Orthoceras scabridum Angelin, 1880

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 61-62

Orthoceras scabridum Angelin, 1880

Figs. 4 , 11C .

Orthoceras regulare v. Schlotheim; Boll 1857: 69–70, pl. 3: 7a–c.

Cycloceras trochleare Hisinger ; Eichwald 1860: 1223–1225, pl. 51: 23 a, b.

Orthoceras regulare Schlotheim ; Karsten 1869: 46, pl. 15: 8a–e (fide Angelin, 1880).

Orthoceras trochleare (Hisinger) ; Karsten 1869: pl. 17: 7a, b.

* Orthoceras scabridum sp. nov.; Angelin 1880: 4, pl. 4: 6–9, pl. 7: 8–10.

pars Orthoceras scabridum Angelin ; Rüdiger 1889: 5–8, pl. 1: 1, 2a, b, pl. 2: 9.

Orthoceras scabridum Angelin ; Holtedahl 1909: 40–42.

Orthoceras scabridum Angelin ; Yü 1930: 54.

Geisonoceras scabridum Angelin ; Troedsson 1931: 16–17.

Geisonoceras constrictum sp. nov. [nomen nudum]; Troedsson 1931: 16.

“ Geisonoceras ” scabridium (Angelin) [nom. null.]; Sweet 1958: 9–10.

“ Geisonoceras ” scabridium (Angelin) [nom. null.]; Sweet 1959: 296.

Orthoceras scabridum Angelin ; Hucke and Voigt 1967: 56.

“ Geisonoceras ” scabridum (Angelin) ; Neben and Krüger 1971: pl. 30: 4–6.

Orthoceras scabridum Angelin ; Dzik 1984: 97, 105, pl. 24: 1, 2, 6–8, text−figs. 35c, d, 39.6.

Holotype / paratype: Two specimens illustrated in Angelin (1880): pl. 4:

6, 7, 9 from Öland are housed at the NRM. The adult chambers of the phragmocone, the living chamber and the recrystallized shell are preserved.

Material.— Forty−three specimens housed in the NRM and the NMB, and five specimens at the MMW, most of them with preserved adult chambers of the phragmocone and living chamber. Sometimes the shell is partly preserved .

Description.—Conch form: Cross section of the conch slightly compressed or circular in juvenile specimens. Maximum diameter: 21 mm in the aperture (mean diameter of the aperture 17 mm). Mean diameter of last chamber of the adult phragmocone: 16 mm. Apical angle of the conch 5.1° (mean). Apical angle of the illustrated types in Angelin (1880): 4°.

Ornamentation: Shell with fine, narrowly spaced transversal striae (<1 mm). The raised spaces between the striae appear imbricated (see Fig. 6A ). The striae turn apical at the lateral sides. The striae of the ventral side fall back in comparison to that of the opposite side (see Fig. 4E for shape of the aperture, which resembles the outline of the growth

n 12

10

8

6

4

2

0

0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 e lines.). Inner shell smooth, sometimes irregular, punctured as in Orthoceras regulare . The cast shows a normal line at the ventral side of the phragmocone.

Phragmocone: Chambers height 0.5 (mean) of the respective diameter, sometimes with strong variations during the whole ontogeny. Sutures straight, septa strongly concave. Impressions of the mural areas display fine longitudinal lines. Short orthochoanitic septal necks. Subcentral siphuncle. Siphuncular diameter 0.1 of the respective diameter of the conch. Cameral and endosiphuncular deposits not observed.

Living chamber: Adult living chamber long conical in shape. Adult aperture with lateral openings (see Fig. 4E ). Adult living chamber with two, more or less developed (crescent shaped), lateral impressions and a connected constriction at the last third (see Fig. 4 ). The ventral side of this constriction is slightly flattened, so that a cross section of the adult chamber through the area of the constriction is triangular. Aperture widens rapidly adorally to the constriction.

Remarks.— Angelin (1880) gave the first detailed description of Orthoceras scabridum . He observed the narrow, lowly elevated and inaequidistant striae of the shell and the impressions of the living chamber: “ Concameratio ultima foveis transversalibus distantibus duabus vel tribus constricta. ” ( Angelin 1880: 4). Rüdiger (1889) mentioned many more important details of the shell of Orthoceras scabridum . He observed the imbricated nature of the many so−called fine “Ringürtel” of the shell. He very carefully described the dorsomyarian muscle attachment scars, which are preserved in many cases in the deepest parts of the living chamber casts in many cases. Furthermore, he mentioned a variation in the typical constrictions/impressions of the living chamber illustrated in Fig. 4 . Troedsson (1931 a) described an extreme form which occurs in the Seby Limestone of Öland, under the name “ Geisonoceras constrictum ”(nomen nudum). The lateral impressions of these variations (see Fig. 4D ) are much less or only incipiently developed and only a ring−like furrow is visible. The two specimens examined in the NRM were considered to fall within the variation of Orthoceras scabridum . Until now there is no evidence that these specimens belong to a preceding taxon of Orthoceras scabridum , because older specimens found in the Segerstad Limestone show the typical shape of the adult living chamber. Instead it is interesting to consider how closely Orthoceras scabridum in some cases resembles Orthoceras bifoveatum ( Fig. 3D ). The adult Orthoceras scabridum is usually smaller than Orthoceras bifoveatum ; its relative siphuncle diameter is significantly greater and the apical angle of the shell is clearly smaller (see Fig. 5 ).

Distribution.— Orthoceras scabridum occurs in the Segerstad, Seby, and Folkeslunda Limestone (Aserian–Lasnamägian) of Öland, Dalarna, Västergotland, Östergotland, and in the erratics of northern Germany and Poland. Holtedahl (1909) mentioned it in the Coelospheridium Beds (Kukrusian) of the Lake Mjösa district in Norway.

Figures

Fig. 4. Variation of the body chamber impressions and aperture in adult Orthoceras scabridum Angelin (A–D, × 0.8; E, × 1). A. NRM−Mo 3140. B. NRM−Mo 155513. C. NRM−Mo 155520. D. NRM−Mo 154202. E. NRM− Mo 155508.

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

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. 5. Histogram of the entire number of observed Orthoceras (n = 66) classified by the expansion rate (e). The frequency peak between e = 0.04 (a = 2.3°) – 0.05 (a = 0.9°) represents O. bifeovatum Noetling and O. regulare Schlotheim, the frequency peak between e = 0.09 (a = 5.1°) – 0.1 (a = 5.7°) represents O. scabridum Angelin.