Cymatoceras cf. bifidum Shimansky, 1975

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

Cymatoceras cf. bifidum Shimansky, 1975

Fig. 6B .

cf. Cymatoceras bifidum Shimansky, 1975: 108 , pl. 23: 1.

Material.— A single deformed specimen with shell preserved (no. PIW 2002IV−4 from locality 0.6 in the Naiba River valley , Sakhalin) .

Description. —Involute nautiloid with broadly rounded venter. The umbilical shoulder is rounded, grading into a steeply inclined to overhanging umbilical wall. The funnel−likeumbilicusisnarrowbutnotoccluded( Fig.6B 3 ).Due to lateral compaction, only maximum diameter (~ 157 mm), whorl height (~ 90 mm) and umbilical diameter (~ 20 mm) could be measured (Wh/D = 0.57; U/D = 0.13). Maximum breadth of whorl seems to be in the lower part of the flanks. The flank is covered by band−like ribs which broaden up to mid−flank, reaching a width of up to 12 mm. Intercalated groovesare2–4mmwide.Bifurcationsorintercalatoriesappear on the outer flanks and on the ventrolateral shoulders ratherirregularly.Ontheventer,theribsareregularlyspaced and approximately 5–7 mm wide ( Fig. 6B 2 ). On the inner to middle part of the flanks the ribs are slightly prorsiradiate, thencurvebackwardsandcrosstheventerwithamoderately rounded sinus ( Fig. 6B 2 ). Neither suture nor the position of the siphuncle are visible.

Remarks.—This specimen is very close to Cymatoceras bifidum , especially in the style of ornament (broad ribs on flank, irregular bifurcations), but also the general shell form and the funnel−like, deep umbilicus (U/D = 0.15 in the holotype) are similar. That species was erected by Shimansky (1975) on the basis of a single specimen from the “Santonian” of Naiba, Sakhalin. According to the locality details (see below) and refined stratigraphy of that area, this find has now to be regarded as Lower Campanian. The holotypefiguredbyShimansky(1975)inpl.23:1a,seemsto have suffered a non−fatal shell injury, partly causing the irregular ribbing pattern on the flank. Due to the poor preservation, only some of the specific characters are unequivocally discernible (general ribbing pattern, umbilicus), and, therefore, our specimen is referred to C. bifidum only with reservation. Cymatoceras pacificum Matsumoto and Muramoto, 1983 (92, pl. 18: 1, pl. 19: 1, pl. 20: 1, 2, text−fig. 3) from the lower Upper Campanian of Hokkaido, is a similar species concerning shell parameters. However, its style of ribbing is more regular and the ribs are finer. Furthermore, the whorl section is slightly more depressed in this species and the umbilicus is very narrow to occluded. Cymatoceras kayeanus ( Blanford, 1861) , a common species in the Lower CampanianofMadagascar( Kabamba1983),issimilarinrespect to the form of the ventral sinus. It, however, has more densely spaced, finer ribs, and is slightly more evolute and clearly compressed.

Occurrence.—OurspecimenisfromtheKrasnoyarkaFormation,Member1,locality0.6intheNaibaRivervalley( Pachydiscus (P.) aff. egertoni Zone, lower Upper Campanian; see Figs.2,3).Theholotypeof C. bifidum comesfromtheLower CampanianofSakhalin,NaibaRiver,neartheRiverSeimestuary (left tributary of Naiba) ( Shimansky 1975).

Figures

Fig. 6. A. Cymatoceras pseudoatlas (Yabe and Shimizu, 1924b). PIW2002IV−2 from the Krasnoyarka Formation, Member 2, locality 12 (Canadoceras multicostatum Zone, Upper Campanian), Naiba River valley, Sakhalin; apertural view [see also Fig. 5B]. B. Cymatoceras cf. bifidum Shimansky, 1975. PIW2002IV−4 from the Krasnoyarka Formation, Member 1 (Pachydiscus (P.) aff. egertoni Zone, Upper Campanian), locality 0.6, Naiba River valley, Sakhalin; in lateral (B1, B3) and ventral (B2) views. Scale bar in the middle is for A, B1, and B3. Scale bars 2 cm.

Fig. 3. Synoptic ammonite and inoceramid bivalve biostratigraphy of the areas discussed in the text (compiled after Zonova et al. 1993; Toshimitsu et al. 1995; Shigeta et al. 1999; Yazykova 2002; Yazykova et al. 2002). Abbreviation: Sant., Santorians.

Fig. 2. Stratigraphic log of the Upper Santonian to Lower Maastrichtian of Sakhalin; numbers of lithostratigraphic members, nautiloid occurrences and biostratigraphic events (first and last appearance data, FAD/LAD) are indicated (modified after Yazykova 2002: fig. 2).