Isorthoceras angelini, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 54-57

Isorthoceras angelini sp. nov.

urn:lsid:zoobank.org:act:D60B4D34-B324-4F4B-817D-2EBE1F0F09BA

Figs 13D , 14D , 17H , 23G , 25

Diagnosis

Isorthoceras with weakly curved conch with circular or slightly compressed cross section; angle of expansion decreases with conch size, ca. 7–9° in premature growth stages; ornamented with fine longitudinal raised lirae and irregularly spaced transverse lirae; transverse lirae form shallow sinus on concave side of conch curvature; conch very weakly, irregularly undulated; mature size ca. 15 mm; position of siphuncle subcentral in early growth stages and eccentric at convex side of conch curvature in late growth stages, septal necks are suborthochoanitic-cyrtochoanitic.

Etymology

In honour of Nils Peter Angelin (1805–1876), a Swedish geologist and paleontologist, for his pioneering work on the Swedish palaeozoic fossils (e.g., Angelin & Lindström 1880).

Type material

Holotype

PMU 26774 .

Paratypes

Twelve paratypes: PMU 26775–26786, Kallholn, Dalarna, Boda Limestone, latest Katian.

Type locality and horizon

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

Description

The holotype, PMU 26774, is a 40 mm long fragment of a phragmocone with a circular cross section and a diameter which is 5.8–9 mm of a length of 30 mm (angle of expansion 9.1) ( Fig. 23G ). The

conch is ornamented with fine raised longitudinal lirae, which have a distance of ca. 0.2 mm, and with transverse lirae, which have a distance of 0.1 mm. The spacing of both the longitudinal and transverse lirae is slightly variable. Additionally, the surface of the shell undulates slightly, with a varying distance of ca. 2 mm. The transverse ornamentation forms a very shallow sinus on the concave side of the conch curvature. The conch is very slightly curved with the siphuncle eccentric to subcentral in position. It shifts slightly to the convex side of the conch curvature during the growth of the organism. At the apical end of the fragment, the smallest distance of the siphuncle from the conch margin is 2.7 mm (SPR 0.46) and the chamber height is 3 mm (ca. 0.5 of corresponding conch height).

The details of the siphuncle and septal necks are seen in PMU 26784, a portion of phragmocone, increasing in diameter from ca. 6.8 to 8.7 mm ( Figs 13D , 14C ). At the adapical end of the specimen the siphuncle is 1.9 mm from the conch margin (SPR 0.28) and the distance between septa is 2.9 mm (0.43 of corresponding conch cross section). The siphuncle is nearly tubular, with very slightly inflated connecting rings. It has a diameter of 0.15 of the corresponding conch cross section. The septal necks are very short, poorly preserved, and appear to be suborthochoanitic or cyrtochoanitic. Episeptal and mural deposits occur ( Figs 13D ).

The nine available measurements of angles of expansion reveal a clear tendency of decreasing angle of expansion with increasing conch size. The maximum values are reached by the fragment of the lectotype, the average is 7.1° and the smallest measurements are on the specimen with a maximum diameter of 15 mm (PMU 26786, Fig. 25 ).

PMU 26785 and 26786 are the largest fragments, both consisting of parts of body chambers that are nearly tubular. The ornamentation of these fragments is characterized by longitudinal and transverse lirae and a very shallow, irregular undulation. Pronounced, irregularly spaced growth lines also occur.

Comparison

This species of Isorthoceras is unique because of its combination of a relatively small adult size of only 15 mm, its angle of expansion of 7–9° and its eccentrically positioned siphuncle.

A very similar, yet undescribed species occurs in the Kullsberg Limestone, early-mid Katian of Dalarna, Sweden. This species differs from I. angelini sp. nov. in having a more pronounced transverse ornamentation.

A reticulate ornamentation occurs also in Orthoceras lepteanarum Angelin in Angelin & Lindström, 1880 , a species which is known from a single specimen (NRM Mo 152344) from the Boda Limestone of Ostbjörka. This specimen is a poorly preserved small fragment of a phragmocone with a diameter of 38 mm. It is very likely to represent a species of Isorthoceras and may be closely related or conspecific with Isorthoceras tenuitextum ( Hall, 1847) . The ornamentation in NRM Mo 152344 is partly preserved and consists of elevated, irregularly spaced, longitudinal and transverse lirae. The distance between the longitudinal striae (ca. 1.5 mm) is considerably larger than that of the transverse elements, resulting in a surface pattern of minute transverse rectangles. Compared with I. angelini sp. nov., the longitudinal striae are significantly more widely spaced, both in absolute dimensions (more than 1 mm in O. lepteanarum against ca. 0.1–0.2 mm in I. angelini sp. nov.) and in relative dimensions (more than 5 transverse lirae occur per distance similar to the distance of two longitudinal striae in I. (?) lepteanarum comb. nov. against ca. 1.5–2 in I. angelini sp. nov.).

