Cymatoceras pseudoatlas ( Yabe and Shimizu, 1924b )

Wilmsen, Markus & Yazykova, Elena A., 2003, Campanian (Late Cretaceous) nautiloids from Sakhalin, Far East Russia, Acta Palaeontologica Polonica 48 (3), pp. 481-490 : 485-487

Cymatoceras pseudoatlas ( Yabe and Shimizu, 1924b)

Figs. 5 , 6A .

Nautilus (Cymatoceras) pseudo−atlas YabeandShimizu,1924b:42 ,pl.5.

Cymatoceras pseudoatlas Yabe and Shimizu ; Kummel 1956: 426.

Cymatoceras sp. ; Shimansky 1975: 113, pl. 24: 1.

Cymatoceras pseudoatlas (Yabeand Shimizu) ; Matsumoto and Muramoto 1983: 90, pl. 17: 1.

Material.—TwolargespecimensfromSakhalin,slightlydeformedbutwithshellpreserved (no.PIW 2002IV−2 fromthe Naiba River valley, locality 12; no. PIW 2002IV−3 from the Manuj River valley, locality 431).

Description.—Very involute, nautiliconic nautiloid with slightly depressed whorl section showing a broadly rounded venter ( Fig. 6A ). Even though both specimens are slightly deformed, standard measurements are as follows:

Maximum breadth is on the lower flanks close to the umbilical shoulder. The umbilical shoulder is rounded and the umbilicalwallsteep to overhanging.The umbilicusisnarrow to occluded. The shell shows conspicuous, rather fine, flattened ribs (mean width 2–4 mm) separated by narrow, 1–2mmwidegrooves.Theribsoriginateattheumbilicalwall, are slightly prorsiradiate on the lowerflank, curve backwards nearthemiddlepartandcrosstheexternalpartwitharounded, shallow ventral sinus ( Fig. 5A 2, B 2 ). They bifurcate preferentially on the lowerto middle partofthe flanks( Fig.5B 1),and may occasionally also divide on the ventrolateral part or the venter. Due to shell preservation, the suture cannot be seen; the position of the siphuncle is not visible either.

Remarks.—In terms of dimensions and shell proportions as well as ribbing pattern, these specimens are very close to C. pseudoatlas as figured by Yabe and Shimizu (1924b: pl. 5) andMatsumotoandMuramoto(1983:pl.17:1).Thisspecies waserected byYabe andShimizu(1924b) inordertodistinguish more irregularly ribbed representatives from Cymatoceras atlas ( Whiteaves, 1876) , which appears to be more finely ribbed and is stronger depressed than C. pseudoatlas (observations by M.W. on specimens of C. atlas in the collectionsoftheNaturalHistoryMuseum,London;Wb/Whof a specimen figured by Sharpe 1853: pl. 4: 1, is 1.32). However,thesespeciesappearcloselyrelatedand C. pseudoatlas may have been derived from C. atlas which is known from the Cenomanian to Santonian of England ( Wright and Wright1951); Kennedy(2002) indicatesonlyaCenomanian distribution for C. atlas . A Campanian specimen from the Naiba River valley described and figured by Shimansky (1975:pl.24:1)as Cymatoceras sp. isherealsoplacedin C. pseudoatlas .Itshows thesame ornament and shellformand may be of the same age as the specimen described here. Cymatoceras bifidum Shimansky, 1975 , from the Campanian of Sakhalin, has broad, flat ribs on the flanks which bifurcate near the ventrolateral shoulder (see also below). Cymatoceras carlottensis ( Whiteaves, 1900: 269) from the (?Lower) Cretaceous “Lower Shales” of Skidegate Inlet, Maple Island (British Columbia), is a generally similar speciesbutappearstobesomewhatcoarserribbedandischaracterizedbyaventraldepressionorgroveandaslightlydeeper sinus. Cymatoceras loricatum ( Schlüter,1876) fromtheUpper Campanian of northern Germany shows imbricated ribs with intervening fine lirae.

Occurrence.— Cymatoceras pseudoatlas is known from the Upper Santonian of Japan ( Inoceramus amakusensis Zone ; Matsumoto and Muramoto 1983) and from the lower Upper Campanian of Sakhalin ( Shimizu 1929). Our specimens are from the Upper Campanian Krasnoyarka Formation, Member 2, from localities 431 and 12 ( Canadoceras multicostatum Zone ; see Figs. 2 , 3 ).

Figures

Fig. 5. Cymatoceras pseudoatlas (Yabe and Shimizu, 1924b). A. PIW2002IV−3 from the Krasnoyarka Formation, Member 2, locality 431 (Canadoceras multicostatum Zone,UpperCampanian),ManujRivervalley,Sakhalin,inlateral(A1)andventral(A2)views. B.PIW2002IV−2fromtheKrasnoyarkaFormation,Member2,locality12(Canadoceras multicostatum Zone,UpperCampanian),NaibaRivervalley,Sakhalin,inlateral(B1)andventral(B2)views. Scale bar 2 cm.

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. 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).

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.