Megaxinus stironensis, Kiel & Taviani, 2018

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 : 559-562

Megaxinus stironensis sp. nov.

Figs. 4 , 5 .

1901 Megaxinus bellardianus (Mayer) ; Sacco 1901: 75–76, pl. 17: 30,

36; not 29, 32–35, 37.

Etymology: For the Stirone River.

Type material: Holotype: MGGC.22009, a shell with both valves preserved . Paratypes: articulated and single shells MGGC.22010–22013, MSF 2143 , NRM Mo184001–184003; all from the type locality and horizon .

Type locality: The Stirone River seep deposit, Northern Apennines, Italy .

Type horizon: Late Pliocene part of the Lugagnano Formation.

Material.—The type material; hundreds more seen in the field.

Dimensions.—The largest specimens including the holotype reach 45 mm in length and height, individual valves are 13 mm thick.

Diagnosis.—Average-sized, moderately inflated Megaxinus , of equal height and length, with small lunule, two incisions on anterodorsal margin, strong and narrow hinge plate, and very long anterior adductor muscle scar.

Description.—Shells of average size for the genus, almost circular, of equal height and length, moderately inflated (T/L = 0.29), with blunt, elevated, prosogyrate umbo. Posterodorsal area broad, bordered by indistinct, broad, shallow groove resulting in a slight angulation on the posterior margin, another indistinct, broad and shallow groove running from the umbo to the center of the ventral margin, where it results in a shallow indentation. Lunule very small, surrounded by two incisions on the anteroventral margin; ligament external, elongate in a sunken groove. Outer shell surface with irregular commarginal growth increments. Hinge plate strong and narrow, edentulous; nymph plate long and thin. Inner shell surface with few or many pustules that are arranged radially toward the ventral margin; in some specimens the pustules are so numerous that they give the interior a granulate appearance; pallial blood vessel scar usually distinct; anterior adductor muscle scar broad and very elongate, tapering ventrally, reaching the ventral quarter of the shell, close to pallial but detached from it for ¾ of its length; posterior adductor muscle scar indistinct, irregular ovate in outline. Pallial line entire, crenulate; area beyond pallial line crenulate close to pallial line but mostly smooth.

Remarks.—This is the most common species at the Stirone River seep deposit and it also occurs in large numbers in shell beds in the surrounding mudstone. Sacco (1901: 75– 76) assigned numerous specimens of middle Miocene to late Pliocene age to Megaxinus bellardianus ( Mayer, 1864) . These specimens show quite a diversity of shapes, some being similar and others being dissimilar to the specimens from the Stirone seep deposit. Given the rich and cryptic present-day diversity of this genus and related lucinid genera ( Glover and Taylor 1997; Taylor and Glover 2005) we prefer to apply to a more narrow species concept to the Neogene Megaxinus than Sacco’s (1901) approach of lumping most specimens into M. bellardianus , although a full revision of Neogene Megaxinus is beyond the scope of this study. Mayer (1864: 27) wrote that the type specimen of M. bellardianus is the specimen illustrated by Michelotti (1847: pl. 4: 10). That specimen is of Miocene age and the provided drawing indicates that it has a more pronounced posterodorsal ridge, and a more pointed and less prosogyrate beak than the specimens from the Stirone seep deposit concerned here. Thus due to the differences in shell shape, we consider the Stirone specimens to be distinct from M. bellardianus and introduce the new species M. stironensis for them. Among the many specimens of M. bellardianus illustrated by Sacco (1901: pl. 17: 30, 36) resemble the Stirone specimens in every aspect, and interestingly are from the late Pliocene (Piacenzian) of Vezza d’Alba, and hence of similar age as the Stirone specimen. We include those specimens from Vezza d’Alba in Megaxinus stironensis sp. nov.

The extant Megaxinus yemenensis Glover and Taylor, 1997 differs from M. stironensis by being elongate-oval rather than round, the extant M. appendiculatus , M. unguiculus , and the Neogene Italian M. rostratus ( Pecchioli, 1864) and M. transversus ( Bronn, 1831) differ from M. stironensis in having a truncate posterior margin and a more pointed anterior margin (i.e., Sacco 1901; Glover and Taylor 1997). Among the geographically close, extant species from the Gulf of Oman, M. omanensis ( Smith, 1906) differs from M. stironensis by having a higher and more angular shell and a smaller and less prosogyrate beak, whereas M. arabicus Glover and Taylor, 1997 has a lower shell with pointed anterior margin and a narrower anterior adductor muscle scar ( Glover and Taylor 1997).

Similar specimens associated with Pliocene whale-fall communities around Italy ( Dominici et al. 2009; Danise et al. 2010, 2014) have been assigned to Megaxinus incrassatus du Bois de Montpéreux, 1831 . However, there are three reasons why we find this questionable. First, the type specimen of M. incrassatus is from a middle Miocene locality on the Volyn’ -Podollya Plateau in the western Ukraine. The specimen was described as “not well preserved” but the illustration shows a very nicely preserved shell ( du Bois de Montpéreux 1831: 58, pl. 6: 1–3). The illustration is thus most likely an idealized reconstruction that should not be taken at face value. Second, specimens of Megaxinus incrassatus from the type locality or nearby illustrated by Friedberg (1934 –1936: pl. 20: 1, 2), which are probably the best available approximation to the type specimens, have a shorter anterior adductor muscle scar and much smaller umbones. Third, the type locality is of Miocene rather than Pliocene age, and Sacco (1901) noted that M. incrassatus is rare in the Miocene of Italy, and was unknown to him from Pliocene deposits. The specimen illustrated from the Orciano Pisano whale fall ( Danise et al. 2010: fig. 7A) has a shallower lunule and a longer anterodorsal margin than Megaxinus stironensis , and may be yet a different (or new) species.

Stratigraphic and geographic range. —Late Pliocene of northern Italy.

Figures

Fig. 4. The lucinid bivalve Megaxinus stironensis sp. nov. from the late Pliocene of Italy, Stirone River seep deposit. MGGC.22009, holotype, left (A) and right (B) valves in dorsal views (A1, B1), views on the interior including hinge, muscle attachment scars and blood vessel attachment scar (A2, B2), and views of exterior (A3, B3).

Fig. 5. The lucinid bivalve Megaxinus stironensis sp. nov. from the late Pliocene of Italy, Stirone River seep deposit. A. MGGC.22010, interior of left valve. B. MGGC.22011, exterior (B1) and interior (B2) of left valve of a large specimen. C. MGGC.22012, exterior of right valve (C1) and dorsal view (C2) of articulated specimen. D. NRM Mo184001, apical view of articulated specimen. E. MGGC.22013, internal mold showing strong radial striation on the thin veneer of remaining shell; this type of preservation is not uncommon at the Stirone River.