440
Journal of Paleontology 91(3):434–443
Table 1. Measurements (in mm) of Costinuculana magharensis n. gen. n. sp. from the Middle Bathonian of Gebel Maghara, Sinai. Abbreviations given in Figure 3.
Specimen
BSPG2014 V 1 (holotype) BSPG2014 VI3a BSPG2014 VI4a BSPG2014 VI4c BSPG2014 VI2 BSPG2014 VI3b Range Mean
L 16
10.5–17.3 14.13
17.3 10.5 15.5 14.2 11.3
H
7.8 7
5.2 6.5 7
5.2–7.8 6.45
5.2 I
3.1–6.1 4.48
6.1 5.2 3.1 4.7 4.3 3.5
D
4.1–6.5 5.5
6.4 6.5 4.3 6.2 5.5 4.1
H/L 0.49
I/L
0.40–0.49 0.46
0.40 0.49 0.42 0.49 0.46
0.29–0.38 0.31
0.38 0.30 0.29 0.30 0.30 0.31
D/L 0.4
0.36–0.41 0.39
0.38 0.41 0.40 0.39 0.36
N. ribs 12
13 8
8–15 11
15 10 8
Figure 6. Hinge of the right valve of Costinuculana magharensis n. gen. n. sp. from the middle to upper Bathonian Kehailia Formation of Gebel Maghara; BSPG2014V I5c.
the sediment-water interface. Because the posterior end of the shell presumably remained above the sediment-water interface, the thick posterior-most oblique ribs probably had another function. Most likely, they strengthened the thin and elongated rostrum against durophagous predators such as ammonites. The inflated valves with their broadly convex anterior margin in addition helped the bivalve to maintain a stable position within the soft substrate and to keep the posterior end above the sediment-water interface.
Paleosynecological remarks
Based on the cluster analysis of Abdelhady and Fürsich (2014, p. 179, fig. 3), Costinuculana magharensis n. gen. n. sp. is a member of their Nicaniella (N.) pisiformis association. This association is found mostly in fine-grained sediments (mainly marly silt and marl) of the middle to upper Bathonian Kehailia Formation of Gebel Maghara. The dominant organisms of this association are bivalves (75.4%), whereas gastropods, brachio- pods, corals, and echinoids are subordinate. According to Abdelhady and Fürsich (2014), the trophic nucleus consists of 12 species and is dominated by small, shallow-burrowing Nicaniella (N.) pisiformis J. de C. Sowerby (31.5%; Fig. 7.3), Nuculoma variabilis (J. de C. Sowerby) (10.9%), and Ryderia decorata (Douvillé) (8.3%). Shallow-infaunal organisms represent 67.8% of the total benthic fauna followed by epifaunal ones 28.2% (Fig. 7.1). With respect to the feeding habits, suspension-feeders dominate (54.2%), followed by deposit- feeders (28.1%), omnivores (12.7%), and microcarnivores (4.9%).
The fine-grained sediment and the abundance of shallow- infaunal organisms and deep-burrowing bivalves (altogether
67.8%) indicate that the Nicaniella (N.) pisiformis association occupied a soft substrate. Due to the activities of mobile taxa (40.7%; Abdelhady and Fürsich, 2014), the softness of substrate probably increased and thus became less attractive for epifaunal organisms. The Nicaniella (N.) pisiformis association reflects low-energy conditions, in which physical breakage was insignificant. All specimens of Costinuculana n. gen. except one are articulated, which implies a relatively low degree of reworking. According to the high percentage of suspension- feeders (54.2%), water energy must have been sufficient enough to keep nutrients in suspension. Moreover, the high diversity both in terms of evenness and species richness and the presence of stenohaline groups, such as ammonites and echinoids, indicate normal-marine, fully oxygenated low-stress conditions. For more details about the paleo-community see Abdelhady (2014, p. 170) and Abdelhady and Fürsich (2014, p. 179–184).
Conclusions
(1) The bivalve Costinuculana magharensis n. gen. n. sp. (Nuculanida, Nuculanidae) is described from the middle to upper Bathonian Kehailia Formation of Gebel Maghara, North Sinai, Egypt.
(2) The main morphological feature that distinguishes Costi- nuculana n. gen. from other nuculanid genera is the irregular opisthocline ribs on the rostrum, which cover an area ~45% of the total valve length from the posterior end. Other diagnostic features are the morphology of the escutcheon and lunule.
(3) Costinuculana magharensis n. gen. n. sp. differs from related species by its small size and its well-developed irregular opisthocline ribs on the rostrum, moderately inflated valves, narrow, lanceolate, and sharply demarcated lunule, and wide and deep escutcheon with well-developed riblets.
(4) The conspicuous oblique ribs in C. magharensis n. sp. probably helped the bivalve to maintain a stable position in the soft substrate. The oblique and bifurcated ribs increased the friction between the valves and sediment. In addition, these ribs probably reinforced the thin, elongated rostrum against durophagous predators (e.g., ammonites).
(5) Paleosynecologically, the Nicaniella (N.) pisiformis asso- ciation and the fine-grained substrate reflect low-energy conditions. The species diversity and evenness of the association indicate a stable environment suitable for the colonization by several guilds.
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