Siljanoceras, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 76-77

Genus Siljanoceras gen. nov.

urn:lsid:zoobank.org:act:71F25FD0-6E8D-4753-9CA2-B0137EC6D22B

Type species

Siljanoceras varians gen. et sp. nov., Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian, Ordovician.

Diagnosis

Rapidly expanding curved conchs with circular to slightly depressed conch cross section; mature body chamber with clearly depressed cross section; nearly straight in late growth stages, more strongly curved in early growth stages, continuously expanding in height, slightly bulbous in width, with decreasing width near aperture; cross section more rounded on dorsal side and slightly flattened on venter; ornamented with fine longitudinal striae and irregularly spaced faint growth lines, which form shallow sinus on venter; siphuncle eccentrically positioned at convex side of conch curvature; siphuncular segments barrel-shaped, widely expanded; septal necks achoanitic.

Etymology

From Lake Siljan, Dalarna, Sweden.

Remarks

The decreasing conch curvature during maturity, the position and shape of the siphuncle, the ventromyarian periphract ( Fig. 34 ) and the longitudinal ornamentation are characters that are typical of the Uranoceratidae and justify an assignment of this new genus within this family.

An apical fragment, PMU 26918, questionably is from a Siljanoceras gen. nov. ( Fig. 35 E-F). It can be described as following: The tip of the fragment is cup-shaped, at ca. 2 mm from the tip the conch height is 4.5 mm, the width 4.3 mm. The conch surface is ornamented with a shallow rounded annulation (ca. 3 annuli per millimetre) and faint longitudinal striae. The annuli have a shallow hyponomic sinus on the concave side of the shell curvature. The conch is very slightly curved and expands with an angle of ca. 33° and is nearly circular in conch cross section at a conch diameter of ca. 9 mm. The siphuncle is positioned close to the conch margin and is ca. 0.08 of the conch diameter.

Comparison

Among other openly coiled Barrandeocerida , this genus is unique in its combination of a siphuncle that lies toward the conch wall on the convex side of the conch curvature and the conch cross section being more rounded on the dorsum (concave side of conch curvature). In its non-annulated, longitudinally striated shell surface, it is most similar to Uranoceras Hyatt, 1884 , but differs from this genus in being more openly coiled and more rapidly expanding.

Stratigraphic and geographic range

Boda Limestone, late Katian, Dalarna, Sweden.

Species included

Siljanoceras varians sp. nov., Siljanoceras sp. A.

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.