Isorthoceras dalecarlense Kroger 2011

Isorthoceras dalecarlense Kröger et al., 2011

Figs 4B, 26

Isorthoceras dalecarlense Kröger et al., 2011: 45, fig. 9c, g.

Diagnosis

Smooth, nearly straight Isorthoceras with circular cross section and comparatively low angle of expansion of 6°, chamber distance 0.3 of corresponding cross section, depth of septal curvature ca. 0.26 of corresponding cross section. Adult size> 35 mm in cross section. Siphuncle eccentric in early, subcentral

in later growth stages, siphuncular segments slightly expanded within chambers with diameter ca. 0.1 of conch cross section. Septal necks short, on border-line between cyrtochoanitic and suborthochoanitic. In apical parts of siphuncle thin parietal deposits that ventrally fuse to irregular endosiphuncular lining at extreme apical part of conch. Cameral deposits not known.

Type locality and horizon

Osmundsberget quarry, Rättvik Municipality, Siljan District, Sweden (locality Osmundsberget 1 of Ebbestad & Högström 2007); nautiloid bed, top of Glisstjärn Formation, Hirnantian, Late Ordovician.

Material examined

Nineteen specimens (PMU 26790–26808) from Kallholn, Siljan District, Sweden; Boda Limestone, latest Katian, Ordovician.

Comparison

The species was described in detail by Kröger et al. (2011). Here, details of the variability of the external conch features can be added. Fourteen specimens are assigned to this species. They are similar in all aspects to I. wahlenbergi Niko, 2008, but differ from the type of I. wahlenbergi in having a significantly lower angle of expansion.

The angle of expansion varies strongly within the smooth Boda Limestone specimen of Isorthoceras (Fig. 26).A slender form (I. dalecarlense) and a wide angled variety (I. wahlenbergi) can be distinguished (compare Fig. 4B, and 4C). But the distinction is complicated by the ontogenetic change of angle of expansion in both varieties. In both varieties the angle of expansion is initially low, reaches maximum values in premature growth stages and declines in fragments of the mature body chamber.

Nevertheless, in the conch size/angle of expansion-plot (Fig. 26) two groups can be distinguished in each individual growth section, but not in juveniles and mature specimens. This overlapping in the plot is an effect of the method of measurement, which does not distinguish between long and short fragments (the angle of expansion is measured in fragments with different lengths). Thus, in smaller fragments of juvenile and mature conchs a distinction between the two species is impossible, but longer conch fragments will always reveal a strong difference in intermediate growth stages.

As a result of these measurements it is clear that some of the specimens measured by Kröger et al. (2011: fig. 10) represent I. wahlenbergi. Consequently, I. wahlenbergi also occurs within the Hirnantian Glisstjärn Formation (see Fig. 26).

Balashov (1975) described smooth, slender orthocerids from the Molodov Horizon of Podolia as Michelinoceras dnestrovense. The shape of the septal necks and the siphuncular segments, as well as the chamber spacing indicate that these specimens are better assigned to Isorthoceras. The chamber spacing of the figured specimen of I. dnestrovense (Balashov, 1975) comb. nov. varies from 0.17 to 0.2 and thus is considerably narrower than in I. dalecarlense.

Stratigraphic and geographic range

Boda Limestone, Glisstjärn Formation, late Katian-Hirnantian, Dalarna, Sweden (see discussion of I. dalecarlense, above).