Cyrtorizoceras thorslundi, Kröger, 2013

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 21-23

Cyrtorizoceras thorslundi sp. nov.

urn:lsid:zoobank.org:act:3B69DD96-E0F9-4516-9383-6DC8FF7D0BFD

Figs 7C , 9 G-H, J, 10A

Diagnosis

Cyrtorizoceras with adult body chamber height of ca. 35 mm and angle of expansion of 12–13°; conch cross section compressed, ovate, with slightly angular anti- and prosiphuncular margins and with width/ height ratio of ca. 0.8–0.9; ornamented with fine, rounded, directly transverse striae; adult body chamber length 30 mm with angle of expansion similar to premature conch, shallow constriction and adoral widening at adult peristome; shallow angular sinus at prosiphuncular side; mature portions of conch slightly less coiled than adapical parts; apex large, cap-shaped.

Etymology

In honour of Per Thorslund (1900–1991), Swedish geologist and paleontologist and professor at Uppsala University (1950–1966), for his important contributions on the geology of the Siljian region (e.g., Thorslund 1935).

Type material

Holotype

PMU 26647 .

Paratypes

Twelve additional specimens (paratypes): PMU 26648–26659 from Kallholn, Dalarna, Sweden, Boda Limestone, Boda Core Member, late Katian.

Type locality and horizon

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

Description

The holotype is a complete juvenile specimen with a conch length of ca. 40 mm, maximum conch width at peristome 11 mm, conch height 13 mm, a ca. 15 mm long body chamber and a nearly complete apical portion ( Fig. 9J ). Apically the conch grows from the absolute tip toward a conch height of 4 mm within the first 2 mm. The conch surface of the holotype is ornamented with rounded striae, which run straight and form a shallow sinus at the prosiphuncular side; ca. five occur per one millimeter at the adoral parts of the conch.

In specimen PMU 26648, which is a fragment of the phragmocone, the septa and the siphuncle are exposed. The septa are simply curved, and the sutures form shallow lateral lobes 1.9 mm distant where

the corresponding conch height is 10.5 mm (0.18 of corresponding conch height). The siphuncle is nearly, but not exactly marginal, its width is 0.6 mm at a corresponding conch height of 6.6 mm (ca. 0.9 of conch height). The siphuncular segments are slightly expanded and the connecting ring is comparatively thin.

The angle of expansion varies only very little among the eight specimens, with 11.5° (PMU 26649) to 13.3° (holotype).

The adult conch height of 34 mm and width 28 mm is indicated by three body chambers of similar size with distinct peristomal shell thickenings (PMU 26656, 26657, 26659; Figs 9 G-H, 10A). These fragments of the body chambers are ornamented with fine transverse striae that form a distinct shallow sinus at the prosiphuncular side. The striae are ca. 1 mm wide.

Remarks

The material described above consists of eight juvenile specimens and three adult body chambers. No fragments of intermediate growth stages are known. The fragments are assigned to Cyrtorizoceras thorslundi sp. nov. because they have a similar conch curvature and nearly identical angles of expansion, cross section shapes and ornamentation.

Comparison

This species differs from the type of the genus in having a more slender body chamber, a less pronounced ornamentation and a shallower conch curvature. Cyrtorizoceras thorslundi sp. nov. is unique amongst the other species assigned to the genus in its combination of a fine transverse ornamentation with a relatively weak conch curvature.

The poorly preserved fragment, described by Teichert (1930) as Beloitoceras (?) estonicum from the Pirgu Regional Stage of Hosholm, Estonia, is very similar to C. thorslundi sp. nov. in general conch shape and should very probably be synonymised with the latter. However, better preserved material is needed for a definitive comparison.

The small C. tenue Strand, 1934 from the Gastropod Limestone of the Oslo region is very similar to C. thorslundi sp. nov. with respect to the general conch shape, size and ornamentation. However, in C. tenue the angle of expansion is considerably higher (ca. 17°), and the conch is more strongly curved.

The juvenile specimens of C. thorslundi sp. nov. can be distinguished from specimens of Beloitoceras in the Boda limestone by its broader conch curvature and low angle of expansion.

Stratigraphic and geographic range

Boda Limestone, late Katian, Dalarna, Sweden.

Figures

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. 9. Oncerida of the Boda Limestone. Note the high variability in conch curvature and angle of expansion in Beloitoceras. A-B. Beloitoceras sinuososeptatum (Roemer, 1861). A. PMU 24769. B. PMU 26638, Kallholn. C-D. Beloitoceras siljanense Frye, 1987. C. PMU 26644. D. PMU 26645, Kallholn. E-F. Apex of Beloitoceras sp., PMU 26646, Kallholn. E. Adapical view. F. Lateral view. G-H. Cyrtorizoceras thorslundi sp. nov., PMU 26657, Osmundsberget, mature body chamber. G. Lateral view. H. Adapical view. I. Beloitoceras siljanense Frye, 1987, PMU 26643, Kallholn, with major conch repair at adoral part. J. Cyrtorizoceras thorslundi sp. nov., PMU 26647, holotype, Kallholn, complete juvenile specimen. Scale bars: A-D, G-J = 10 mm; E-F = 2 mm.