Hybolophus nelsoni ( Grzybowski, 1899 )

Devries, Thomas J., 2016, Fossil Cenozoic crassatelline bivalves from Peru: New species and generic insights, Acta Palaeontologica Polonica 61 (3), pp. 661-688 : 678-680

Hybolophus nelsoni ( Grzybowski, 1899)

Fig. 10I .

1870 Crassatella gibbosa Sowerby ; Nelson 1870: 203, pl. 7: 9; not Crassatella gibbosa ( Sowerby, 1832) .

1899 Venus nelsoni sp. nov.; Grzybowski 1899: 639, pl. 19: 2, 2a.

1922 Crassatellites (Scambula) nelsoni (Grzybowski) ; Spieker 1922: 128, pl. 7: 8.

1932 Eucrassatella (Hybolophus) nelsoni (Grzybowski) ; Olsson 1932: 87, figs. 1, 4.

1922 Crassatellites charanensis sp. nov.; Woods 1922: 112, pl. 19: 6; pl. 20: 1, 2, 3; not Eucrassatella (Hybolophus) charanensis (Woods, 1922) ; DeVries 1986: 429, pl. 13: 5, 8, 10 (= Hybolophus gibbosus [ Sowerby, 1832]).

1925 Crassatellites pizzaroi sp. nov.; Hannah and Israelsky 1925: 46, pl. 7: 1.

1932 Eucrassatella (Hybolophus) gibbosa tucilla subsp. nov.; Olsson 1932: 88, pl. 6: 6.

Type material: Grzybowski’s (1899) holotype not examined by the author. The whereabouts of every Grzybowski molluscan type from

Peru is unknown. They do not reside in the collections of his home institution, the Jagiellonian University in Kraków (Jolanta Gruza, personal communication 15 February 2016). Emended description based on illustrations of Nelson (1870) and Grzybowski (1899) and Spieker’s

(1922) specimen.

Type locality: Caleta Grau , Peru .

Type horizon: Miocene, indeterminate formation.

Material.— USNM 562398, ridge at head of Quebrada Heath, northern Peru ( Spieker 1922).

Emended diagnosis.—Shell inflated. Anteriodorsal margin straight to slightly convex. Posterior produced. Primary and secondary posterior ridges diverging by more than 10°, latter close to margin of escutcheon.

Emended description.—Shell to at least 70 mm long, ovate anteriorly, produced posteriorly, beak located two-fifths of length from anterior end,L:H ratio 1.2−1.4, T:H ratio 0.3, maximum inflation ventral to beak. Anteriodorsal margin straight to slightly convex, posteriodorsal margin concave. Anterior margin and ventral margin rounded, latter not constricted posteriorly; posterior margin weakly produced, bluntly truncate. Weakly angular primary posterior ridge diverging 15−20° from posteriodorsal margin, secondary posterior ridge obsolete or close to escutcheon. Lunule cordate, about 60% length of anteriodorsal margin. Escutcheon about 60% length of posteriodorsal margin, as wide or narrower than lunule. Beak orthogyrate to prosogyrate. Umbo flattened, opisthogyrate, with broad commarginal ribs. Remainder of exterior with irregular commarginal growth lines. Left hinge not available. Right anterior pseudocardinal tooth narrow, diverging from lunule margin, inclined 35° anteriorly. Right cardinal tooth wedge-shaped, inclined anteriorly 20°. Right posterior pseudocardinal tooth lamellar, diverging 10° posteriorly from midpoint of cardinal tooth. Inner ventral margin smooth.

Remarks.— Hybolophus nelsoni encompasses Miocene specimens from several localities in northern Peru. The oldest are attributed to the lower Miocene lower Zorritos Formation by Spieker (1922), although the specimens may have been collected from the middle Miocene Cardalitos Shale ( Grzybowski 1899; Woods 1922; Hanna and Israelsky 1925; Olsson 1932). The youngest were collected from the upper Miocene Tumbez Formation ( Olsson 1932).

Previous authors offer conflicting conclusions about the relationship between Hybolophus nelsoni and H. gibbosus . Quaternary specimens from northern Peru have closely spaced primary and secondary posterior ridges, producing an exceptionally narrow posterior truncation (i.e., the posterior nearly comes to a point), a deeply concave posteriodorsal margin, and an exceedingly opisthogyrate umbo; these are assigned to H. gibbosus . Specimens of H. nelsoni have a wider divergence between the primary and secondary posterior ridges, with the latter close to the margin of the escutcheon. The result is a broader posterior truncation, a straighter posteriodorsal margin, and a less opisthogyrate umbo.

Stratigraphic and geographic range.—Middle to upper Miocene, northern 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.