows a normal line at the ventral side of the phragmocone.

Phragmocone: Chamber height approx. 0.5 (mean) of the respective diameter. Sutures straight. Septa strong by concave. Impression of the mural areas display fine longitudinal lines. Short orthochoanitic septal necks. Siphuncle subcentral. Siphuncular diameter 0.15–0.2of the respective di − ameter of the conch. Hypo− and episeptal and endosiphuncular deposits were observed. Endosiphuncular deposits occur at the most apical parts of the shell ( Dzik 1984). Endosiphuncular deposits in form of highly irregular asymmetrical annuli/lining, living chamber long (max. 120 mm).

Living chamber: Conical adult living chamber with two, more or less developed (half−moon shaped), lateral impressions in the last quarter. The impressions are highly variable in shape (see Fig. 3 ), length and depth. Aperture widens rapidly orad of the constriction.

Remarks.— The species was described in detail by Noetling (1884) and Troedsson (1931 a). The recent observation of a larger amount of material confirms the findings of both authors. Juvenile specimens of the genus Orthoceras show a set of features which allow a good differentiation at the species level. Troedsson (1931 a) says that “The banding (of Orthoceras bifoveatum , the author) [...] is rather similar to that in Geisonoceras scabridum ...” ( Troedsson 1931 a: 29), but it is distinctly coarser. The fine longitudinal striation of the shell of Orthoceras regulare and the dimension of the siphuncle of Orthoceras scabridum are good diagnostic traits for excluding specimens from Orthoceras bifeovatum . The adult size of Orthoceras bifoveatum is intermediate between Orthoceras regulare and Orthoceras scabridum .

Distribution.— Orthoceras bifoveatum occurs in the Seby and Folkeslunda Limestone (Lasnamägian) of Öland and Östergotland and in the Furudal Limestone (Uhakuan) of Dalarna. It is also found the Upper Grey Limestone (Lasnamägian) of the erratics of northern Germany and Poland.

Figures

Fig. 3. Variation of the body chamber impressions in adult Orthoceras bifeovatum Noetling (right flanks, apex down, × 0.7). A. NMB C. 5383, coll. Remelé 1241. B. NRM−Mo 154270. C. NMB C. 5384, coll. Remelé. D. NRM−Mo 154268.

Fig. 6. Schematic sketches of the cross section of the shell. Shell ornamentation of Orthoceras (A), Nilssonoceras (B), and Kinnekulloceras (C) (aperture to the right).