Protocycloceras lamarcki

Protocycloceras lamarcki (Billings, 1859)

Figs 36I, 37A–B

Orthoceras lamarcki Billings, 1859: 362, fig. 11f–h.

Protocycloceras lamarcki – Kröger & Landing 2009: 679–681, figs 10.5, 10.6, 11.6–8 (cum syn). — Evans 2011: 82–94, pl. 13 figs 6–9, 11–39, pl. 14 figs 1–16, pl. 15 figs 9–10, text-figs 22–26, table 14 (cum syn).

Diagnosis

Slightly curved longicones with prominent undulation; conch cross section circular-subcircular; ribs directly transverse, rounded, concave interspaces; ribs and interspaces similar in size and shape; one rib and a concave interspace correspond to one chamber; sutures directly transverse; approximately 4–5 chambers over a length similar to conch cross section; siphuncle eccentric between center of conch and ventral conch margin; siphuncular diameter approximately one third of conch cross section; siphuncular segments concave; septal necks short, loxochoanitic to orthochoanitic; connecting ring thick; endosiphuncular rod in ventral part of siphuncle, fills entire siphuncle in more adapical part; rod wedges out adorally with steep angle toward the venter; diaphragms in adapical portions of siphuncle; epi- and hyposeptal deposits present (from Kröger & Landing 2009).

Material examined

Specimen FMNH-P30409, from Profilstranda section, adjacent to Hinlopenstretet, Spitsbergen, from bed PO 3.7, 0.7 m above base of Olenidsletta Member, and specimen FMNH-P30418 from bed PO 04, 1 m above base of Olenidsletta Member, both V1 a trilobite zone, Blackhillsian, Floian.

Description

Specimen FMNH-P30418 consists of a portion of a phragmocone and 3 mm of the basal part of the body chamber. In total, the length of the specimen exceeds 24 mm. The conch cross section is slightly subcircular with a diameter of 6.5–8.0 mm. The exact angle of expansion is unknown.

The surface is ornamented with rounded, directly transverse ribs, which run parallel to the sutures and are very slightly shifted toward the apex at the prosiphuncular side, forming a very shallow ventral sinus (Fig. 36I). Approximately one rib and one valley occur per chamber, and ca four ribs occur in a distance similar to corresponding conch cross section.

The three adoral chambers counted from body chamber toward apex measure 1 mm, 1.4 mm, and 1.7 mm (rCL = 0.25–0.21) and probably represent adult septal chambers. The siphuncle is at least 2.2 mm in diameter (rSD = 0.28); the distance from the ventral conch margin cannot be measured with certainty but is approximately 1.4–1.8 mm (rSP = 0.18–0.25).

The siphuncular segments are strongly concave and the connecting rings are thick. The septal necks of the two adoralmost chambers are preserved and are relatively long (ca 0.6–0.7 mm) and loxochoanitic (Fig. 37A). An interpretation of the endosiphuncular structure is difficult because of the state of preservation of the specimen, but episeptal and mural cameral deposits are visible in the polished section of the specimen.

A second specimen, FMNH-P30409, is a naturally weathered sagittal cut of a phragmocone, exposing the siphuncle and the central parts of the septa. The conch is straight and slender. The conch cross section is only known from the ventral parts and does not allow any inference on the cross-section shape. The surface is weakly annulated with ca one undulation per chamber. The chambers have a distance of ca 1.2– 0.7 mm. The siphuncle is 0.8 mm in diameter, with weakly concave segments and orthochoanitic septal necks, and has no endosiphuncular deposits. Episeptal cameral deposits are present in specimen FMNH-P30409.

Remarks

The morphological characters of the two specimens, including shape and spacing of annulation, spacing of septa, siphuncular position and relative thickness and the preserved details of the connecting ring and septal necks all are within the well documented (Kröger & Landing 2009; Evans 2011) variability of P. lamarcki. This justifies the assignment of these fragments to P. lamarcki, herein. It should be noted, however, that specimen FMNH-P30418 probably represents an adult specimen (based on adult septal crowding). With an adult size of only 8 mm it is clearly smaller than many specimens measured from the Fort Cassin Formation, New York, USA, which reach conch diameters of more than 30 mm (Kröger & Landing 2009) and from the Durness Group, Scotland, UK (Evans 2011), which reach diameters of ca 25 mm.

Stratigraphic and geographic range

Widespread in North America (see details in Kröger & Landing 2009), Scotland, UK (see review in Evans 2011), and Spitsbergen, herein, Floian Stage.