Trocholitoceras juvenicostatum Ulrich, Foerste, Miller & Furnish 1942

Trocholitoceras juvenicostatum Ulrich, Foerste, Miller & Furnish, 1942

Figs 44C, 48

Trocholitoceras juvenicostatum Ulrich, Foerste, Miller & Furnish, 1942: 82, pl. 51 fig. 6, pl. 53 figs 5–6, pl. 56 figs 1–3.

Trocholitoceras with relatively narrow whorls and prominent ribs throughout entire growth; sutures directly transverse with dorsal lobe; siphuncle located close to the dorsal wall in mature portions of the phragmocone (compiled from Ulrich et al. 1942: 82).

Six specimens (FMNH-P30323, P30324, P30330, P30332, P30344, P30347), all from Profilstranda section, Ny Friesland, Svalbard, bed PO 131, 128 m above base of Olenidsletta Member, Blackhillsian, Floian.

Specimen FMNH-P30323 (Fig. 48A–B) is the most complete specimen assigned this species; it is a fragment of a phragmocone, which is partly heavily recrystallized and part of a 140° long continuously growing body chamber, and which grows with a WER of 2.03. At the base of the body chamber the conch diameter is 37 mm and the whorl is 16.5 mm wide and 11 mm high (rW = 1.05). The slightly

depressed cross section has flattened flanks, a rounded venter and a shallow imprint zone (0.6 mm at whorl height 11 mm) (Fig. 44C).

The conch is ornamented with prominent ribs which run obliquely transverse across the whorl and form a deep hyponomic sinus over the venter; ca 20 ribs occur per volution and the distance of the ribs is ca 4 mm at the base of the body chamber. Additionally, the conch surface is finely striated with ca 5 striae per mm, which run parallel to the ribs. The annulation appears to be constant in amplitude and relative spacing over the entire preserved part of the specimen. On the imprint of the inner shell a wrinkle layer is visible (Fig. 48A).

The septal perforation is ca 2.9 mm in diameter and positioned sub-dorsally, ca 1.5 mm from the dorsum at the base of the body chamber. The chamber distance is visible in specimen FMNH-P30324, which is a small fragment of a phragmocone with a conch diameter of 22 mm, where at a conch width of 12 mm

the septal distance is ca 2 mm at the venter. The sutures are straight and directly transverse. In specimen FMNH-P30332 the whorl width is 15 mm where the whorl height is 13 mm (rW = 1.15).

In the type material of T. juvenicostatum, from the St Armand limestone (Middle Ordovician, see Salad Hersi et al. 2007 for age constraint of St. Armand limestone), Quebec, Canada, described by Ulrich et al. (1942), the rW decreases from 1.2 at a whorl height of 6.5 mm to 0.91 at a whorl height of 12 mm, and the conch grows with a WER of 2.5. This is slightly different from our material. However, the type material of T. juvenicostatum is slightly tectonically distorted and the measurements therefore reflect some preservation effects. Also, little is known about intraspecific variation of this species. The assignment of the specimens from the Olendisletta Member to T. juvenicostatum is justified by the presence of prominent ribs, the general similarity of the conch growth rate, shape of the whorl cross section and the subdorsal position of the siphuncle. The other species of Trocholitoceras with prominent ribs, T. phillipsburgense Ulrich, Foerste, Miller & Furnish, 1942, differs in having a nearly smooth surface in early volutions. Trocholitoceras juvenicostatum was previously known from the St. Armand limestone (Middle Ordovician) of Quebec.