Pholadomya gigantea (J. de C. Sowerby, 1836 )

Lazo, Darío G., 2007, The bivalve Pholadomya gigantea in the Early Cretaceous of Argentina: Taxonomy, taphonomy, and paleogeographic implications, Acta Palaeontologica Polonica 52 (2), pp. 375-390 : 378-383

Pholadomya gigantea (J. de C. Sowerby, 1836)

Figs. 4A–E, 6A–E.

1836 Pholas giganteus sp. nov.; J. de C. Sowerby 1836: 338, pl. 14: 1.

1840 Pholadomya elongata sp. nov.; Münster in Goldfuss 1840: 270, pl. 157: 3.

1842 Pholadomya elongata Münster in Goldfuss 1840; Agassiz 1842: 57–58, pl. 1: 16, 17, pl. 2 (2): 1–6.

1842 Pholadomya favrina sp. nov.; Agassiz 1842: 59, pl. 2 (1): 1, 2.

1842 Pholadomya scheuchzeri sp. nov.; Agassiz 1842: 58–59, pl. 2(1): 3–7, pl. 2 (2): 7.

1844 Pholadomya elongata Münster in Goldfuss 1840; d’Orbigny 1844: 350–351, pl. 362: 1–3.

1852 Pholadomya favrina Agassiz, 1842 ; Pictet and Roux 1852: 403– 404, pl. 29: 1a, b.

1864 Pholadomya elongata Münster in Goldfuss 1840; Pictet and Campiche 1864: 74–77, pl. 104: 1–4.

1884 Pholadomya alternans Roemer, 1841 ; Weerth 1884: 34, pl. 8: 1; pl. 9: 11 [?non Roemer 1841: 76].

1884 Pholadomya cf. P. gigantea (J. de C. Sowerby, 1836) ; Weerth 1884: 34–35, pl. 8: 2a, b.

1895 Pholadomya elongata Münster in Goldfuss 1840; Maas 1895: 279–280, pl. 9: 1, 2.

1897 Pholadomya elongata Münster in Goldfuss 1840; Karakasch 1897: 81–82, pl. 2: 9a, b.

1900 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Burckhardt 1900: 23, pl. 23: 5, 6.

1900 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Müller 1900: 44, pl. 21: 1.

1903 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Burckhardt 1903: 76, pl. 15: 1, 2.

1909 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Woods 1909: 246–249, pl. 40: 14, pl. 41: 1.

1924 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Gillet 1924: 145–146, text−fig. 90.

1928 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Lees 1928: 639.

1931 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Weaver 1931: 313–315, pl. 36: 209–211.

1940 Pholadomya cf. P. gigantea (J. de C. Sowerby, 1836) ; Imlay 1940: 155, pl. 18: 5.

1948 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Tavani 1948: 66, pl. 19 (10): 12.

1956 Pholadomya cf. P. gigantea (J. de C. Sowerby, 1836) ; Alencaster de Cserna 1956: 20–21, pl. 3: 9.

?1959 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Corvalán Díaz 1959: 35–36, pl. 2: 7.

1960 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Muromtsieva and Janin 1960: 221, pl. 28: 5–7.

1961 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Prozorovskii et al. 1961: 148–149, pl. 17: 2.

1967 Pholadomya cf. P. gigantea (J. de C. Sowerby, 1836) ; Bramer 1967: 106, pl. 7: 1, 2, 4, 6, 7, 8.

1974 Pholadomya (Pholadomya) gigantea (J. de C. Sowerby, 1836) ; Dimitrova 1974: 116, pl. 56: 1; pl. 58: 1, 2; pl. 59: 1.

1974 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Oekentorp and Siegfried 1974: 162, pl. 18: 2.

