Cameroceras turrisoides, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 8-13

Cameroceras turrisoides sp. nov.

urn:lsid:zoobank.org:act:3166A3D0-2EBC-40C4-AEF2-F6DFF3152F30

Figs 4A , 5 , 6 , 7D

Endoceras sp. B. – Strand 1934: 10.

Cameroceras sp. – Evans 1994: 16, fig. 6.1-2 (non fig. 6.3).

Diagnosis

Slender orthocones with average angle of expansion of ca. 6°, elliptical conch cross section, which is slightly compressed in earlier growth stages and slightly depressed in late growth stages; ornamented with irregularly spaced lirae, which are slightly oblique, shifted toward the apex on the prosiphuncular side and forming very shallow, broad hyponomic sinus; additionally shell is slightly irregularly undulated; sutures directly transverse, with distance (cameral depth) varying from 0.17 to 0.3 of the corresponding conch cross section; siphuncle eccentric with SPR of ca. 0.2; siphuncle diameter relatively small with 0.3–0.4 of the corresponding conch cross section; siphuncular segments slightly concave in longitudinal section, septal necks holochoanitic; apex large with angle of expansion of ca. 25°, a length of ca. 50 mm and a conch height of 24 mm, apically slightly curved with siphuncle on concave side of conch curvature.

Etymology

Refers to the similarity of this species to the type of Orthoceras turris Angelin in Angelin & Lindström, 1880 , which differs in having a marginal, slightly wider siphuncle.

Type material

Holotype

PMU 26613 .

Paratypes

Fourteen additional specimens (paratypes): seven ( PMU 26614–26620, NRM Mo 9264) from Kallholn; three ( PMU 26621–26623) from Osmundsberget; and four ( PMU 26624–26627) from Unskarsheden, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian.

Type locality and horizon

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

Description

The holotype is a fragment of a phragmocone with a length of 128 mm, a conch height of 63–77 mm, and a conch width of 68–81 mm (width/height ratio ca. 1.05–1.08, and angle of expansion in width of ca. 2.2°; Fig. 4A ). The cross section is elliptical. The surface of the conch is ornamented with irregularly spaced oblique, shallow lirae. Between the lirae are widely rounded, shallow valleys. The ornamentation is slightly oblique, forming a shallow broad hyponomic sinus on the prosiphuncular side of the conch. The siphuncle of the holotype is circular in cross section with a diameter of 26–31 mm (ca. 0.4 of conch cross section) and eccentrically positioned with a distance of 10–12 mm from conch margin (SPR ca. 0.16).

A total of 12 measurements reveals a varying angle of expansion with a mean of 5.7°, and a maximum of 8.8°, without any clear ontogenetic trend, but highest angles occur in intermediate conch sizes ( Fig. 3 ). The chamber distance was measured in five specimens and varies between 0.13 and 0.33 of the corresponding conch cross section without any significant ontogenetic trend. The diameter of the siphuncle varies between ca. 0.3 and 0.4 relative to the corresponding conch cross section diameter. In general, the initial position of the siphuncle is nearly marginal, but during ontogeny shifts toward the conch axis so that the SPR lies between ca. 0.15 and 0.18. However, in the largest specimen in the collection (PMU 26627), a portion of phragmocone has a maximum diameter of 165 mm and a siphuncle diameter approximately 0.3 of the conch height, and the SPR is only 0.09.

The structure of the siphuncle can be seen in specimen PMU 26624, a piece of phragmocone with a diameter of 75-81 mm ( Fig. 5 ). The septal distance varies between 8 and 14 mm (nine chambers occur in a distance of 97 mm). The septal necks are holochoanitic with a concave to s-shaped profile.

The collection includes two specimens representing apical portions (PMU 26623, NRM Mo 9264) with completely preserved shells ( Figs 6 , 7 ). Both specimens possess large conical apical parts (maximum diameters ca. 25 mm) that differ significantly from the younger growth stages. This apical portion of the shell is markedly conical (apical angle ca. 25°) and has a compressed conch cross section (width/height ratios of ca. 0.88). The shell surface is ornamented with irregular longitudinal grooves and appears thickened and irregularly wrinkled around the circumference in a longitudinal direction. At its adoral end there is a marked change in growth, initiated by a constriction and an abrupt change in the angle of expansion. The conch portion 30 mm adorally of the termination of the apical part of the shell is nearly tubular and ornamented with the characteristic transverse ornamentation of a more mature specimen. This conch portion is additionally weakly longitudinally ornamented with irregular striae. The change in growth is accompanied by a marked increase in shell thickness. In specimen PMU 26623 the shell adapically of the growth change is less the 0.5 mm thick, at the position of the constriction it is only 0.4 mm, and rapidly increases adorally to ca. 1 mm. The diameter of the siphuncle at the position of the constriction is 9 mm. There the siphuncle is nearly marginal, only one millimetre distant from the conch margin.

