Litoceras profilbekkenense, 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 : 67-68

Litoceras profilbekkenense sp. nov.

urn:lsid:zoobank.org:act:847E0904-7D05-46EF-81F3-EDA8A5F740A7

Figs 44B , 46–47

Diagnosis

Litoceras with relatively slender, rapidly enlarging conch, with WER of 2.3; with reniform, depressed conch cross section and WWI of ca 1.3; ornamented with fine growth lines which run obliquely across the flanks and form broad hyponomic sinus; the siphuncle is relatively wide, ca 0.25 of apertural height, and at a distance of 0.25 of the corresponding apertural height from the dorsum.

Etymology

Referring to the Profilbekken River near the type locality.

Type material

Holotype

Specimen FMNH-P30329 ; by monotypy.

Type locality and horizon

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

Description

This specimen is a tectonically distorted fragment of a phragmocone and parts of a ca 180° long body chamber with a conch diameter of 86 mm ( Fig. 46 ). The base of the body chamber is at a conch diameter of 58 mm, where the conch has ca three volutions, a whorl height of 21 mm, an apertural height of 20 mm and a conch width of 28 mm. The WER of the conch at the base of the body chamber is 2.3 (see Fig. 47 for comparison). In cross section the whorls are reniform and show a very broad, almost flattened venter and rounded flanks.

The conch surface is ornamented with pronounced growth lines and is otherwise smooth or with very low ribs. The septal perforation at the base of the body chamber is subcentrally positioned at a distance

of ca 5 mm from the dorsal margin and has a diameter of ca 5 mm. The sutures are simple and directly transverse.

Remarks

This specimen falls within the range of variability of Trocholitoceras walcotti with respect to the IZR, WWI, and general conch cross section shape (see Fig 47 ); however, at a comparable conch size it has a much larger WER than T. walcotti and the siphuncle is wider and subcentrally positioned, as seen in Litoceras . Litoceras calciferum (Billings, 1865) , L. insolens (Billings, 1865) , L. versutum (Billings, 1865) , and L. huygenae Flower, 1968 have broader whorls. In L. whiteavsi Hyatt, 1894 the rounded flanks are more convex. Litoceras avus Barrande, 1870 (see Flower 1979) and L. adamsi Flower, 1968 have a similar WER (2.3), but the whorl cross section and exact position of the siphuncle is not known from the former, and the whorl cross section is wider in the latter. Litoceras was previously restricted to the Middle Ordovician. Notably, several specimens of Litoceras with comparatively high whorls, which are still wider than those of L. profilbekkenense sp. nov., occur in basal Whiterockian strata in Nevada, USA ( Flower 1968). These strata are only slightly younger than division V2 of the Olenidsletta Member ( Fortey 1980; Fortey & Droser 1999; Loch & Ethington 2017), where L. profilbekkenense sp. nov. occurs. Therefore, L. profilbekkenense sp. nov. may be seen as a morphological and stratigraphical link between the predominantly Floian Trocholitoceras and the predominantly Middle Ordovician Litoceras .

Figures

Fig. 44. Cross sections of coiled cephalopods from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen with details of siphuncular position and impression zone of conch. A. Deltoceras beluga sp. nov., FMNH-P30322, from bed PO 07. B. Litoceras profilbekkenense sp. nov, FMNH-P30329, holotype, from bed PO 131, C. Trocholitoceras juvenicostatum Ulrich et al., 1942, FMNH-P30324, from bed PO 131. D. Trocholitoceras walcotti Hyatt, 1894, FMNH-P30328, from bed PO 131. Scale bars = 10 mm.

Fig. 46. Litoceras profilbekkenense sp. nov., FMNH-P30329, holotype, from bed PO 131, from the Olenidsletta Member, Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen. A. Lateral view of left side of fragment of body chamber. B. Ventral view and whorl cross section with siphuncular perforation preserved. C. Lateral view of right side. Scale bar = 10 mm for all figures.

Fig. 47. Diagrams of whorl expansion rate (WER), whorl width index (WWI), and relative depth of impression zone (IZR) ofTrocholitoceras walcotti Hyatt, 1894,T. latum Ulrichet al., 1942, andLitoceras profilbekkenense sp. nov. Measurements from specimens from the Olenidsletta Member (Olen. Mbr), Floian, Ordovician, Profilstranda section, Ny Friesland, Spitsbergen, and from type specimens based on Ulrich et al. (1942). See Supp. file 2 for details of measurements.