Hybolophus gibbosus ( Sowerby, 1832 )

Devries, Thomas J., 2016, Fossil Cenozoic crassatelline bivalves from Peru: New species and generic insights, Acta Palaeontologica Polonica 61 (3), pp. 661-688 : 680-681

Hybolophus gibbosus ( Sowerby, 1832)

Fig. 10H, J, K .

1832 Crassatella gibbosa sp. nov.; Sowerby 1832: 56.

1843 Crassatella gibbosa Sowerby ; Reeve 1843: vol. 1, pl. 1: 1a, b. 1909 Crassatellites gibbosus (Sowerby) ; Dall 1909: 260.

1930 Crassatellites rudis sp. nov.; Li 1930: 251, pl. 3: 16.

1961 Crassatellites (Hybolophus) gibbosus (Sowerby) ; Olsson 1961: 179, pl. 25: 1a, b.

1986 Eucrassatella (Hybolophus) charanensis (Wood, 1922) : DeVries 1986: 429, pl. 13: 5, 8, 10. 1986 Eucrassatella (Hybolophus) gibbosa ( Sowerby, 1832) ; DeVries 1986: 427, pl. 13: 6, 9.

1986 Eucrassatella (Hybolophus) ovata sp. nov.; DeVries 1986: 431, pl. 13: 1, 3, 7, 11, 12.

2012 Eucrassatella gibbosa ( Sowerby, 1832) ; Coan and Valentich-Scott 2012: 398, pl. 130; not Crassatellites charanensis Wood, 1922 (= Hybolophus nelsoni Grzybowski, 1899 ).

See Coan (1984) for exhaustive synonymy.

Holotype: British Museum of Natural History , Zoology, 1953.4.15.15.

Type locality: Western coast of South America.

Type horizon: Recent.

Material.— UWBM 101880, Los Organos, northern Peru, Recent, L 55.4, H 38.4, T 13.7; UWBM 101881, Los Organos, northern Peru, Recent, L 36.4, H 24.7, T 11.1; UWBM 101882, Perlas Islands, Pacific coast of Panama, dredged 12−18 m, Recent, L 57.1, H 38.6, T 13.1; UWBM 101883, Perlas Islands, Pacific coast of Panama, dredged 12−18 m, Recent, L 46.4, H 32.4, T 11.1; OSU 37597, DV 245-2, Punta Organos Chico, Peru, late Pleistocene, L 54.2, H 42.6, T 14.9; OSU 37598, DV 245-2, L 48.9, H 32.2, T 11.6; OSU 37599, DV 241-2, near Los Organos, Peru, middle Pleistocene, L 49.8, H 38.2, T (15); OSU 37600, DV 328-3, west of Cerro El Nuro, Peru, late Pliocene, L (73), H 56.2, T 19.9; OSU 37601, DV 328-3, L (70), H (56), T 25.6; OSU 37602, DV 243-1, northeast of Cerro El Nuro, Peru, late Pliocene; OSU 37603, DV 244-2, south of El Alto, Peru, late Pliocene, L 58.0, H 43.4, T 13.7; OSU 37604, DV 244-2, L 54.8, H 38.7, T 12.0; OSU 37606, DV 244-2, H (41).

Emended diagnosis.—Closely spaced primary and secondary posterior ridge. Pointed posterior margin. Posteriodorsal margin deeply concave.

Description.—Shell to 90 mm long, ovate produced posteriorly, beak about one-third length from anterior end, L:H ratio 1.3−1.5, T:H ratio 0.2−0.25, maximum inflation ventral to beak. Anteriodorsal profile straight to slightly concave, posteriodorsal profile deeply concave. Anterior and ventral margin rounded, latter often constricted posteriorly; posterior margin produced, narrowly truncate. Angular primary posterior ridge diverging 15° from posteriodorsal margin, weaker angular secondary posterior ridge diverging only a few degrees less. Lunule cordate, escutcheon elongate, both equally wide and half the length of respective margins. Beak orthogyrate. Umbo flattened, strongly opisthogyrate, with broad, widely spaced commarginal ribs. Exterior with variably developed regular and irregular anterior commarginal ribs. Remainder of exterior with irregular commarginal growth lines. Left anterior tooth narrow, wedge-shaped, inclined anteriorly 50°, separated entirely from lunule by groove. Left posterior cardinal tooth smaller, wedge-shaped, inclined anteriorly 10−20°. Right anterior pseudocardinal tooth thick, diverging from lunule margin, inclined anteriorly 25−35°. Right cardinal tooth wedge-shaped, inclined anteriorly 20−30°. Right posterior pseudocardinal tooth lamellar, diverging 10° posteriorly from midpoint of cardinal tooth. Inner ventral margin smooth.

Remarks.—Specimens of Hybolophus gibbosus are found in northern Peru on the Mancora, Talara, and Lobitos tablazos of early, middle, and late Pleistocene age, respectively, and in the Golf Course Member of the upper Pliocene Taime Formation ( DeVries 1986, 1988). A large unfigured specimen is reported from the Pliocene Canoa Formation of southwestern Ecuador ( Pilsbry and Olsson 1941). Hybolophus aviaguensis peruviana ( Olsson, 1932) from the middle Miocene ( Dunbar et al. 1990) Montera Formation of the northern Peruvian Sechura Basin has an equally concave posteriodorsal margin and equally strongly opisthogyrate umbo, but the beak is more anteriorly located and the anterior margin consequently is more tightly circular in profile.

Stratigraphic and geographic range.—Pliocene, Canoa Formation, Manabí Province, Ecuador; Taime Formation, northern Peru. Pleistocene, Mancora, Talara, and Lobitos Tablazo, northern Peru. Recent, Baja California to Piura, Peru.

Figures

Fig. 10. Hybolophus crassatellines from Peru. A−G. Hybolophus maleficae sp. nov., middle Miocene, Pisco depositional sequence, south-central Peru. A. UWBM 101862, paratype, exterior of left valve. B. UWBM 101858, holotype, interior of left valve. C. UWBM 101860, paratype, interior of left valve. D. MUSM INV 222, paratype, exterior of left valve. E. UWBM 101864, dorsal margin of paired valves, anterior at right. F. UWBM 101859, paratype, interior of right valve. G. UWBM 101865, dorsal margin of paired valves, anterior at right. H, J, K. Hybolophus gibbosus (Sowerby, 1832), early to middle Pleistocene, tablazos, northern Peru. H. OSU 37600, exterior (H1) and interior (H2) of right valve. J. OSU 37597, exterior (J1) and interior (J2) of left valve. K. OSU 37601, dorsal margin of left valve, anterior at left. I. Hybolophus nelsoni (Grzybowski 1899), USNM 562398, late Miocene, Quebrada Heath, northern Peru; exterior of right valve. Scale bars 10 mm.