Pleurophopsis unioides Van Winkle, 1919

Kiel, Steffen, 2007, Status of the enigmatic fossil vesicomyid bivalve Pleurophopsis, Acta Palaeontologica Polonica 52 (3), pp. 639-642 : 639-641

Pleurophopsis unioides Van Winkle, 1919

Fig. 1 .

1912 Unio sp. ; Maury 1912: 50, pl. 8: 18, 19.

1919 Pleurophopsis unioides sp. nov.; Van Winkle 1919: 24, pl. 3: 12.

1969 Pleurophopsis unioides Palmer, 1919 ; Keen 1969: N664, fig. E138−6.

2005 Pleurophopsis unioides Van Winkle ; Gill et al. 2005: 205, fig. 7A, B.

Material.—The description below is based on (i) the neotype PRI 28452, (ii) a rubber cast of the neotype, (iii) Van Winkle’s illustration, and (iv) own observations on subsequently collected material (see Gill et al. 2005). Because only the lower portion of the hinge is preserved, characters like nymph and nymphal ridge, escutcheon, and lunule can not be evaluated.

Description.—Shell elongate elliptical, beak small, pointed and slightly prosogyrate, positioned in anterior quarter of shell; anterior margin evenly rounded, ventral margin straight or slightly convex, posterior margin rounded but more pointed than anterior margin, posterodorsal margin almost straight to slightly convex, only slightly inclined ventrally; after slight angulation at about three quarters of total shell length, shell margin slopes gently toward its rounded posterior end. Anterior adductor scar deeply impressed, almost straight on posterior side where bordered by strong vertical ridge; rounded on anterior side. Anterior pedal retractor scar deep, triangular, located directly above strong ridge bordering anterior adductor scar; posterior muscle scars not observed. Pallial line impressed on anterior side of shell, fading away on posterior side of shell; presence or absence of pallial sinus unknown. Interior of shell shows fine radial striations between pallial line and ventral shell margin.

Only lower portion of hinge dentition preserved. Right valve: cardinal tooth 1 strong, triangular in ventral view; cardinal 3b elongate at base, 3a unknown; socket for cardinal 2b of left valve rectangular and deep. Left valve: cardinal 2a thin; cardinal 2b strong, flat on its ventral side. Ligament, nymph, potential escutcheon, and lunule not discernable due to poor preservation. Posterodorsal margin shows some variation in its slope.

Comparisons.—Whereas Van Winkle’s (1919) specimen shows a long, inclined and almost evenly convex posterodorsal margin that reaches close to the posterior end without angulation, the angulation is quite pronounced in some better−preserved specimens. Compared to other elongate vesicomyids, P. unioides resembles Calyptogena elongata Dall, 1916 , type species of Ectenagena Woodring, 1938 , in terms of shell shape, strongly impressed anterior adductor scar and its associated ridge, and hinge dentition. The only discernable difference is the flat base of cardinal 2b in P. unioides , whereas cardinal 2b of C. elongata has a small indentation (compare fig. 10E of Boss and Turner 1980). Also the Recent Californian Ectenagena extenta Krylova and Moskalev, 1996 has a similar deeply impressed anterior muscle adductor scar, pedal retractor scar, and hinge dentition, especially when seen in ventral view ( Krylova and Moskalev 1996: fig. 1D). The Recent Japanese Calyptogena (Ectenagena) phaseoliformis Métivier, Okutani, and Ohta, 1986 also has a similar hinge dentition, only its cardinal 2a is more strongly developed than in P. unioides ( Métivier et al. 1986) . Another Japanese species with similar hinge and general shell shape is C. (E.) kaikoi Okutani and Métivier, 1986 . Horikoshi (1989) documented some ontogenetic variation in its hinge teeth configuration, but these do not differ significantly from the hinge dentition preserved in P. unioides . Horikoshi (1989) also documented an ontogenetic change in the shape of the ventral shell margin of C. (E.) kaikoi , from convex or straight in small specimens, to slightly concave in a larger specimen.

The general shape and hinge dentition of the Miocene Japanese Adulomya uchimuraensis Kuroda, 1931 (see Kanno et al. 1998) is very similar to that of P. unioides . Adulomya uchimuraensis is more elongate than P. unioides , has a lower beak, and also the ridge posterior to the anterior muscle−attachment scar is not as strongly developed as in P. unioides . The fossil Pleurophopsis peruviana Olsson, 1931 from Eocene or Oligocene seep carbonates in northern Peru and southern Ecuador has almost no teeth on its hinge and its systematic position is doubtful ( Kiel and Peckmann 2007).

Figures

Fig. 1. The vesicomyid bivalve Pleurophopsis unioides Van Winkle, 1919 from middle Miocene strata of Trinidad, Neotype (PRI 28452). A. Lateral view of left valve, note that the number written on specimen is the former Cornell University number. B. Dorsal view of hinge area. C–H. Silicone rubber casts of neotype illustrating hinge features. C. Hinge and anterior adductor scar of left valve in lateral view. D. Close−up of C. E. Ventral view of left valve hinge. F. Hinge and anterior adductor scar of right valve in lateral view. G. Close−up of F, slightly tilted ventro−lateral view. H. Ventral view of right valve hinge. Abbreviations: aas, anterior adductor scar; aprs, anterior pedal retractor scar; card., cardinal tooth; LV, left valve, RV, right valve.