Cymatoceras cf. honmai Matsumoto and Miyauchi, 1983

Wilmsen, Markus & Yazykova, Elena A., 2003, Campanian (Late Cretaceous) nautiloids from Sakhalin, Far East Russia, Acta Palaeontologica Polonica 48 (3), pp. 481-490 : 487-488

Cymatoceras cf. honmai Matsumoto and Miyauchi, 1983

Fig. 7 .

cf. Cymatoceras honmai Matsumoto and Miyauchi, 1983: 225 , pl. 48:

2; pl. 49: 1, 2.

Material.— A single, wholly septate, strongly weathered internal mould (specimen PIW 2002IV−5) from locality 35 in the Manuj River valley , Sakhalin .

Description.—Involute, depressed nautiloid with broadly rounded,nearlyflatventer.Dimensionsandproportionsareas follows: D = ~ 52 mm; Wb = 39.6 mm; Wh = 33.3 mm; U = ~ 5 mm; Wb/D = 0.76; Wh/D = 0.64; Wb/Wh = 1.19; D/U=~0.10.Thewhorlsectionisbroadlyellipticalwithmaximum breadth in the lower part of the flanks. The flanks are onlyweaklyconvexandgentlyconvergetowardsthebroadly rounded ventrolateral shoulders. The umbilicus is deep and narrowwithsteepumbilicalwalls;theumbilicalshouldersare rounded. The siphuncle is positioned subcentrally, slightly nearer to the venter than to the dorsum. The prorsiradiate sutureisoflowsinuosityandcharacterizedbyaninconspicuous saddle at the umbilical shoulder, a broad, shallow lateral lobe and a shallow lobe on the broad venter, separated by a saddle at the ventrolateral shoulder. The septa are fairly densely spaced (ca. 11–12 septa per half whorl).

Remarks.—Withrespect tooutlineofwhorl section, proportions of shell, density of septa per whorl, position of siphuncle and degree of involution, our specimen is very close to Cymatoceras honmai . However, due to its incomplete preservation and slightdifferences in sutural sinuosity, it is assigned to C. honmai with a query.

Occurrence.—Lower Campanian ( Anapachydiscus (N.) naumanni Zone) of Sakhalin, Member 10 of the Bykov Formation,locality35(seeFigs.2,3).Thetypesof Cymatoceras honmai are from the lower Upper Campanian near Soya, northern Hokkaido (member “E” of Campanian strata of Matsumoto and Miyauchi 1983).

Figures

Fig. 7. A. Cymatoceras cf. honmai Matsumotoand Miyauchi, 1983 (specimenPIW2002IV−5 fromtheLower CampanianBykov Formation,Member10, locality35attheManujRiversection,Sakhalin)inlateral(A1),ventral(A2),andseptal(A3)views. B.Reconstructionofwhorlsection. C.Sutureline(not to scale) (all figures natural size except C).