Ordogeisonoceras foerstei Kröger 2013, comb. nov.
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Orthocerida
Geisonoceratidae
Ordogeisonoceras
Ordogeisonoceras foerstei

description: Ordogeisonoceras foerstei (Strand, 1934) comb. nov.
Figs 13H, 19E
Geisonoceras foerstei Strand, 1934: 13, pl. 3, fig. 9.
Diagnosis
Slender orthocones with apical angle less than 5°, slightly compressed to circular conch cross section; ornamented with slightly oblique transverse striae, which are slightly imbricated in direction of growth, distance between lirae variable, but generally less than one millimeter; siphuncle eccentric with SPR of ca. 0.2l; siphuncle diameter ca. 0.1 of corresponding conch cross section. (Adopted from Strand 1934.)
Type locality and horizon
Frognøya Island, Ringerike, Norway; exact stratigraphic horizon unknown, Late Ordovician.
Material examined
One specimen from Unskarsheden, Dalarna (PMU 26759); eight specimens from Kallholn, Dalarna (PMU 26760–26767); all from Boda Limestone, late Katian.
Description
Specimen PMU 26763 is a portion of phragmocone ca. 50 mm long, with a conch height of 34–37 mm (angle of expansion 2.9°). The conch surface is ornamented with slightly oblique (angle of ca. 15° to the normal of the conch axis) transverse lirae with a frequency of ca. 4 per millimeter (Fig. 19E). The lirae slope in adapical direction toward the prosiphuncular side, forming a shallow sinus on the prosiphuncular side. The lirae are rounded and slightly imbricate in an adoral direction. The septa are directly transverse and ca. 20 mm apart (0.54 of corresponding chamber height) with a convexity of 13 mm (ca. 0.35 of corresponding conch height). The septal necks are orthochoanitic to suborthochoanitic, ca. 3 mm long (Fig. 13H). The siphuncular diameter is ca. 3.5 mm at the septal foramen and the connecting rings are nearly tubular. No endosiphuncular or cameral deposits occur.
Amongst the specimens from the Boda Limestone, several are of a relatively large size. The largest fragment, PMU 26760, is a nearly complete fragment of a body chamber with a diameter increasing from 115 to 145 mm and a length of 444 mm.
The shape of the conch cross section varies slightly between the specimens from circular to slightly compressed (minimum width/height ratio 0.97). The position of the siphuncle is highly eccentric in all specimens, but varies between SPRs of 0.13 and 0.25, with a tendency to shift toward the center during growth.
The total length of the body chamber of 150 mm is preserved in specimen PMU 26761, which is 40 mm in diameter at the last septum and 48 mm at the aperture (angle of expansion 3.1°). In this specimen the distance of the transverse lirae is relatively wide, varying from 0.5 to ca. 0.8 mm.
Details of the siphuncle and septal necks are preserved in specimen PMU 26763 (Fig. 13H); a portion of phragmocone is 35 to 37 mm in diameter. The septa of this specimen are crushed and only parts of the siphuncle and septal necks are preserved. The septal necks are orthochoanitic at the side directed toward the conch margin and suborthochoanitic at the side directed toward the conch centre and very short (ca. 0.1 of the length of the corresponding siphuncular segment = chamber height). Weak episeptal deposits occur.
Remarks
The type specimen consists of a body chamber only. The specimens from the Boda Limestone are similar to the type specimen in possessing a slightly oblique, imbricated transverse ornamentation, a similar conch cross section, as well as a similar siphuncle diameter and position. Consequently, they are here assigned to O. foerstei comb. nov. The Boda specimens reveal a suborthochoanitic septal neck and a nearly tubular shape of the siphuncle, which are diagnostic characters of Ordogeisonoceras.
Comparison
The type of the genus differs from O. foerstei comb. nov. in having periodically alternating distances between the transverse lirae.
Michelinoceras? expansum Blake, 1882 from the late Katian Killey Bridge of Ireland (Evans 1993) is similar in having a slightly oblique transverse ornamentation with ca. four lirae per one millimeter. It is probably conspecific with O. foerstei comb. nov. But the internal characters are too poorly known in order to synonymize O. foerstei with this species.
Geisonoceras fustis Barskov, 1980 from the latest Ordovician of Kazakhstan, which is similar in general conch shape and ornamentation, differs in having a centrally positioned siphuncle and shorter, more curved septal necks.
Stratigraphic and geographic range
Late Ordovician, Ringerike District, Norway (Strand 1934); Boda Limestone, Siljan District, Sweden; late Katian, Ordovician.
description: Published as part of Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, pp. 1-110 in European Journal of Taxonomy 41 on pages 48-49, DOI: 10.5852/ejt.2013.41, http://zenodo.org/record/3814221