1985 Pholadomya aff. P. gigantea (J. de C. Sowerby, 1836) ; Etayo−Serna 1985: 21–22, pl. 1: 24.

1988 Pholadomya gigantea (J. de C. Sowerby, 1836) ; Dhondt and Dieni 1988: 59–60, pl. 13: 20, text−fig. 25.

2001 Pholadomya aff. P. gigantea (J. de C. Sowerby, 1836) ; Aguirre 2001, pl. 5: 6.

Emended diagnosis.— Pholadomyidae without anterior gape. Shell less than 2 mm−thick, possibly three−layered and aragonitic, outer and middle layers forming radial ribs, inner layer not undulating. Ornamented with commarginal growth lines and 17 to 44 radial ribs evenly distributed on each valve. Rather constant number of radial ribs on a single specimen from juvenile to adult stage. Ribs meet ventrally in opposite position giving a crenulated appearance to ventral margin.

Description.—Shell equivalve, individual valves very inequilateral, posteriorly elongated, gaping at the posterior end (siphonal gape), without anterior or pedal gape. Anterior margin rounded in lateral aspect, but rounded to angular in dorsal aspect. Ventral margin gently convex. Posterior margin short rounded to truncate. Dorsal margin straight to gently concave. Maximum height around umbo. Maximum width near the middle of the height. Beaks without point of contact. Valves not overlapping. Shell less than 2 mm−thick, possibly three−layered, outer and middle layers forming radial ribs, inner layer not undulating.

Ornamented with commarginal growth lines and coarse radial ribs. Twenty−six radial ribs on average, but highly variable in number between specimens, from 17 to 44. Rather constant number of radial ribs on a single specimen from juvenile to adult stage. Radial ribs straight to slightly curved, serrate near the posterior end; sharp and asymmetric in cross−section, dorsal slope steep and ventral slope gentle, especially near the shell margins. Medial and dorsal shell sectors ornamented with flatter radial ribs, most anterior and dorsal parts without radial ribs. Radial ribs not impressed on the internal mould except possibly towards the shell margin. Ribs meet ventrally in opposite position giving a crenulated appearance to ventral margin. Interspaces concave to widely concave, increasing in width toward the shell margin, wider than radial ribs.

Pallial line anteriorly punctuated and posteriorly continuous. Large pallial sinus. Posterior adductor muscle scar rounded and large. Other internal characters described by Runnegar (1974: text−fig. 6) are not preserved.

Occurrence.— Pholadomya gigantea ranges from the Tithonian?–Valanginian up to the Late Aptian ( Corvalán Díaz 1959; Dhondt and Dieni 1988). It has been recorded in Argentina, Bulgaria, Caucasus, Chile, Colombia, Crimea, England, Ethiopia, France, Germany, Mexico, Oman, Sardinia, Spain, Switzerland, Tanzania, Tunisia, and Turkmenistan (see Appendix 1). In Argentina this species is recorded throughout the Agrio Formation from the Pseudofavrella angulatiformis to the Paraspiticeras groeberi ammonite zones, Late Valanginian up to Early Barremian ( Fig. 5 ). Previous records in the Quintuco Formation are subject to some doubt and are going to be revised in future field works.

Discussion and affinities.—A large number of Early Cretaceous species of Pholadomya have been published from different parts of the world, especially from Europe, and most of these studies are somewhat outdated and type materials need revision, for instance d’Orbigny’s (1844–1847) and Pictet and Campiche’s (1864–1867) collections. However, due to its strong variability in the number of radial ribs many species of Pholadomya have been placed later in synonymy by a number of authors (see Woods 1909; Dhondt and Dieni 1988). Most of these species are included in the present synonymy list. To avoid spurious and extensive discussion of taxa, the author decided to compare P. gigantea only to local and widely distributed species of Pholadomya of Early Cretaceous age.

Pholadomya agrioensis Weaver, 1931 (Weaver 1931: 316–317, pl. 36: 205, 206) is endemic to the Neuquén Basin and is recorded from the upper part of the Quintuco Formation in the Neuquén Basin, ranging from Berriasian to Middle Valanginian ( Fig. 2 ). It differs from P. gigantea by reaching a smaller size (L <60 mm) and having fewer and thicker radial ribs (10 to 12) bearing small pustules, a blunt anterior margin and a wide umbonal−ventral sulcus separating the anterior portion of the valve from the rest of the radially ribbed flank (see Figs. 6F–H, 7 ). In addition, radial ribs meet ventrally in a concordant position. Pholadomya agrioensis is closely related to P. gigantea , even though a number of differences have been indicated. Their stratigraphic ranges do not overlap (see Fig. 2 ). Further differences in functional morphology between both species are discussed below.

