Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880 ) Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 44-46

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.

Figures

Fig. 2. Polished median section of a fragment of a phragmocone of Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26747, Unskarsheden. The chamber is filled with epi- and hypsoseptal deposits (a), a layered wackestone with bird’s eye structures (b), a concentration of ostracods (c) and a sparitic space (d). Picture on the right is detail of picture on the left. Left scale = 10 mm, right scale = 1 mm.

Fig. 13. Polished median sections of Orthocerida of the Boda Limestone. A. Dawsonoceras fenestratum (Eichwald, 1860), PMU 26706, Kallholn. B-C. Striatocycloceras isbergi sp. nov. B. PMU 26681, Kallholn. C. PMU 26661, Osmundsberget. D. Isorthoceras angelini sp. nov., PMU 26784, Kallholn. E. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype, Kallholn. F. Nathorstoceras adnatum sp. nov., PMU 26733, holotype, Kallholn. G. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo, 190102c, Osmundsberget.H.Ordogeisonocerasfoerstei(Strand,1934)comb.nov.,PMU26763,Kallholn. I. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26740, Kallholn. Scale bars: A-D, I = 1 mm; E-H = 5 mm.

Fig. 18. Shell ornamentation of selected Cephalopoda of the Boda Formation. A. Order, gen. et sp. indet., PMU 26950, Kallholn, lateral view. B. Palaeodawsonocerina? nicolletoides sp. nov., PMU 26727, holotype, Kallholn. C. Palaeodawsonocerina senckenbergi (Teichert, 1930), PMU 26718, Kallholn. D. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26745, Unskarsheden. E. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo 150126, holotype, Osmundsberget. F. Warburgoceras longitudinale (Angelin in Angelin & Lindström, 1880) comb. nov., NRM-PZ Mo 154065, Unskarsheden. G. Redpathoceras depressum sp. nov., PMU 26931, Kallholn. Scale bar = 5 mm for all figures.

Fig. 19. Orthocerida of the Boda Limestone. A. Nathorstoceras adnatum sp. nov., PMU d1440, holotype, Kallholn, lateral view. B-C. Nathorstoceras kallholnense sp. nov., PMU 26730, Kallholn, complete body chamber. B. Lateral view. C. view from prosiphuncular side. D. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26747, Unskarsheden, lateral view. E. Ordogeisonoceras foerstei (Strand, 1934) comb. nov., PMU 26762, Kallholn, lateral view. F. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype, lateral view, note siphuncle on left side. Scale bar = 10 mm for all figures.

Fig. 22. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26740, from Unskarsheden, Boda Limestone. Same specimen as in Fig. 13I.