Anodontia Link, 1807

Kiel, Steffen, Sami, Marco & Taviani, Marco, 2018, A serpulid-Anodontia-dominated methane-seep deposit from the upper Miocene of northern Italy, Acta Palaeontologica Polonica 63 (3), pp. 569-577 : 575

Genus Anodontia Link, 1807

Type species: Anodontia alba Link, 1807 (type by monotypy); Recent, Caribbean Sea.

“ Anodontia ” mioinflata sp. nov.

Fig. 7 .

Etymology: Combination of the age of the species (Miocene) with the globosity of the shell.

Type material: Holotype: MGGC.22003, well-preserved, articulated shell . Paratypes: MGGC.22004 (two disarticulated valves embedded in rock matrix), MSF 2141 (internal mold of right valve) , MSF 2142 (articulated specimen with shell remains). All from the type locality and horizon .

Type locality: The Ca’ Fornace seep deposit in the Sintria creek valley, northern Italy .

Type horizon: Presumably late Miocene.

Material.— The type material, and 11 specimens housed at MSF from the type locality and horizon .

Dimensions.—The holotype is 19 mm long, 17.5 mm high, and 13.5 mm wide (articulated); the largest specimen is 23 mm long.

Diagnosis.—Subcircular, strongly inflated shell (T/L-ratio ca. 0.81–0.88); posterodorsal area large, marked by distinct ridge running from umbo to posteroventral margin; anterior adductor muscle scar elongate and long, reaching well below midline of shell, deviating from the pallial line by ca. 20°.

Description.—Solid, medium-sized, subcircular, strongly inflated shell (L/H-ratio ca. 1.1; T/L-ratio ca. 0.81–0.88); beaks prominent but only slightly elevated above hinge line; both anterior and posterior margins slightly truncate, ventral margin well-rounded; posterodorsal area large, marked on the exterior by a fine double line, and on the interior by a distinct ridge; lunule indistinct, if any; surface sculpture of fine commarginal growth lines. Anterior adductor muscle scar thin, elongate, reaching down to about 66–70% of shell height, detached from pallial line for most of its length, at an angle of ca. 20°.

Remarks.—A large number of species often differentiated through subtle morphological shell characters have been recognized in the edentulous lucinid genus Anodontia ( Taylor and Glover 2005) . Most living species in Anodontia inhabit shallow settings, rarely reaching down to 500 meters and none is known from bathyal seep habitats. Fossil specimens of this genus in the Mediterranean basin are currently assigned to the extant A. fragilis Philippi, 1836 and relates to shallow water situations ( Chirli 2015); however, some fossil attributions of A. fragilis are questioned by Taylor and Glover (2005) and identified as Leucosphaera . Our material consists of very globose shells definitely different from any known fossil representative, and its deep-water seep habitat is almost unique although the genus has been recently discovered also in a Pliocene cold seep ( Kiel and Taviani 2018). We believe that this warrants the institution of a new species. The preservation state of our material suggests a prudential collocation in “ Anodontia ” sensu lato.

Figures

Fig. 7. The lucinid bivalve “Anodontia” mioinflata sp. nov. from the late Miocene Ca’ Fornace seep deposit, Italy. A. MGGC.22003, holotype, an articulated specimen, view on the dorsal side (A1) and on the right valve (A2), arrow indicating lower end of anterior adductor muscle scar. B. MSF 2141, paratype, right valve embedded in rock matrix, arrows indicate anterior adductor muscle scar. C. MSF 2142, paratype, view on posterodorsal side, arrows indicate the double line delimiting the posterodorsal area. D. MGGC.22004, paratype, the outer side embedded in rock matrix and carbonate mineral deposits on the inside, but showing the general shell outline; left valve (D1) and right valve (D2).