Dawsonoceras fenestratum ( Eichwald, 1860 )

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 30-32

Dawsonoceras fenestratum ( Eichwald, 1860)

Figs 13A , 14B , 16 A-B

Cycloceras fenestratum Eichwald, 1860: 1231, 1232 , pl. 48: 14a-b.

Dawsonoceras fenestratum – Kröger & Isakar 2006: 156, figs 8b, 10b, 12c, f-g (with synonymy). ― Kröger 2007: 597 .

Diagnosis

Dawsonoceras with angle of expansion of less than ten degrees in juvenile stages. Tubular adult body chamber has diameter of approximately 25 mm; five to six annulations occur at distance similar to shell diameter; approximately 20 longitudinal lirae occur around shell circumference, approximately fifteen growth lines or transverse striae occur per cycle of annulations; septal necks achoanitic; siphuncle subcentral, expanded within chambers. Mural and hyposeptal cameral deposits are known. (Adopted from Kröger & Isakar 2006.)

Type locality and horizon

Mõnuste (Kirna), western Estonia, Late Ordovician.

Material examined

Two specimens, PMU 26700 and 26701, Osmundsberget, Siljan District; fifteen specimens ( PMU 26702–26716), Kallholn, Siljan District; all from the Boda Limestone, Boda Core Member, late Katian.

Description

The external characters were described in detail by Kröger and Isakar (2006; see also Figs 16 A-B). In specimen PMU 26706 additional details of the siphuncle are preserved. Specimen PMU 26706 is a fragment of a phragmocone with a diameter of 18–21 mm and a length of 28 mm. The chamber distance is relatively constant at ca. 4 mm. The siphuncle is slightly eccentric at the adoral end and ca. 8 mm distant from the conch margin (SPR 0.38). The siphuncular diameter is 1.3 mm at the adapical end (0.07 of the corresponding conch cross section); there the septal perforation is only 0.8 mm wide. The siphuncular segments are barrel-shaped; rapidly increasing in diameter close to the septal perforation, the side directed toward the conch center bends inward slightly, forming a shallow concave connecting ring segment ( Figs 13A , 14B ). The asymmetrical shape of the connecting ring was noted in material previously described by Kröger & Isakar (2006). The septal necks are short and suborthochoanitic. Shallow hyposeptal deposits occur at the adapical five chambers.

Remarks

This species was revised by Kröger & Isakar (2006) and its main features were described based on specimens from the Porkuni Formation of Estonia. Here, additional data regarding the variation of the external characters and features of the siphuncle are noted. The occurrence of hyposeptal cameral

deposits in specimens of Dawsonoceras fenestratum ( Eichwald, 1860) from the Boda Limestone (see below) is included within the diagnosis herein.

The conch measurements reveal a clear tendency of a decrease in angle of expansion with increasing conch diameter ( Kröger & Isakar 2006). The mature body chamber of D. fenestratum is nearly tubular, whereas in juvenile conchs the maximum expansion reaches 10° (PMU 26708 with cross section diameters of 9–14 mm; compare Fig. 16 A-B). This is in agreement with the measurements listed by Kröger & Isakar (2006). Additionally, the relative distance of the transverse annuli is highly variable, but lacks a clear ontogenetic trend and varies between 0.16 and 0.3 of the corresponding conch diameter (mean 0.25, n=12). The number of longitudinal lirae is fixed in each specimen, but varies among different specimens from 20 to 31.

These external features are well within the range of variability of the Trenton dawsonoceratid Orthoceras beauportense Whiteaves, 1898 , from which the internal characters are unknown. It is probable that this species should be synonymized with D. fenestratum . Chen & Zou (1984) reported a similar species from the late Katian Sheshan Formation of North China, which they assign to Anaspyroceras beauportense (Whiteaves, 1898) but provided no details regarding internal characters. This specimen probably should also be assigned to D. fenestratum .

Stratigraphic and geographic range

Nabala/Vormsi Regional Stages, Estonia (questionable, see Kröger & Isakar 2006); Porkuni Formation, Estonia; Boda Limestone, Siljan District, Sweden; late Katian-Hirnantian, Ordovician.

Figures

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. 14. Reconstructions of the details of the septal necks and siphuncular segments of Orthocerida of the Boda Limestone. Note the different septal thicknesses. A. Striatocycloceras isbergi sp. nov., PMU 26681, Kallholn. B. Dawsonoceras fenestratum (Eichwald, 1860), PMU 26706, Kallholn. C. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo 150102c, Osmundsberget. D. Isorthoceras angelini sp. nov., PMU 26784, Kallholn.

Fig. 16. Dawsonoceratidae and Proteoceratidae of the Boda Limestone. A-B. Dawsonoceras fenestratum (Eichwald, 1860), Kallholn. A. PMU 26702, nearly mature specimen with adorally decreasing angle of expansion. B. PMU 26712, juvenile specimen. C. Gorbyoceras sp. A, PMU 26772, Osmundsberget. D. Goryboceras sp. B, PMU 26773, Kallholn. E. Dawsonoceras stumburi sp. nov., PMU 26717, holotype, Kallholn. F. Gorbyoceras alternestriatum (Strand, 1934) comb. nov., PMU 26771, Unskarsheden. Scale bars: A-C, E = 10 mm; D = 10 mm; F = 10 mm.