Protocycloceras undetermined

Protocycloceras sp.

Fig. 36A

Specimen FMNH-P30417 from Profilstranda section, bed PO 123.3, 120.3 m above base of Olenidsletta Member, V2 a trilobite zone, Blackhillsian, Floian.

The specimen is an orthoconic fragment of a phragmocone with a circular conch cross section 2.0– 5.5 mm in diameter and 17 mm long (angle of expansion 12°). The specimen is ornamented with a transverse annulation, the annuli slope very slightly toward the aperture at the antispihuncular side, ca 3 annuli occur per distance similar to the corresponding conch diameter. Additionally, fine striae occur, which run parallel to the annulations, ca 20 striae occur per one millimeter. The chamber distance is roughly similar to the distance of the annuli; at the adoral end of the specimen the chamber length is 1.5 mm. There the siphuncle is 0.75 mm in diameter (rSD = 0.14) and positioned ca 0.6 mm from the conch margin (rSP = 0.1). The connecting ring segments are slightly concave and relatively thick, and the septal necks are orthochoanitic to loxochoanitic with a length of 0.4 mm, where the chamber length is 1.5 mm.

This specimen differs from P. minor sp. nov. in having a very thin siphuncle (rSD = 0.14, compared with rSD = 0.22 in P. minor). Hence, it is morphologically transitional to Slemmestadoceras attavus (Brøgger, 1882) and Kyminoceras forresti Teichert & Glenister, 1954, which have a thin marginal siphuncle, and C. arennigense and P. lamarcki which have a much thicker siphuncle, that is positioned close to the conch center. However, based on this single, small fragment the erection of a new species would be not justified and it will be left in open nomenclature until more material is available.