Protocycloceras minor Kröger & Pohle 2021, sp. nov.

Protocycloceras minor sp. nov.

urn:lsid:zoobank.org:act: C3D57CD1-2758-40A3-8596-6865D77F3620

Figs 36B–C, E–H, 37C, 38

From the Latin ‘ minor ’, ‘relatively small’, referring to the relatively small size of this species of Protocycloceras.

Holotype Specimen FMNH-P30411.

Paratypes Five additional specimens (FMNH-P30410, P30412 to P30415) all from type horizon and locality.

From Profilstranda section, adjacent to Hinlopenstratet, Spitsbergen, bed PO 131, 128 m above base of Olenidsletta Member, V 2 trilobite zone, Blackhillsian, Floian.

The holotype is a fragment of a body chamber and part of the phragmocone with a nearly circular conch cross section of 9.5 mm in diameter (Fig. 36B, E–F). The preservation does not allow precise measurement of the angle of expansion. The preserved part of the body chamber is 12 mm long. The conch is annulated, with annulations running parallel to the sutures with a distance of ca 2 mm and a chamber length of ca 2 mm (ca 4–5 chambers and annulations per corresponding cross section diameter).

The siphuncle is nearly tubular, has a diameter of 2.1 mm (rSD = 0.22), and is located ca 0.7 mm from the conch margin (rSP = 0.07) (Fig. 37C). The holotype has two complete septa preserved with a distance of 2.2 mm at a corresponding conch cross section of 9.5 mm (rCL = 0.23). The septal necks are poorly preserved but appear to be orthochoanitic and with a length of ca 0.5 mm.

The conch cross section is circular to subcircular and grows with an angle of expansion of ca 3° in the two other measured specimens (FMNH- P30410, FMNH-P30414). The septal perforation was measured in FMNH-P30412 and FMNH-P30415 with some inaccuracy because of poor preservation and varies from 0.29 to 0.34 in the two specimens. The conch surface in all specimens is ornamented with ribs and transverse striae. The ribs are directly transverse and run parallel to the sutures; they are very slightly shifted toward the apex at the prosiphuncular side, forming a very shallow ventral sinus. The ribs are more sharply rounded than the valleys, ca one rib and one concave interspace occur per chamber, and ca 3 ribs occur per distance similar to corresponding conch cross section. The relative spacing of the annulation increases with increasing conch diameter (Fig. 38). Additionally, the surface is ornamented with ca 15–20 fine transverse striae or growth lines per annulation. The largest specimen in the collection has a diameter of ca 12 mm (specimen FMNH-P30415).

The siphuncle is more eccentric (rSP ≈ 0.1 in P. minor sp. nov. compared with 0.25 in P. lamarcki) and thinner (ca ⅓ of the conch cross section in P. lamarcki) than in P. lamarcki. The position and dimensions of the septal perforation are similar to those of Castelloceras arennigense Evans, 2005, from the Bohaul Member of the Ogof Hên Formation, Moridunian Regional Stage (Floian), which is slightly older than division V2 of the Olenidsletta Member. However, the details of the siphuncle are not known from that species, which precludes further comparison.