Plagiostomoceras laevigatum ( Boll, 1857 )

Kröger, Björn, 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia, Acta Palaeontologica Polonica 49 (1), pp. 57-74 : 65-66

Plagiostomoceras laevigatum ( Boll, 1857)

Fig. 12A, B .

* Orthoceras laevigatum sp. nov., non Orthoceras laevigatum ( McCoy, 1844) ; Boll 1857: 71–72, pl. 3: 9.

Orthoceras laevigatum Boll ; Karsten 1869: 47: pl. 16: 2a, b.

? Orthoceras aff. ariel Barrande, 1868 ; Rüdiger 1889: 10–11.

Protobactrites delicatulum sp. nov.; Troedsson 1932: 32, 33, pl. 2: 5, pl. 4: 3, 4, pl. 7: 1.

Polygrammoceras delicatulum Troedsson ; Flower 1952: 36.

Protobactrites delicatulum Troedsson ; Dzik 1984: 105, 106, text−fig.39.9.

Remarks.—The only two specimens at the MMW, which were originally labelled as Orthoceras laevigatum by Boll are badly preserved fragments of (1) a specimen which consist of the mould and parts of the inner shell (which is slightly punctured as in O. bifeovatum ) and (2) a 28 mm long fragment (diameter 15–16 mm), preserving three chambers of the phragmocone without cameral deposits. The shell of the latter is smooth and its apical angle very low. The central siphuncle is tubular as originally described by Boll. Karsten (1869) described two specimens and mentioned their very low apical angle. Rüdiger (1889) placed Orthoceras laevigatum as a synonym of Orthoceras conicum Hisinger , which is a Rhynchorthoceras (see Dzik 1984: fig. 50), Rhynchorthoceras conicum (Hisinger) . It should consequently show heavy cameral deposits. But the paratype does not show cameral deposits. Furthermore, the apical angle of the specimen illustrated by Boll (1857: pl. 3: 9) clearly differs from the original description of O. conicum Hisinger, 1837 . Obviously Orthoceras laevigatum sensu Boll is not a Rhynchorthoceras conicum . The description of Orthoceras aff. ariel Barrande by Rüdiger (1889) matches exactly that of the specimens identified here as Plagiostomoceras laevigatum , but he does not give an illustration and its original material has not been localised so far. Therefore, no final answer can be given to the question as to whether Rüdiger’s Orthoceras aff. ariel is a synonym of Plagiostomoceras laevigatum . But the Silurian Orthoceras ariel Barrande shows a clearly higher apical angle than Plagiostomoceras laevigatum and yet is another species. Troedsson integrated some nautiloids, which exactly match the description of Boll (1858) into his new species Protobactrites delicatulum Troedsson, 1932 (holo − type: NRM Mo−14454a). It is therefore regarded as a subjective junior synonym of P. laevigatum .

It is noteworthy that the name “ Orthoceras laevigatum ” was wrongly used to label Cycloceras laevigatum M'Coy, 1844 by subsequent researchers (e.g., de Koninck 1880). Thus Orthoceras laevigatum Boll is clearly the senior homonym and therefore considered a valid taxon.

Distribution.— Plagiostomoceras laevigatum occurs in the Red Orthoceratite Limestone (Aserian–Lasnamägian) of Öland, Västergötland and of the erratics of Northern Germany.

Family Geisonoceratidae Zhuravleva, 1959 Diagnosis.—The orthoconic, sometimes slightly cyrtoceraconic conch shows a circular to strongly compressed cross section. The shell is smooth, sometimes sculptured. The suture is straight, simple or slightly undulated. The siphuncle is dorsocentral or ventrocentral. The septal necks are suborthochoanitic to orthochoanitic. The connecting rings are slightly convex or cylindrical. Siphuncular deposits in the form of annuli in the septal foramen may grow anteriorly against the connecting rings or fuse with those of adjacent segments to form continuous siphuncular lining. Cameral deposits are well developed.

Remarks.—The Geisonoceratidae include around 20 genera. The family was originally defined by the typical co−occurrence of suborthochoanitic to orthochoanitic septal necks with endosiphuncular deposits forming annuli at the position of the septal foramen. But as showed by Hook and Flower (1977) and Dzik (1984), other orthocerids (e.g., Orthoceras bifeovatum Noetling ) show endosiphuncular deposits strongly resembling those of the Geisonoceratidae . The crucial point of the differentiation between the Geisonoceratidae and the Orthoceratidae lies in the extent of the endosiphuncular deposits. This seems a rather vague character for the definition of a family. But the endosiphuncular deposits of the Orthoceratinae of the Orthoceratidae are not only highly supressed and occur in the very apicad part of the phragmocone but differ in their highly irregular shape from that of the Geisonoceratidae , which developed clear and distinguishable annuli at the septal foramen in a certain stage of deposition.

Figures

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.