Pleurorthoceras osmundsbergense, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 50-51

Pleurorthoceras osmundsbergense sp. nov.

urn:lsid:zoobank.org:act:C6C26456-C557-45FD-BBB8-E2BFC1C0300C

Figs 13G, 14C, 18E, 23A, 24

Diagnosis

Pleurorthoceras with very slightly curved shell, with angle of expansion of ca. 7°, circular conch cross section, and adult diameters of ca. 50 mm; ornamented with fine, inconspicuous, irregularly spaced, slightly oblique growth lines or rounded striae; siphuncle subcentral (SPR ca. 0.46) on convex side of conch curvature. Heavy mural deposits.

Etymology

Refers to the type locality.

Type material

Holotype

NRM Mo 150126 .

Paratypes

Two paratypes: NRM Mo 150102 from Osmundsberget, Dalarna; PMU 26768 from Kallholn, Dalarna, Sweden; all Boda Limestone, Boda Core Member, late Katian.

Type locality and horizon

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

Other material examined

An additional 43 non-type specimens from Osmundsberget are in the collections of the NRM.

Description

The holotype is a nearly complete specimen with a length of 280 mm, with a diameter of 15–48 mm (angle of expansion 6.7°) and a very slightly curved conch ( Figs 18E, 23A). The angle of expansion decreases with size to form a nearly tubular adult body chamber. The shell surface is nearly smooth, but fine, irregularly spaced, rounded striae run obliquely transverse, forming a shallow sinus on the concave side of the conch curvature.

In specimen NRM Mo 150102, which is a fragment of a juvenile phragmocone with a length of 105 mm and a diameter of 22–37 mm (angle of expansion 8.1°), the distance between septa is 0.24–0.36 of the corresponding conch cross section. The siphuncle is nearly central in apical parts, becoming more eccentric at adoral parts (SPR 0.46). The siphuncular segments are barrel-shaped, slightly expanded within the chambers, with a maximum siphuncle diameter of 3 mm at septal foramen at a conch cross section diameter of 22 mm (0.14 of conch cross section). The septal necks are suborthochoanitic ( Figs 13G, 14C). No endiphuncular deposits occur, and remains of heavy mural cameral deposits are preserved in the apical parts of the fragment ( Fig. 24).

Comparison

Pleurorthoceras osmundbergense sp. nov. differs from the genotype and the second North American species, P. selkirkense (Whiteaves, 1892) in having a significantly higher angle of expansion and a slightly curved growth axis. P. subcostatum ( Portlock, 1843) from Killey Bridge Formation, latest Katian ( Evans 1993), differs in having a distinct reticulate ornamentation.

Stratigraphic and geographic range

Boda Limestone, Siljan District, Sweden; late Katian, Ordovician.