436
Journal of Paleontology 91(3):434–443
Figure 2. Chronostratigraphic correction chart of the Jurassic formations of Gebel Maghara, Sinai, Egypt (modified after Abdelhady and Fürsich, 2015a).
Figure 3. Measured parameters and orientation of Costinuculana magharensis n. gen. n. sp.: (1) lateral view of right valve; (2) dorsal view of articulated valves. Abbreviations: L = length, H = height, I = inflation of articulated specimen, D = distance from the anterior end to umbo, nr. ribs = number of oblique ribs along rostrum, RV = right valve, LV = left valve.
Etymology.—Combination of costa (Latin) = rib, and the genus Nuculana, referring to the oblique ribs along rostrum.
Occurrence.—The material of the new genus has been collected from the middle Bathonian ammonite Clydomphalites clydocromphalus Zone.
Remarks.—The significant taxonomic characters of Costinucu- lana n. gen. are not present in the majority of Jurassic nuculanid species that have been identified and described from different localities (e.g., Pandey and Singh, 1981; Fürsich, 1982;
Aberhan, 1994, 1998; Jaitly et al., 1995; Sha et al., 1998). The main diagnostic feature distinguishing Costinuculana from other genera of the family Nuculanidae (e.g., Nuculana Link, 1807) is the presence of well-developed oblique ribs on the posterior part of the flank, which cover an area up to ~45% of total valve length from posterior end (Fig. 4.1–4.9). The genus Ryderia Wilton, 1830 (type species: Leda renevieri Oppel, 1856, p. 215) is similar to the new genus in having an elongated- rostrate shell, a strongly rounded anterior margin, and small and opisthogyrate beaks, but differs in having a narrower and much more elongated rostrum, a narrow and smooth escutcheon, fine and closely spaced commarginal ribs, in lacking posterior oblique ribs, and in being much more compressed than the present material. The commarginal ribs of Glyptoleda Fletcher, 1945 are folded and sub-vertically deflected towards the posterior part of the flank, but the ornamentation style completely differs from that of Costinuculana. In addition, Glyptoleda differs in being much larger than the present genus (L= ~45mm as opposed to 12.35mm on average for C. magharensis n. sp.) and is of Permian origin. Chen et al. (1983) erected two genera, Xiaoshuiculana Chen in Chen et al. and Qiongzhounia Lan in Chen et al. from the Upper Triassic and Pliocene of China, respectively. The genus Xiaoshuiculana differs from Costinuculana n. gen. in having only radial ribs that cover the entire valves and in being shorter. The genus Qiongzhounia resembles Costinuculana n. gen. in having an elongated-rostrate shell and a well-developed umbonal posterior ridge, but differs also in the style of ornamentation. It is orna- mented with faint concentric ribs on the flank and with delicate, weak growth lines on the posterior slope (see Chen et al., 1983, p. 619, fig. 1). The reticulate ornamentation and short rostrum distinguishes Indoculana Kanjilal and Singh (1973) from the Callovian of the Kachchh Basin, India, from Costinuculana n. gen. Ledoides Wen and Lan in Gu et al., 1976 from the Upper Triassic of China differs in having three different kinds of ribs: concentric ribs on the umbonal area, a reticulate pattern on the anterior part, and oblique radial ribs on the posterior part. In addition, Ledoides has a narrow and shorter rostrum. The sub- genus (Gonionucula) Hautmann and Aghababalou in Hautmann et al. (2011, p. 22–24, figs. 2, 3) from the Norian–Rhaetian (Late Triassic) of Iran differs in having chevron-like ribs on the central part of the flank and an additional set of steeply opisthocline to nearly vertical ribs arranged in a narrowstripe on the posterior part of the flank. The latter authors stated that their new subgenus differs fromthe genus AcilaAdams andAdams, 1858 in having an additional set of opisthocline ribs on the posterior part. Apart from its ornamentation, the genus Acila differs from Costinuculana n. gen. in being less elongated (not rostrate), larger, and in having a wide and strongly rounded anterior margin. Pandey and Singh (1981, p. 1296, text-fig. 1) erected a new
species “Indoculana” sadharaensis from the lower Callovian of Gora Dongar, India on the basis of the presence of faint radial riblets along the posterior part of flank. Jaitly et al. (1995, p. 160) regarded the different types of surface ornament just as intraspecific variations of Nuculana (Praesaccella) calloviensis (Kanjilal and Singh, 1973). The second feature regarded by Pandey and Singh (1981) as diagnostic feature of “I.” sadharaensis, is the presence of transverse threads in the escutcheon. Jaitly et al. (1995, p. 160, pl. 2, figs. 13, 14) observed
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