Lyckholmoceras norvegiae Strand, 1934

Lyckholmoceras norvegiae Strand, 1934

Figs 37G View Fig , 38A–B

Lyckholmoceras norvegiae Strand, 1934: 88–91, pl. 11 figs 1a–b, 2.

Beloitoceras heterocurvatum View in CoL Strand, 1934: 77, pl. 10 fig. 11 (non pl. 10 figs 8–10).

Parryoceras strandi Sweet, 1959: 58–59, pl. 9 figs 5–6, pl. 10 fig. 6.

Lyckholmoceras norvegiae – Flower in Flower & Teichert 1957: 59, text-figs c, g. ― Dzik 1984: 67, text-fig. 18.13.

Diagnosis

Lyckholmoceras with compressed conch cross section (CHI ca 1.5), in longitudinal direction moderately curved; mature body chamber almost straight with uncontracted aperture and well-marked hyponomic sinus; sutures with shallow lateral lobes; siphuncular segments slightly expanded within chambers, narrowing adorally, with SCR of ca 1; ornamented with faint, densely irregularly spaced lirae. Small endosiphuncular bullettes are present. (Compiled from Strand 1934: 89–90.)

Material examined

ESTONIA • 1 spec.; Haapsalu holm ; Adila Formation , Pirgu Regional Stage; TUG 1745-2 • 4 specs; Vormsi Island, Hosholm shore (tower locality); Adila Formation , Pirgu Regional Stage; GIT 878-166 View Materials , GIT 878-167 View Materials , GIT 878-187 View Materials , GIT 878-226 View Materials .

Description

The most complete specimen is GIT 878-226, which preserves parts of the mature living chamber and a 70 mm long mold of the phragmocone ( Fig. 39A–B View Fig ). The conch height cannot be measured in this specimen because the prosiphuncular side of the conch is not preserved but the preserved parts show the general conch shape. In lateral view, the body chamber appears straight, the adoral ca 35 mm of the phragmocone has a convex outline at the antisiphuncular side and the adapical part is straight. The straight part of the phragmocone has a conch width of <37 mm. The conch cross section is ovally compressed with a narrow margin at the antisiphuncular side. At the base of the body chamber, the conch width is 38 mm and the height is approximately 50 mm (CHI = 1.3). The peristome is straight and simple on the antisiphuncular side. It is ca 40 mm distant from the base of the body chamber. The part of the phragmocone with width <ca 35 mm is distinctly longitudinally striated; ca 60 striae occur around the circumference. The sutures are 5 mm apart where the conch width is 28 mm, they form shallow lateral lobes.

The conch cross section is preserved in specimen GIT 878-167, which is a small fragment of a phragmocone with only one chamber completely preserved. There, the conch height and width are 32 mm and 25 mm respectively (CHI = 1.28), and the conch cross section is elliptically compressed with narrow dorsal and ventral margins. The septal foramen is located ca 1–2 mm from the conch margin and has a diameter of 3 mm (RSH = 0.09, RSP ca 0.07).

The angle of expansion can be measured in specimen TUG 1745-2, which is a fragment of a phragmocone in which the conch height increases from 20 mm to 32 mm in 37 mm (angle of expansion = 18°). Specimens GIT 878-166, GIT 878-167, GIT 878-187, and TUG 1745-2 have conch heights of less than 40 mm and all show the characteristic longitudinal striation on the mold of the outer shell of the phragmocone (e.g., Fig. 38A–B View Fig ).

The details of the siphuncle and septal necks are preserved in GIT 878-187 ( Fig. 37G View Fig ), and GIT 878- 167. In both specimens, the siphuncle is located close to the margin at the concave side of the conch. The siphuncular segments are expanded tubular. The connecting ring is relatively thick and adnate on the adapical surface of the septum on the siphuncular side nearest to the conch center and adnate at the adoral side of the septa at the siphuncular side nearest to the conch margin. Where the conch width is 32 mm, the chamber length is 4.5 mm, the maximum height of the siphuncle is 4 mm, and the septal foramen is 2 mm (GIT 878-187, RSS = 2, SCR = 0.89). The septal necks are cyrtochoanitic; they are recumbent and adnate at the side of the siphuncle nearest to the conch margin. Small bullettes occur, which expand in adapical direction.

Remarks

The specimens described above are slightly less compressed in conch cross section than the types of the species described by Strand (1934), which have a CHI of 1.36–1.49. This difference can be interpreted as taphonomic effect or as an effect of ontogenetic change in CHI and measurement at different growth stages. More material is needed to evaluate the intraspecific and ontogenetic variability of the CHI. The Estonian specimens are nevertheless placed within L. norvegiae , based on the general similarities in conch dimensions and conch shape with this species.

The specimen described under Parryoceras strandi Sweet, 1959 is identical in conch shape, dimensions, and with regard of the shape and size of the siphuncle-septal neck complex and must be synonymized with L. norvegiae . It remains an open question how the closely related Parryoceras Sweet & Miller, 1957 compares in detail to Lyckholmoceras , because the details of the septal necks and connecting rings are poorly known in the former. Parryoceras euchari Sweet & Miller, 1957 , the type of the genus, apparently differs from species of Lyckholmoceras in having a more strongly constricted mature body chamber than species of Lyckholmoceras .