Striatocycloceras isbergi, Kröger, 2013

Striatocycloceras isbergi sp. nov.

urn:lsid:zoobank.org:act:23541702-00D8-4063-AE65-6799

Figs 12 View Fig B-C, 13B-C, 14 A

Striatocycloceras foerstei – Kröger & Isakar 2006: 151, figs 7q, 10h.

Diagnosis

Slender orthocones with apical angle less than 10° and conch cross section less than 15 mm at maturity; ornamented with eight to ten pronounced, sharply crested annuli at distance equal to corresponding conch diameter; annuli laterally oblique ca. 70° to growth axis; ca. three annuli in distance between septa; siphuncle slightly subcentral; nearly tubular segments; septal necks orthochoanitic.

Etymology

In honor of Orvar Isberg (1884 - 1950), a Swedish paleontologist who published an extensive monograph on the Boda Limestone bivalves ( Isberg 1934).

Type material

Holotype

PMU 26660 .

Paratypes

Thirty-seven paratypes: one specimen ( PMU 26661) from Osmundsberget; 36 specimens ( PMU 26662– 26698), Kallholn, Siljan District, Sweden; all Boda Limestone, Boda Core Member, late Katian.

Type locality and horizon

Kallholn, Siljan District, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

Holotype is a 45 mm long, 10–13 mm wide portion of phragmocone (angle of expansion 4°) with a circular conch cross section ( Fig. 12 B View Fig ). The shell surface is ornamented with sharply crested annuli, of which ca. eight occur at a length equal to the conch diameter. The annuli form sharp, angular crests with wide, rounded grooves. The shell surface is otherwise nearly smooth, with only very faint transverse growth lines visible. The annuli are oblique, at ca. 70° to the growth axis. In specimen PMU 26661 the sutures are directly transverse and crossed obliquely by the annulations. The septa are markedly convex, being 3 mm deep at a conch diameter of 7 mm (0.43 of conch diameter). The siphuncle is centrally positioned, with a septal perforation of 0.9 mm at a diameter of 7 mm (0.13 of conch diameter).

The juvenile specimen, PMU 26664, has a diameter of 2.2–3 mm and a length of 13 mm (angle of expansion is slightly curved and less pronouncedly annulated; Fig. 12 C View Fig ).

The internal characters are well preserved in specimen PMU 26681 ( Figs 13 B View Fig , 14 A View Fig ), which is a portion of a phragmocone with a diameter increasing from 7.5 to 9 mm and a length of 16 mm. The chamber height at the adoral end of this fragment is 3 mm (0.3 of corresponding conch diameter), and the distance of the siphuncle is 3.8 mm ( SPR 0.42). In specimen PMU 26661 the siphuncle is relatively well-preserved. There the siphuncle is nearly tubular and the septal necks are orthochoanitic ( Fig. 13 C View Fig ).

Remarks

The larger number of specimens at the PMU permit an evaluation of conch variability and an estimation of the adult size of this species. Based on these new data, the specimen figured in Kröger & Isakar (2006: figs 7q, 10h) from the Vormsi Regional Stage of Hiiumaa Island, Estonia, must be assigned to Striatocycloceras isbergi sp. nov. It differs from the type specimen of S. foerstei in its smaller size and its more numerous, more sharply crested annuli. In this, it lies within within the range of variability of the specimens of S. isbergi sp. nov. from the Boda Limestone.

Comparison

This species is unique with regard to its narrow annulation. The otherwise similar S. foerstei differs in having six to seven annulations per distance equal to conch diameter and a significantly larger adult size.

Stratigraphic and geographic range

Vormsi Regional Stages, Estonia ( Kröger & Isakar 2006); Boda Limestone, Siljan District, Sweden, late Katian, Ordovician.