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

description: Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov.
Figs 2, 13I, 18D, 19D, 22
Orthoceras wegelini Angelin in Angelin & Lindström, 1880: 5, pl. 6, fig. 6.
Orthoceras saksbyense Teichert, 1930: 277, pl. 5, fig. 2.
Diagnosis
Slender orthocones with angle of expansion of ca. 5° or less, circular or slightly compressed conch cross section; chamber height of ca. 0.3–0.5 of distance similar to corresponding cross section diameter; ornamented with obliquely transverse, irregularly spaced growth bands; distance between growth bands 0.3–3 mm; siphuncular position eccentric (SPR ca. 0.2); siphuncular segments tubular, short orthochoanitic to suborthochoanitic septal necks; weakly developed annulosiphuncular deposits are restricted to the position of the septal necks; hypo- and episeptal deposits occur.
Type material
Lectotype
NRM Mo 152346 (designated herein). Paralectotypes
Two paralectotypes (NRM Mo 152347, 152348) from Östbjörka, Boda Limestone, latest Katian.
Type locality and horizon
Östbjörka, Siljan district, Dalarna, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.
Other material examined
Five specimens (PMU 26734–26738) from Kallholn; 19 specimens (PMU 26739–26757) from Unskarsheden, Siljan District; all Boda Limestone, latest Katian.
Description
The lectotype is a relatively poorly preserved fragment of a phragmocone with an angle of expansion which cannot be measured exactly because of this fragmentary status. It agrees with respect to conch ornamentation, general conch shape and position of the siphuncle with the specimen PMU 26747.
Specimen PMU 26747 is a well preserved portion of phragmocone 83 mm long, with a slightly compressed section (ratio width/height 0.93), and a conch height of 43–49 mm (angle of expansion 4.1°) (Figs 2, 19D). The conch surface is ornamented with irregularly spaced bands with fine deep grooves in between. Bands and grooves are parallel to each other. The distance between the grooves varies between 0.2 and 0.3 mm in this specimen (Fig. 18D). The bands are oblique, forming a shallow sinus on the prosiphuncular side. At the apical end of the specimen, the siphuncle is positioned at ca. 9 mm from the conch margin (SPR 0.21). There the siphuncular perforation is 4 mm wide (ca. 0.09 of corresponding conch height). The most adapical septum is moderately convex with a height of curvature of ca. 10 mm (0.23 of corresponding conch height).
The mean of the angle of expansion of eight measured specimen is ca. 4.2° (minimum 2.19°, maximum 5.19°). The eccentricity of the siphuncle is relatively constant; in three measured specimens it varies between a SPR of 0.17 to 0.22.
The largest specimen, PMU 26752, with a cross section diameter of 92 mm, possesses a pronounced constriction on the cast of the body chamber at ca. 35 mm adapically of the adoral end of the fragment.
The shape of the siphuncle and septal neck is preserved in specimens PMU 26745 (Fig. 18D), 26740 (Figs 13I, 22), 26741 and 26746. These fragments have conch diameters between 24 and 58 mm. In all five specimens the siphuncle is nearly tubular, very slightly constricted at the position of the septal perforation and the septal necks are short orthochoanitic to suborthochoanitic, with a length of ca. 0.14 of the length of the siphuncular segment (= chamber height). At the position of the septal neck, shallow annulosiphuncular deposits are developed at parts of the septal necks which are directed toward the conch center. Hyposetal, episeptal and mural deposits occur (see, e.g., Fig. 2).
Remarks
The transverse ornamentation is subdued and visible only in well preserved specimens. In slightly weathered specimens, the conch surface appears perfectly smooth and in steinkerns with high probability it is not preserved. Therefore, poorly preserved specimens could be mistakenly assigned to the smooth Pleurorthoceras. Teichert (1930) described a smooth longicone under the name of Orthoceras saksbyense with exactly the same conch dimensions, shape and distance of septa, position and dimension of siphuncle and septal necks which have no signs of ornamentation. Therefore, with a very high probability this species is conspecific with Geisonoceras wegelini Angelin in Angelin & Lindström, 1880 comb. nov. and is synonymised with the latter, herein.
Comparison
Geisonoceras is a genus which has been widely used as a wastebasket taxon for a variety of straight longicones with a transverse ornamentation. Currently it comprises more than fifty species. Probably, most of them are not closely related to the genotype. G. wegelini comb. nov. is very similar to the genotype with respect to its ornamentation and internal characters; it differs in having a strictly orthoconic shell and only poorly developed annulosiphuncular deposits. The latter may be an artifact, as only relatively large fragments with internal structures preserved are known from G. wegelini comb. nov.
Stratigraphic and geographic range
Vormsi Regional Stages, Estonia (Teichert 1930); 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 44-46, DOI: 10.5852/ejt.2013.41, http://zenodo.org/record/3814221