Pholadomya agrioensis is similar to P. dominicalis Sharpe, 1856 and P. moeschii (Weerth, 1884) . These species form a homogenous plexus, although a thorough revision of type specimens is needed. Pholadomya dominicalis Sharpe, 1856 ( Sharpe 1856: 194–195, pl. 22: 6) is recorded in the Sundays River area in the Algoa Basin, South Africa (Uitenhage Group) and is probably Early Cretacoues in age (see Kitchin 1908: 33). Pholadomya moeschii Weerth, 1884 (Weerth 1884: 35, pl. 8: 4) is recorded in the Early Cretaceous of Germany and the description was based on a small−sized specimen. This proposed plexus had a somewhat different life habit from that of P. gigantea (see below).

Pholadomya picteti Mayer−Eymar, 1893 ( Mayer−Eymar 1893: 263–264, pl. 2: 1; Tavani 1948: 146–147, pl. 10: 9) from the Early Cretaceous of Somaliland is very different from P. gigantea . It has a subtrigonal outline that is not posteriorly elongated and it lacks prominent radial ribbing. This species has been recorded also in the Berriasian of Tunisia, Barremian of Algeria and Aptian of Colombia and Spain ( Gillet 1924: 147; Dietrich 1938: pl. 22: 4; Glaçon and Mongin 1951: 435, pl. 15: 11; Mongin 1979: 137; Etayo−Serna 1985: 21).

Pholadomya sanctaecrucis Pictet and Campiche, 1864 (Pictet and Campiche 1864: 82–83, pl. 105: 6, 7) and Pholadomya valangiensis Pictet and Campiche, 1864 (Pictet and Campiche 1864: 83–84, pl. 106: 1–3) from the Valanginian of Switzerland and France have an oval shape, a smaller siphonal gape, and they are ornamented only with commarginal lines. Both species are morphologically undistinguishable (personal observation, Museé Cantonal de Geologie, Lausanne) and must be placed in synonymy and probably removed to the genus Homomya Agassiz, 1843 . Pholadomya sanctaecrucis was also recorded in the Quintuco Formation in Neuquén and in the Barril Viejo Shale in Coahuilla, Mexico (Weaver 1931: 317– 318, pl. 36: 202, 203; Imlay 1940: 155, pl. 12: 3–5). Pholadomya valangiensis has been recorded also in the Valanginian of Colombia ( Etayo−Serna 1985: 22).

Pholadomya speetonensis Woods, 1909 (Woods 1909: 248–249, pl. 41: 4) from the Speeton Clay (probably from the C Beds, Hauterivian) of England has a suboval to rounded outline and is less elongated (L/H approximately 1.36) than P. gigantea . Pholadomya aff. P. speetonensis was also recorded in the Late Valanginian of Colombia ( Etayo−Serna 1985: 22).

Figures

Fig. 5. Stratigraphic log of the Agrio Formation in Agua de la Mula locality (Neuquén Basin, Argentina) showing the occurrence of Pholadomya gigantea (J. de C. Sowerby, 1836) and CPBA number. Ammonite zonation and age from Aguirre−Urreta and Rawson (1997, 2003).

Fig. 2. Schematic correlation chart of the Mendoza Group between Neuquén and Mendoza Provinces, Early Cretaceous of the Neuquén Basin, Argentina, showing ranges of Pholadomya species. Modified from Uliana et al. (1977); stages from Aguirre−Urreta and Rawson (1997, 2003); ranges are based on Weaver (1931), Damborenea et al. (1979), and author’s observations. Abbreviations: Bar., Barremian; Kim., Kimmeridgian.

Fig. 7. Main differences between Pholadomya gigantea (Sowerby, 1836) (A) and Pholadomya agrioensis Weaver, 1931 (B). Schemes in ventral view.