Isorthoceras tenuitextum ( Hall, 1847) , which is common in late Katian sediments of Eastern Laurentia, is similar in ornamentation, but differs in having an adult size of more than 25 mm and a less, but more consistently expanding conch with an angle of expansion of ca. 6°.

Isorthoceras albersi (Miller & Faber, 1894) , which is the most common species within the Lexington Limestone, Kentucky, USA (see Frey 1995: 60), has a similar ornamentation and angle of expansion, but differs in having a larger adult size.

Stratigraphic and geographic range

Boda Limestone, late Katian, Dalarna, Sweden.

Figures

Fig. 13. Polished median sections of Orthocerida of the Boda Limestone. A. Dawsonoceras fenestratum (Eichwald, 1860), PMU 26706, Kallholn. B-C. Striatocycloceras isbergi sp. nov. B. PMU 26681, Kallholn. C. PMU 26661, Osmundsberget. D. Isorthoceras angelini sp. nov., PMU 26784, Kallholn. E. Nathorstoceras kallholnense sp. nov., PMU 26729, holotype, Kallholn. F. Nathorstoceras adnatum sp. nov., PMU 26733, holotype, Kallholn. G. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo, 190102c, Osmundsberget.H.Ordogeisonocerasfoerstei(Strand,1934)comb.nov.,PMU26763,Kallholn. I. Geisonoceras wegelini (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26740, Kallholn. Scale bars: A-D, I = 1 mm; E-H = 5 mm.

Fig. 14. Reconstructions of the details of the septal necks and siphuncular segments of Orthocerida of the Boda Limestone. Note the different septal thicknesses. A. Striatocycloceras isbergi sp. nov., PMU 26681, Kallholn. B. Dawsonoceras fenestratum (Eichwald, 1860), PMU 26706, Kallholn. C. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo 150102c, Osmundsberget. D. Isorthoceras angelini sp. nov., PMU 26784, Kallholn.

Fig. 17. Shell ornamentation of selected Orthocerida of the Boda Formation. A. Isorthoceras cf. elongatocinctum (Portlock, 1843), PMU 26825, Kallholn, adoral part of the specimen. B. Dawsonoceras stumburi sp. nov., PMU 26717, holotype, Kallholn. C. Gorbyoceras sp. A, PMU 26772, Osmundsberget. D. Isorthoceras cf. elongatocinctum (Portlock, 1843), same specimen as in A, adapical part of specimen. E. Isorthoceras junceum (Hall, 1847), PMU 26834, Kallholn. F. Gorbyoceras alternestriatum (Strand, 1934) comb. nov., PMU 26771, Unskarsheden. G. Isorthoceras curvilineatum sp. nov., PMU 26787, holotype, Osmundsberget. H. Isorthoceras angelini sp. nov., PMU 26774, holotype, Kallholn. I. Isorthoceras suave (Angelin in Angelin & Lindström, 1880) comb. nov., PMU 26835, Kallholn. Scale bar = 5 mm for all figures.

Fig. 23. Orthocerida of the Boda Limestone. A. Pleurorthoceras osmundsbergense sp. nov., NRM-PZ Mo 150126, holotype, Osmundsberget. B. Isorthoceras suave (Angelin inAngelin & Lindström, 1880) comb. nov.,PMU26835,Kallholn.C.Isorthocerasjunceum(Hall,1847),PMU26833,Kallholn.D-E.Isorthoceras cf. elongatocinctum (Portlock, 1843), Kallholn. D. PMU 26814, juvenile growth stage. E. PMU 26825, larger specimen with irregularly spaced transverse ornamentation. F. Isorthoceras curvilineatum sp. nov., PMU26787,holotype,Osmundsberget.G.Isorthocerasangelinisp.nov.,PMU26774,holotype,Kallholn. H. Geisonoceras? sp., PMU 26758, Kallholn. I. Isorthoceras heroyense (Strand, 1934) comb. nov., PMU 26831, Kallholn. Scale bars: A = 10 mm; B-I = 10 mm.

Fig. 25. Variability of the angle of expansion in fragments of different diameters of Isorthoceras angelini sp. nov. Note the tendency of decreasing angle of expansion with diameter. The adult diameter is reached at ca. 15 mm.