Isorthoceras wahlenbergi Niko, 2008

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 66-67

Isorthoceras wahlenbergi Niko, 2008

Figs 4C , 26 , 28C

Isorthoceras wahlenbergi Niko, 2008: 195 , pl. 6, fig. 1.1-6.

Diagnosis

Isorthoceras with circular conch cross section; smooth shell surface; weakly curved growth axis with siphuncle slightly eccentric on concave side of curvature; angle of expansion ca. 9°; mature body chamber with maximum diameter of 31 mm and two constrictions on body chamber cast; position of siphuncle shifting sightly toward the center during ontogeny; siphuncular segments pyriform to fusiform; with weakly developed endosiphuncular deposits; septal necks short, cyrtochoanitic. (Adopted from Niko 2008.)

Type locality and horizon

Jutjärn Quarry, Siljan district, Dalarna, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.

Material examined

Fifty-five specimens in the collections of the PMU: two specimens ( PMU 26842, 26843), Unskarsheden, Dalarna; 53 specimens ( PMU 26844–26896), Kallholn, Dalarna; all from Boda Limestone, late Katian.

Description

The species was described in detail by Niko (2008). Because I. wahlenbergi is one of the most common cephalopods in the Boda Limestone. Fifty-three specimens are available for study in the collections of the PMU. This material provides valuable additional data relating to the variability of some of the characters. Measurements of the angle of expansion reveal an increase in the expansion rate up to diameters of ca. 10 mm, when an a mean angle of ca. 10° is reached ( Fig. 26 ). The nearly mature conch expands more slowly and reaches its adult diameter at ca. 30 mm. The mean of all 47 measurements is 9.0° (1 st quantile 8.1°, 3 rd quantile 10.2°, n=47).

The chamber distance was measured in ten specimens. The chamber height is relatively small, with a mean height 0.23 of the corresponding conch height (1 st quantile 0.22, 3 rd quantile 0.26, n=10).

Remarks

Three fragments of I. wahlenbergi are preserved with a longitudinal color pattern. In specimen PMU 26862 the shell is perfectly smooth; at a conch cross section of ca. 18–22 mm dark reddish, irregularly spaced longitudinal bands occur with a distance of ca. 1–2 mm (ca. 100 would occur around circumference, Fig. 28C ). In PMU 26850, a portion of the phragmocone with a diameter increasing from 17 to 13 mm, the smooth shell is partly ornamented with striae with a width of ca. 0.2 mm and a distance of ca. 2 mm (ca. 50 would occur around circumference). The bands are irregularly carved within the surface of the shell, which is otherwise smooth, and are interpreted as diagenetically altered traces of a color pattern ( Fig. 4C ). A similar longitudinal color pattern is known from other species of Isorthoceras , such as I. tenuitextum ( Hall, 1847) , I. romingeri ( Foerste, 1932) (see Ruedemann 1919; Foerste 1932: pl. 10; Kröger et al. 2009), I. heroyense comb. nov. and I. suave comb. nov. (see above, Fig. 28 ). Because color pattern is relatively rarely preserved, its relevance for species differentiation is not known at the present.

Comparison

This species constitutes a large Isorthoceras . It is similar in size to I. dalecarliense Kröger et al., 2011 , from which it differs in having a smaller angle of expansion of ca. 6° ( Fig. 26 ). I. sociale from the Maquoketa Formation, late Katian of the north-central USA is similar in dimension and general outline, but differs in having a less expanded siphuncle and a smaller adult size of only 25 mm.

Holland (1993) described and figured a number of specimens from the late Katian Kildare Limestone of Ireland under the name ‘ Orthoceras ’ gregarium J. De C. Sowerb, 1839. These specimens are similar to I. wahlenbergi in general conch shape, in septal spacing and siphuncle position, as well as in having a smooth surface, and probably must be synonymized with the latter. However, because the internal characters are not known from the Kildare Limestone cephalopods, a synonymization is impossible at this point.

Stratigraphic and geographic range

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

Figures

Fig. 4. Orthoconic cephalopods of the Boda Limestone. A. Cameroceras turrisoides sp. nov., PMU 26613, holotype, Unskarsheden, lateral view. B. Isorthoceras dalecarlense Kröger et al., 2011, PMU 26794, Kallholn. C. Isorthoceras wahlenbergi Niko, 2008, PMU 26893 and 26894, Kallholn. D. Order, gen. et sp. indet., PMU 26950, Kallholn, lateral view. Scale bar = 10 mm for all figures.

Fig. 26. Variability of the angle of expansion in fragments of different diameters of Isorthoceras wahlenbergi Niko, 2008 (black points) and Isorthoceras dalecarlense Kröger et al., 2010 (grey points). The black circles refer to specimens figured in Kröger et al. (2011: fig.11) and identified as I. dalecarlense, therein. The complete dataset, presented herein, demonstrates that a definite species distinction by angle of expansion and size alone is impossible in fragmentary specimens with angles of expansion between 6° and 8°. Additionally, the complete dataset reveals that specimens with conch diameter> 7 mm and angle of expansion> 8° in Kröger et al. (2011: fig. 11) must correctly be interpreted as fragments of I. wahlenbergi.

Fig. 28. Color marks in Isorthoceras Flower, 1962, Boda Limestone. Longitudinal striae of varying width occur in several specimen of Isorthoceras; see text for discussion. A. Isorthoceras heroyense (Strand, 1934) comb. nov., PMU 26831, Kallholn. B. Isorthoceras suave (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26835, Osmundsberget. C. Isorthoceras wahlenbergi Niko, 2008, PMU 26862, Kallholn. Scale bar = 10 mm for all figures.