438
Journal of Paleontology 91(3):434–443
Holotype.—Articulated specimen no. BSPG 2014V 1 (Fig. 5.13–5.17) from the middle Bathonian (Middle Jurassic) of Gebel Maghara, Egypt.
Paratypes.—Eight articulated specimens no BSPG 2014V I2, I3a, b, I4a, b, I5a, b from the middle Bathonian of Gebel Maghara, and one right valve no BSPG 2014V I5c.
Type locality.—Gebel Maghara, North Sinai, Egypt. Diagnosis.—See genus.
Description.—Shells small (L = 10.5–17.3mm; Table 1), elongated-rostrate in outline, strongly inequilateral, equivalved, moderately inflated (I/L = 29–38%), and posteriorly elongated with ribbed rostrum (Fig. 4). Antero-dorsal margin short, strongly convex, higher than postero-dorsal one, and meeting anterior margin in strongly rounded curve. Postero-dorsal margin long, slightly concave. Anterior margin strongly convex, meeting ventral margin in rounded angle. Posterior margin rostrate, its end broken off. Ventral margin broad, irregular, moderately convex toward anterior margin, slightly concave posteriorly, and forming rostrum with postero-dorsal margin. Umbones broad and strongly convex. Beaks small, strongly incurved posteriorly and situated, on average, 39% of total shell length from anterior end (Table 1). Escutcheon wide, moderately deep, ornamented with fine riblets, and delimited by blunt ridge; running from umbo to postero-dorsal margin (Figs. 4.5, 5.16, 5.17). Lunule narrow, lanceolate, deep, and demarcated (Fig. 5.12, 5.19). Hinge with numerous chevron- shaped teeth, points directed towards umbones (Figs. 5.21, 6). Anterior hinge carrying slightly more teeth (14) than posterior hinge (≥11). Ornamentation consisting of numerous well- developed commarginal ribs on anterior and central parts of valve, but irregular and opisthocline ribs along rostrum (9–12), separated by wide and deep interspaces (Fig. 4.1–4.9). Commarginal ribs well developed ventrally (Fig. 5.18), asym- metrical in cross-section with slightly convex, broad ventral sides and steeply inclined and slightly concave dorsal sides (Fig. 7.2). Oblique ribs variable in shape and size, straight to folded, bifurcated ventrally and occasionally postero-dorsally (Figs. 4.3, 4.4, 4.6–4.8, 5.7–5.9, 5.13, 5.14), covering an area ~45% of total valve length from posterior end. Close to ventral margin, these ribs are thicker, bifurcating, and are asymmetrical in cross-section with a steeply concave side anteriorly and slightly convex side in opposite direction (Fig. 7.2).
Etymology.—After the type locality Gebel Maghara, Sinai Peninsula, Egypt.
Remarks.—See genus.
Mode of life and functional morphology of Costinuculana n. gen.
Morphologically, Costinuculana n. gen. is narrowly elongated, rostrate, has a broadly convex anterior margin, and is moder- ately inflated with strong, oblique ribs along the rostrum. The long narrow valve (like in Ryderia) indicates a rapid burrower (Stanley, 1970). Recent nuculanids are shallow infaunal bivalves with inhalant and exhalant siphons, which usually burrow with their long axis vertical or steeply inclined (Cox, 1960). The posterior end is maintained above the sediment-water interface and the posterior siphons are never covered with sediment. They are deposit-feeders, feeding from the sediment surface by palp proboscides (Cox, 1960). These palp proboscides extend immediately ventral of the siphons and collect food from the surface of the substrate (Yonge, 1939, p. 86). With respect to ribbing style, most ribs in bivalves are com-
marginal (“concentric”) or radial (longitudinal) (Checa and Jiménez-Jiménez, 2003). In a third, less-common ribbing pattern, the ribs are oblique (Seilacher, 1972) or discordant (Stanley, 1969), whereby ribs migrate along the margin with growth. According to Stanley (1969, 1970) and Seilacher (1972, 1973), one of the functions of oblique ribs in bivalves is to facilitate burrowing in soft sediments. For a burrowing-related function, ribs show a gentle slope in the burrowing direction and a steep, in some cases inverted, slope in the opposite direction (frictional asymmetry; Seilacher, 1972). For some other cases, Seilacher (1972) assumed that the oblique ribs were non-functional or had functions unrelated to burrowing. Later, Stanley (1981) and Savazzi (1982, 1983) showed experimentally that the radial ribs of several cardiids and of one arcid act to reduce the number of burrowing sequences required to complete burrowing. Costinuculana magharensis n. gen. n. sp. has two kinds of
ribs, commarginal and oblique ribs. The commarginal ribs are asymmetrical in cross-section, with a wide and slightly convex side in the burrowing direction, and a steeply inclined and slightly concave side in the opposite direction (Fig. 7.2). In contrast, the oblique ribs are thick and asymmetrical in cross- section close to the ventral margin, with a steeply concave side in the burrowing direction and a slightly convex side in the opposite direction (Fig. 7.2). At the beginning of burrowing, when the bivalve erected itself, the asymmetrical commarginal ribs facilitated the burrowing process (burrowing-related func- tion). Once most of the bivalve had buried itself, the posterior oblique ribs provided stability for the organism in the soft sub- strate when the ribs were nearly perpendicular to the direction of burrowing (Fig. 7.2). In this case, the concave sides of the oblique ribs caught the sediment when the shell rotated forward during the burrowing process and kept the posterior end above
Figure 5. Costinuculana magharensis n. gen. n. sp. from the middle-upper Bathonian Kehailia Formation of Gebel Maghara, Egypt. All specimens are in shell preservation and articulated except Figure 5.21. In all specimens the tip of the rostrum is broken off: (1) right valve view; (2) dorsal view; BSPG2014V I4a; (3) left valve view showing thick irregular ribs with wide and deep interspaces; (4) right valve view with well-developed commarginal ribs; BSPG2014V I3b; (5) left valve view showing thick irregular ribs with sharp crests; (6) dorsal view; (7) right valve view with bifurcated ribs close to the ventral margin; BSPG2014V I4b; (8) right valve view; (9) close-up showing an inverted V-shaped rib close to the ventral margin; BSPG2014V I4c; (10) left valve view showing slightly folded ribs posteriorly and bifurcated ventrally; (11) right valve view; (12) dorsal view showing the lanceolate and demarcated lunule; BSPG2014V I3a; (13) left valve view, holotype; (14) close-up of Figure 5.13 showing the irregular and bifurcated ribs close to the ventral margin, (15) right valve view with serpulid; (16) dorsal view showing the moderately inflated valves, (17) close-up of Figure 5.16 showing wide and ornamented escutcheon, delimited by blunt ridges; BSPG2014V 1; (18) right valve view showing thick and well-developed commarginal ribs close to the ventral margin; (19) dorsal view; (20) left valve view; BSPG2014V I2: (21) right valve view showing numerous chevron-shaped teeth; BSPG2014V I5c. Scale bars = 2mm.
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