Isorropodon aff. perplexum Sturany, 1896

Kiel, Steffen & Taviani, Marco, 2018, Chemosymbiotic bivalves from the late Pliocene Stirone River hydrocarbon seep complex in northern Italy, Acta Palaeontologica Polonica 63 (3), pp. 557-568 : 564-565

Isorropodon aff. perplexum Sturany, 1896

Fig. 10 .

Material.—Two fragmentary specimens, preserving the hinge area of a left and a right valve (MGGC.22019 and MGGC.22020); from the late Pliocene of Italy, Stirone seep complex.

Description.— Prodissoconch roundish, about 220 μm wide. Beak somewhat elevated, slightly prosogyrate; surface smooth; lunular incision present. Hinge plate narrow, three teeth in each valve; in right valve, cardinal 1 thin just anterior of beak, elongate, subparallel to dorsal margin, cardinal 3a commencing slightly anterior of beak, elongate and very thin, slightly thickened anteriorly, parallel to dorsal shell margin, bends slightly ventrally at its posterior end; cardinal 3b just posterior of 3a, equally thin, very slightly oblique to dorsal shell margin; in left valve, cardinal 2a short, parallel to anterior shell margin, connected through a thin ridge with the stout, triangular cardinal 2b; 4b elongate, thin, parallel to dorsal shell margin.

Remarks. —The hinge dentition of the two available fragmentary specimens agrees well with that of extant Mediterranean Isorropodon perplexum ( Sturany, 1896) as figured by Cosel and Salas (2001) and also the prodissoconch shown here on Fig. 9B 2 is similar in size and shape to that of Isorropodon perplexum . A single left valve identified as Isorropodon aff. perplexum has been reported from late Miocene (Messinian) sediments near Torino, northern Italy ( Janssen and Krylova 2012) and seems to have a slightly shorter hinge plate than the late Pliocene specimens figured here. Two extant Isorropodon species are known from mud volcanoes in the Gulf of Cadiz in the NE Atlantic ( Oliver et al. 2011). Among them, Isorropodon megadesmus Oliver, Rodrigues, and Cunha, 2011 differs from the Pliocene Isorropodon aff. perplexum by its higher hinge plate and a curved cardinal 3a; and Isorropodon sp. indet. has a hinge similar to that from the Stirone River but its cardinal 3b appears more undulating. Also the hinge dentition of the extant Isorropodon nyeggaensis from the North Atlantic off of northern Norway ( Krylova et al. 2011) is similar to that of the Stirone specimens, although its nymph appears to be broader. At this Norwegian cold seep site, this vesicomyid (as Calyptogena sp. ) occurs with Acharax and thyasirids in ca. 700 m water depth ( Ivanov et al. 2010).

Figures

Fig. 10. The vesicomyid bivalve Isorropodon sp. from the late Pliocene of Italy, Stirone River seep deposit. A. MGGC.22019, hinge area of left valve. B. MGGC.22020, hinge area of right valve (B1), close-up of prodissoconch (B2).

Fig. 9. The lucinid bivalve Anodontia sp. from the late Pliocene of Italy, Stirone River seep deposit. A. MGGC.22017, smaller specimen, close-up of edentulous hinge area of left valve (A1), dorsal view (A2), and view of outer side of left valve (A3). B. MGGC.22018, larger specimen, left valve, exterior view.

Fig. 2. The solemyid bivalve Acharax doderleini (Mayer, 1861) from the late Pliocene of Italy, Stirone River seep deposit; MGGC.22007 view on exterior of right valve.