Cycloplectoceras hinlopense, Kröger & Pohle, 2021

Kröger, Björn & Pohle, Alexander, 2021, Early-Middle Ordovician cephalopods from Ny Friesland, Spitsbergen - a pelagic fauna with Laurentian affinities, European Journal of Taxonomy 783 (1), pp. 1-102 : 61

Cycloplectoceras hinlopense sp. nov.

urn:lsid:zoobank.org:act:17CC19A8-797C-4718-AE09-C6DEEF037258

Fig. 41C–D

Diagnosis

Strongly ribbed Cycloplectoceras with rapidly enlarging conch with WER of ca 3.7 and LER of ca 2, with adult size> 30 mm.

Etymology

From Hinlopen Strait, referring to the type region of this species.

Type material

Holotype Specimen FMNH-P30327 ; by monotypy.

Type locality and horizon

From Profilstranda section, Ny Friesland, Svalbard, bed PO 131, 128 m above base of Olenidsletta Member, Blackhillsian, Floian.

Description

Specimen FMNH-P30327 is a fragment of a phragmocone consisting of ca two whorls with a total diameter of 26 mm. The conch grows with a WER of 3.66 and a LER of 2. The whorl is 13 mm wide and high at a conch diameter of 26 mm and is slightly flattened on the lateral flanks and venter; a 0.3 mm deep imprint zone is present at the dorsum.

The conch is strongly annulated with ribs than run obliquely transverse across the flanks and forming a deep and broad ventral lobe. Prominent growth lines or striae are present, ca 8 per one millimetre, and are parallel to the ribs.

The siphuncular perforation has a diameter of 1.4 mm and is positioned at a distance of 1.3 mm from the venter (rSP = 0.1). The septal spacing is relatively narrow; where the conch is 22 mm in diameter the sutures have a distance of 2 mm apart over the venter.

Remarks

This specimen is assigned to Cycloplectoceras because it is a ribbed tarphycerid with rapidly enlarging whorls, both in whorl height and in whorl width. The conch cross section is nearly circular, similar as in known species of Cycloplectoceras . Pionoceras Ulrich et al., 1942 is another annulated tarphycerid with high WER and rapid growth of the whorl width. Pionoceras , however, differs in having broadly subtrapezoidal whorl cross sections and a very broad venter. The fragment described above is, however, relatively small, and ontogenetic growth changes are frequent among tarphycerid species, and especially among species of Pionoceras . It is therefore possible that this specimen represents a fragment of a larger individual, which if fully preserved would exhibit an adult morphology with an aspect of Pionoceras . More material is required to fully document the ontogeny, and consequently to further support the generic assignment of this species.

Cycloplectoceras hinlopense sp. nov. differs from C. miseri Ulrich, Foerste, Miller & Furnish, 1942 in being larger (the adult size of C. miseri is only 20 mm). The other known species of Cycloplectoceras , C. funatum Ulrich, Foerste, Miller & Furnish, 1942 , has a smaller rate of increase in relative conch width (LER = ca 1.2, compare Ulrich et al. 1942: pl. 6 fig. 7) and is less strongly annulated.

Figures

Fig. 41. Coiled cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Deltoceras beluga sp. nov., FMNH-P30322, from bed PO 07. B. Trocholitoceras walcotti Hyatt, 1894, FMNH-P30328, from bed PO 131. C-D. Cycloplectoceras hinlopense sp. nov., FMNH-P30327, holotype, from bed PO 131. Scale bar = 10 mm for all figures.