Plagiostomoceras fragile, Kröger, 2004

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 64-65

Plagiostomoceras fragile sp. nov.

Figs. 11D , 12G, F .

? Orthoceras sp. nov. ex aff. longicameratum Foord (1888); Teichert

1928: 125, 126.

Holotype: The specimen NRM Mo−155529, which shows the last four chambers and the whole living chamber.

Type locality: Böda, Stora Mossen of Öland ( Sweden).

Type horizon: Folkeslunda Limestone (Lasnamägian) of the Orthoceratite Limestone.

Derivation of the name: From Latin fragilis, fine, breakable:referring to the small size of the adult conchs.

Material.— Ten specimens available in the NRM, 5 with preserved adult chambers and living chamber; sometimes the shell is partly preserved.

Diagnosis.— A fragile Plagiostomoceras (no more than 13 mm in diameter in adult size) with very low apical angle and extremely long chambers.

Description.—Conch form: Cross section slightly compressed. Mean diameter of last chamber of the adult phragmocone: 9 mm. Apical angle of the conch very low 1.2–2.4°, mean e: 1.7°.

Ornamentation: Shell with very fine, low elevated, narrowly spaced, oblique, transversal striae. Inner shell irregular, punctured as in Orthoceras regulare . Mural areas with fine longitudinal lines.

Phragmocone: Chamber height 0.7–1.5 of the respective diameter. Sutures straight. Septa strongly concave. Orthochoanitic septal necks. Subcentral siphuncle. Siphuncular diameter 0.09–0.1 of the respective diameter of the conch. Neither cameral or endosiphuncular deposits are observed.

Living chamber: Adult living chamber long conical, bulging slightly outwards. Aperture simple, straight with a slight constriction at the very adoral part.

Comparison.— Plagiostomoceras fragile can be detected by its small adult size, low apical angle and high chambers. Orthoceras ariel Barrande, 1870 and Orthoceras expectans Barrande, 1870 show a greater adult diameter, a lower chamber height and a smooth shell. The siphuncle of Orthoceras expectans is significantly thinner than that of Plagiostomoceras fragile . Parts of the phragmocone differ from the juvenile parts of Orthoceras regulare and Orthoceras bifeovatum in a thin siphuncle, and from Orthoceras scabridum in a weaker sculpture and longer chambers.

Paratype: A small fragment of a phragmocone (length 23 mm, diameter 15–16 mm) from the erratics of the Red Orthoceras Limestone (Aserian?) of northern Germany in the Boll Collection ( MMW number) originally labelled by Boll as Orthoceras laevigatum . The recrystallized outer shell is preserved.

Material.— Two specimens are available in the NMB , one at the MMW and 21 at the NRM (paratypes 154093–96, 14454, 144548) .

Description.—Conch form: Cross section slightly compressed. Maximum diameter: 21 mm. Apical angle of the conch very low being about 1.5–2°.

Ornamentation: Shell is smooth with slight irregular slightly oblique growth lines and a faint longitudinal striation which is considerable only in very well preserved specimens. Growth lines forming a sinus at the antisiphonal side of the conch.

Phragmocone: Chamber height ~0.5 of the respective diameter. Sutures straight. Septa strongly concave. Orthochoanitic septal necks. Subcentral siphuncle. Siphuncular diameter 0.08 of the respective diameter of the conch. Neither cameral or endosiphuncular depositions observed; living chamber and apical parts of the phragmocone not known.

Distribution.— Plagiostomoceras fragile occurs only in the Folkeslunda Limestone of Öland.

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.

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.