Nilssonoceras nilssoni ( Boll, 1857 )

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 68-70

Nilssonoceras nilssoni ( Boll, 1857)

Figs. 10B , 12C, F , 13H, C.

* Orthoceras nilssoni sp. nov.; Boll 1857: 69, pl. 3: 6.

Orthoceras wahlenbergii sp. nov; Boll 1857: 70, pl. 3: 8.

Orthoceras nilssoni Boll ; Karsten 1869: 46, pl. 16: 1.

Orthoceras wahlenbergii Boll ; Karsten 1869: 47, pl. 16: 3.

Orthoceras centrale Hisinger 1837 ; Barrande 1874: 694, pl. 438: 6–8.

Orthoceras tortum sp. nov.; Angelin 1880: 4–5, pl. 4: 12–15,17, pl. 6: 12, 13.

? Orthoceras centrale Hisinger 1880 ; Angelin: 4, pl. 5: 1–3.

? Orthoceras centrale Hisinger ; Remelé 1883: 29.

Orthoceras nilssoni Boll ; Rüdiger 1889:1–3.

Orthoceras wahlenbergii Boll ; Rüdiger 1889: 8–9.

“ Orthoceros ” nilssoni (Boll) ; Jaanusson and Mutvei 1951: 631, 634.

“ Orthoceros ” nilssoni (Boll) ; Jaanusson 1960: 279.

Orthoceras nilssoni Boll ; Hucke and Voigt 1967: 56.

“ Geisonoceras ” nilssoni (Boll) ; Neben and Krueger 1971: pl. 15: 12, 13.

“ Orthoceras ” nilssoni (Boll) ; Dzik 1984:92–95, 97, text−fig. 34, pl.22: 2.

“ Orthoceras ” wahlenbergi (Boll) ; Dzik 1984: 94, text−fig. 34, pl. 21: 1–8.

Eosomichelinoceras tortum (Angelin) ; King 1999: 147, fig. 5A, B.

Paratype: A fragment of a phragmocone (18–24 mm in diameter, 115 mm in length) in the collection of the MMW designated originally by Boll as Orthoceras nilssoni from the Red Orthoceratite Limestone (Aserian?) of the erratics of Neubrandenburg / Germany .

Material.—One−hundred and thirty−three specimens, mainly from the Remelé Collection in the NMB, where it was labelled as Orthoceras tortum or Orthoceras nilssoni , and the Holm Collection in the NRM under the name of Orthoceras nilssoni . Five specimens came from the collection of erratics in GIH and GIG, three specimens labelled O. wahlenbergi and one specimen Orthoceras nilssoni from the original Boll Collection in the MMW.

Description.—Conch form: Very slightly cyrtoconic conch. Cross section laterally slightly compressed. Apical angle of the conch of the specimen as figured in Boll (1857): 2.3°. Apical angle of the entire material (n = 121) varies greatly: mean 3.4°, with a maximum angle of 8.5° and a minimum rate of 1.7° (see Fig. 8 ).

Ornamentation: Shell transversally striated by sharp, acute striae ( Fig. 6B ). The distance between the striae strongly varied in the course of the ontogeny of the specimens and between the specimens (0.3–1 mm). Slightly undulating shell in juvenile specimens. Sometimes widely punctured.

Phragmocone: Chamber height x: 0.75, max: 1.2, min: 0.25 (n = 95) of the respective diameter. Boll gives a chamber height of 0.83 of the respective diameter. Suture straight. Shape of the septa mammoidal. Mural faces sometimes slightly longitudinal striated. Orthochoanitic septal necks. Ovate septal foramina. Endosiphuncular annuli which are very strong in juvenile chambers and fusing into an intrasiphuncular lining in the apical part of the conch ( Fig. 12F ). The irregular, bullet−like hypo− and episeptal deposits vary in the 18 cut and polished specimens in their grade of exposition. Sometimes cameral deposits lack also in juvenile chambers, but never in very apical parts of the shell. Sometimes it is difficult to detect the endosiphuncular annuli if the endosiphuncular lining is strong and strongly recrystallized, thus cuts of specimens can show endosiphuncular lining, endosiphuncular annuli and cameral deposits in any variety of these features. The position of the siphuncle is centromarginal to subcentral. A column of measurements (n = 9) gives a mean position of 0.33 (if the centre of the conch is 0.5, the rim 1 or 0) with a minimum of 0.22 and a maximum of 0.4. The thickness of the siphuncle is in the mean 0.14 of the respective diameter of the shell (n = 9, max = 0.17, min = 0.13).

Living chamber: The adult living chamber displays a long simple conus: 208 mm at a diameter of 25 mm. The maximum diameter of Nilssonoceras nilssoni is 83 mm, although the majority of specimens' fragments are smaller than 10 mm in diameter; Nilssonoceras nilssoni therefore after Kinnekulloceras kinnekullense is the largest of the Orthocerataceae of the Orthoceratite Limestone.

Remarks.—According to Boll (1857) Orthoceras wahlenbergi differs from Orthoceras nilssoni only in the apical angle. As shown in Fig. 8 the apical angle of Nilssonoceras conforms to a simple Gauss distribution with a major peak at a = 3.4, which represents N. nilssoni sensu stricto, and a minor peak at 4.6, which represents the values of Orthoceras wahlenbergii Boll. There is, however, no reason to split the species according to the apical angle. Dzik (1984) mentioned that “ O. wahlenbergi and O. nilssoni may be merely conspecific morphotypes”. This opinion will be followed here and Orthoceras wahlenbergi Boll assigned to the species Nilssonoceras nilssoni and accepted only as a variation of the latter ( N. nilssoni var. wahlenbergi ). Rüdiger (1889) shows that O. tortum is a synonym of N. nilssoni and the figured specimen in King (1999) exactly illustrates an adoral part of a phragmocone of N. nilssoni . Unfortunately the holotype of O. tortum Angelin was not found in the collection of the NRM. The only available paratype originally labelled by Angelin as O. tortum , does not have the outer shell preserved.

Comparison.— Nilssonoceras nilssoni differs from Orthoceras sodale Barrande, 1870 in its shape of the striae and from Orthoceras primum Barrande, 1870 in the cross section of the shell. (The shell of Orthoceras primum is additionally to the striation undulated.) Nilssonoceras nilssoni differs from Geisonoceras rivale (Barrande, 1859) in the shape of the striae and the endosiphuncular deposits, and it differs from Kinnekulloceras kinnekullense in different sculpture.

Distribution.—The oldest evidence is given by Jaanusson and Mutvei (1951) in the highest Holen Limestone (Kundan) of Dalarna and by King (1999) of Öland. Nilssonoceras nilssoni also occurs in the Segerstad, Seby, and Folkeslunda Limestone, and the Furudal Limestone (Aserian–Uhakuan) of Öland, Närke, Dalarna, Västergötland, and Östergotland, and in the erratics of northern Germany and Poland.

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.

Fig. 8. Histogram of the entire number of observed specimens of Nilssonoceras nilssoni (Boll) (n = 130) classified by the expansion rate (e).

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).