Siljanoceras varians, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 77-81

Siljanoceras varians sp. nov.

urn:lsid:zoobank.org:act:E4162038-CB02-4BDC-B8B3-EDE3417D2980

Figs 34 , 35 A-D, G-I, 36C-D, 37, 38

Diagnosis

Siljanoceras gen. nov. with angle of expansion of ca. 25°, with circular to slightly depressed conch cross section; siphuncle eccentrically positioned with distance of ca. 0.3 of corresponding conch diameter from conch margin at convex side of conch curvature; siphuncular segments barrel-shaped, widely expanded within chambers with diameters of 0.16–0.2 of corresponding conch cross section; septal necks achoanitic.

Etymology

Refers to the relatively high morphological variability, Latin variabilis, of this species.

Type material

Holotype

PMU 26909 .

Paratypes

Seven paratypes ( PMU 26910–26916), Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Type locality and horizon

Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Description

The holotype is a fragment of a slightly curved mature body chamber ( Figs 34 , 35 A-B, G). The elliptically compressed body chamber has an apical height of 58 mm and width of 67 mm (width height ratio = 1.16), an adoral height of 92 mm against a width of 105 mm. The conch margin is flattened at the convex side of the conch curvature and more rounded at the concave side. In lateral view the conch continuously expands with an angle of 23° and its width expansion decreases over the entire length of the specimen with an overall angle for the preserved portion of expansion of 25°. In the adoral 10 mm of the fragment the conch width decreases slightly, giving the body chamber a slightly bulbous shape.

The siphuncle is eccentrically positioned with diameter of the septal perforation of 3 mm (0.05 of corresponding conch height). The siphuncle is situated at a distance of ca. 18 mm from the conch margin at the convex side of the conch curvature, where the conch height is 58 mm (SPR 0.31).

The shell surface is nearly smooth. Fine, irregularly spaced, rounded growth lines or striae form a very shallow salient on the lateral sides and faint traces of longitudinal striae are preserved. The suture of the last chamber is transverse. The septum is shallow and cap-shaped.

At the base of the body chamber the periphract is preserved as an impression on the internal mould ( Fig. 34 ). The periphractic imprint forms a narrow ca. 5 mm wide band with an irregular margin on the lateral and dorsal (antisiphuncular) sides. Ventrally, a pair of adjacent impressions, each ca. 13 mm wide, occur. The fine, longitudinally and transversely striated ornamentation is best preserved in specimen PMU 26913 ( Fig. 37 ). In this specimen the longitudinal elements are preserved as fine, raised striae with a distance of ca. 0.2 mm at a conch height of ca. 30 mm.

The chamber height is ca. 0.16 of the corresponding conch height (based on measurements on specimens PMU 26913 and 26915). The sutures are straight and directly transverse.

Details of the internal morphology are preserved in specimen PMU 26915, which is a fragment consisting of five chambers and part of a body chamber, with conch height increasing from 19 to 35 mm ( Fig. 36C ). The septa are directly transverse and ca. 0.17 of the conch height in distance. The septal perforation is eccentrically positioned at a distance of 5 mm apically, and 7 mm adorally (SPR ca. 0.25–0.26). The septal necks are achoanitic. The siphunclular segments are barrel-shaped, widely expanded. In the adapical-most chamber the siphuncle is significantly less expanded (0.16 of corresponding conch cross section) than in the adoralmost chamber (relative siphuncular diameter 0.2).

Remarks

Some of the specimens assigned to this species were slightly crushed during deposition and as a consequence show aberrant conch cross sections. Therefore, the variability in conch cross section shape and angle of expansion is difficult to evaluate. Generally, a trend of increasing compression of the conch cross section can be ascertained ( Fig. 35 ). Based on the limited material available it seems reasonable

to assume that the conch cross section varies from nearly circular to slightly depressed ( Fig. 38 ) and the angle of expansion varies across the sample between 23 and 31° ( Fig. 38 ).

Stratigraphic and geographic range

Boda Limestone, late Katian, Dalarna, Sweden.

Figures

Fig. 34. Muscle attachment scars from ventromyarian periphract in Siljanoceras varians sp. nov., PMU 26909, holotype, Kallholn. Scale bar = 10 mm.

Fig. 35. Siljanoceras gen. nov., Kallholn, Boda Limestone. A-D. Siljanoceras varians sp. nov. A. PMU 26909, holotype, mature body chamber, view from prosiphuncular side. B. Same specimen, lateral view. C. PMU 26913, lateral view. D. Same specimen, adapical view. E-F. Apical fragment of Siljanoceras? sp., PMU 26918. E. Lateral view. F. Adapical view. G-I. Siljanoceras varians sp. nov. G. Same specimen as in A-B, adapical view. H. PMU 26914, lateral view. I. Same specimen view from prosiphuncular side. Scale bars: A-D, G-I =10 mm; E-F = 2 mm.

Fig. 37. Detail of ornamentation of Siljanoceras varians sp. nov., PMU 26913, Kallholn, Boda Limestone; position of apex down. Scale bar = 10 mm.

Fig. 36. Polished median sections of Siljanoceras gen. nov., Boda Limestone. A. Detail of siphuncular segment of Siljanoceras sp. A, PMU 26917, Kallholn. B. Same specimen, total view. C. Siljanoceras varians sp. nov., PMU 26915, Kallholn; detail of apical-most siphuncular segment, section not perfectly in median plane, with adapical septal perforation not preserved. D. Same specimen, total view. Scale bars: A, C = 1 mm; B, D = 5 mm.

Fig. 38. Morphological variation of Siljanoceras gen. nov. Black dots = Siljanoceras varians sp. nov.; grey dot = Siljanoceras sp. A.