Trocholitoceras walcotti Hyatt 1894

Trocholitoceras walcotti Hyatt, 1894

Figs 41B, 44D, 47

Trocholitoceras walcotti Hyatt, 1894: 480–482, pl. 6 figs 12–20.

Trocholitoceras latum Ulrich et al., 1942: 82–83, pl. 42 figs 1–3.

Trocholitoceras walcotti – Ruedemann 1906: 478. — Ulrich et al. 1942: 80–81, pl. 31 fig. 3, pl. 53 figs 3–4. — Dzik 1984: 44, text-fig. 12.34.

Trocholitoceras with moderately expanding whorls with WER decreasing during growth from ca 2 to 1.7; whorl cross section moderately involute with average IZR 0.26, and depressed with WWI decreasing with conch size from more than 2 to less than 1.5; conch surface ornamented with prominent transverse growth lines and low rounded ribs at mature body chamber which curve apicad on the lateral side and form a deep, rounded hyponomic sinus; siphuncle subcentral in earliest growth, subdorsal during later growth; mature conch with diameter ca 90 mm with 4–5 volutions (compiled from Ulrich et al. 1942: 80–81).

Three specimens (FMNH-P30325, FMNH-P30328, FMNH-P30333) from Profilstranda section, Ny Friesland, Svalbard, bed PO 131, 128 m above base of Olenidsletta Member, Blackhillsian, Floian.

The most complete specimen is FMNH-P30328 with a presumed conch diameter of more than 80 mm and the last volution slightly evolute (Fig. 41B). At a conch diameter of 42 mm the whorl is 20 mm wide, 19 mm high, with an apertural height of 12 mm, and an impression zone with a depth of 6–7 mm. The depression of the whorl decreases with increasing diameter, with WWI nearly 2.5 at an apertural height of 6 mm to 1.5 at an apertural height of 20 mm. The WER is 1.96 at a conch diameter of 42 mm (Fig. 47). The IZR varies between ca 0.1 and 0.4 (Fig. 47). The whorl cross section has broadly rounded flanks and venter (Fig. 44D). The conch surface is ornamented with prominent transverse growth lines. The siphuncle is 2 mm distant from the dorsal margin in specimen FMNH-P30325, where the apertural height is 6.5 mm, while its and diameter is ca 1 mm. In specimen FMNH-P30328 the siphuncle is nearly marginal at whorl heights greater than 8 mm and has a diameter ca ¹/10 of the apertural height.

Measurements taken from the median cross section of the types of T. walcotti and T. latum (Ulrich et al. 1942: text-fig. 22) reveal a large overlap in WER, IZR, and in the relative whorl width (WWI, Fig. 47) between both species. This is especially relevant in the case of the WWI, because T. latum was originally distinguished from T. walcotti based on the broader and lower whorls of the former (Ulrich et al. 1942: 83). The wide overlap suggests synonymy between the two species, with T. walcotti having priority. The two specimens from the Olenidsletta Member are slightly less involute, have less depressed whorls at growth stages less than 20 mm, but fall within the variability of the WER of T. walcotti and are also similar in whorl cross section shape, ornamentation and position of the siphuncle. Altogether this similarity justifies the assignment of these specimens to T. walcotti. Trocholithoceras walcotti (and T. latum) were previously known from the Fort Cassin Limestone, Vermont and the St. Armand Limestone, Quebec.