The apex is eccentrically positioned marginally on the prosiphuncular side of the conch. The extreme tip is not preserved, but the slight decrease of the apical angle at the apicalmost preserved part may represent the fragment of a subspherical initial shell ( Fig. 6B ).

Remarks

The apex of C. turrisoides sp. nov. is exceptional in form and size, but not entirely unknown. A very similar apex was described as Endoceras akpatoense Foerste & Cox, 1936 , from the latest Katian of the Arctic Canadian Island Akpatok. This specimen differs in having even larger dimensions and a less curved extreme apical part. Based on the genus diagnosis of Cameroceras of Teichert (1964, see above) this species must be recombined as Cameroceras akpatoense (Foerste & Cox, 1936) comb. nov.

Comparison

This very large species of Cameroceras is most similar to Endoceras windriverense Miller, 1932 (based on the generic diagnosis of Cameroceras , see above, this species must be recombined as Cameroceras windriverense (Miller, 1932) comb. nov.) from the latest Katian Lander Formation from Canada, which has a relative siphuncle diameter of 0.35 and an SPR of 0.18, but differs in having a lower angle of expansion and in having a nearly circular conch cross section. Additionally, the ornamentation of this species is not known.

Endoceras sp. B. Strand (1934) is interpreted as conspecific with C. turrisoides sp. nov., even though the conch surface of this specimen is not preserved, because it is identical in having a relatively thin siphuncle (0.4 of conch cross section), which is positioned at a distance from the conch margin (SPR 0.18) which is well within the range of the Boda specimen.

Although the conch surface is not known in the Cameroceras sp. reported by Evans (1994: fig. 6.1-2) from the Bardahessaigh Formation of Ireland, it is synonymized with C. turrisoides sp. nov. herein. This is justified by the size of the siphuncle (0.35 of conch cross section) and the relative distance of the siphuncle (SPR 0.14), which are both within the range of variation of the Boda specimen.

Stratigraphic and geographic range

Bardahessaigh Formation, mid-late Katian, Ireland ( Evans 1994); Bønsnes Formation, Ringerike district, Norway ( Strand 1934); Boda Limestone, late Katian, Siljan District, Sweden.

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. 5. Polished median section of Cameroceras turrisoides sp. nov., PMU 26624, Unskarsheden, Boda Limestone. A. Total fragment; note that the light, layered, micritic filling of the siphuncle is not an endosiphuncular deposit. B. Enlarged detail of the connecting ring and the holochoanitic septal neck. Scale bars: A = 10 mm; B = 5 mm.

Fig. 6. Apex of Cameroceras turrisoides sp. nov., NRM-PZ Mo 9264, Kallholn, Boda Limestone. A. View from prosiphuncular side. B. Lateral view. C. Complete specimen, prosiphuncular view; note shell repair on adoral part. D. Lateral view. Scale bars: A-B = 10 mm; C-D = 10 mm.

Fig. 7. Breviconic cephalopods of the Boda Limestone and details of an endocerid. A. Dalecarlioceras constrictum Frye, 1987, PMU 24777, holotype, Kallholn. B. Dalecarlioceras dalecarlicum (Frye, 1987) comb. nov., PMU 24774, holotype, Kallholn. C. Cyrtorizoceras thorslundi sp. nov., PMU 26658, Kallholn; note the morphological transition between D. constrictum and C. thorslundi sp. nov.; the adult size increases in Dalecarlioceras and the mature body chamber is gibbous. D. Cameroceras turrisoides sp. nov., PMU 26623, Osmundberget, lateral view of apical fragment with abrupt transition between embryonic and juvenile conch. E. Strandoceras sphinx (Schmidt, 1858), PMU 26629, Kallholn. Scale bars: A-C, E = 10 mm; D = 10 mm.

Fig. 3. Variation in relative siphuncle size, position and angle of expansion in Cameroceras hasta (Eichwald, 1860) (grey points) and Cameroceras turrisoides sp. nov. SPR = the minimum distance of the connecting ring from the conch margin, divided by the corresponding conch height.