Akardita subrevoluta (de Stefani, 1888 )

Perna, Rafael La, Brunetti, Mauro M. & Bella, Giano Della, 2018, Systematic position of two Pliocene carditids with description of Akardita n. gen. and A. iberica n. sp. (Bivalvia: Carditidae), Zootaxa 4379 (2), pp. 215-230 : 220-222

Akardita subrevoluta (de Stefani, 1888)

Fig. 3A – J

1878 Cardita revoluta Seguenza, 1877 , sensu de Stefani & Pantanelli: 43. 1888 Cardita subrevoluta de Stefani : 189, pl. 10, figs 11 – 13.

1893 Venericardia subrevoluta (de Stefani, 1888) —Pantanelli: 155.

1899 Actinobolus antiquatus var. rhodiensis (Fischer, 1877) , sensu Sacco: 18, pl. 5, figs 11, 12.

Type materia l. The species was described from Poggiarone, in the area of Siena. No type material has been found so far in the de Stefani and Pantanelli collections housed in the University Museums of Florence and Modena.

Material examined. 38 valves, Tavernelle, lower Pliocene (Della Bella coll. GDB-090), 35 valves (Brunetti coll., U828BNF1); 3 valves, Poggio alla Fame, lower Pliocene (Brunetti coll. U828PGF). Illustrated material ( Fig. 3A–J ) housed in MZB (47006).

Description. Shell small, slightly exceeding 20 mm in length, relatively robust, equivalve, moderately inflated. Shape slightly elongate, ovate-subquadrate, moderately inequilateral, well rounded anteriorly, poorly to moderately convex ventrally, barely truncate posteriorly. Umbo small, protruding, prosogyrate. Lunule small, slightly concave; escutcheon markedly elongate, deeply sunken. Ligament external, elongate, opisthodetic.

Radial sculpture consisting of 24–26 poorly convex ribs. Radial interspaces narrow, shallow, poorly impressed ventrally. Growth striae distinct, closely set, crossing radial ribs and interspaces, producing a fine, irregular and decussate pattern. Commarginal sculpture mainly lamellose near ventral margin. Posterior slope distinct, with about five, notably finer ribs. Early sculpture consisting of beaded radial ribs, with slightly deeper and larger interspaces.

Hinge plate moderately robust, slightly arched. Right hinge with three cardinal teeth: anterior tooth small, poorly developed; central tooth robust, triangular, elongate, strongly oblique; posterior tooth elongate, medially grooved, parallel to posterior-dorsal margin. Left hinge with two cardinal teeth: anterior tooth smaller, somewhat triangular, slightly oblique; posterior tooth larger, elongate, parallel to posterior-dorsal margin. Lateral dentition only represented by a small, tubercle-shaped anterior lateral tooth on both valves.

Inner margin coarsely crenulate anteriorly and ventrally, more finely posteriorly. Pallial line entire, moderately convex. Anterior adductor muscle scar slightly larger, reniform; posterior scar roughly triangular; both well distinct.

Distribution. Akardita subrevoluta is only known from the lower Pliocene of northern Italy.

Remarks. The present species was first reported as Cardita revoluta Seguenza, 1877 , from the Pliocene of Siena by de Stefani & Pantanelli (1878), who added a description. Ten years later, de Stefani (1888: 189, pl. 10, figs 11–13) noted some differences from Seguenza's species and described it as new, re-proposing the early description: " Testa cordata, subquadrata; latus anticum rotundatum, latus posticum obtuse-angulatum, margo pallearis convexiusculus: costae longitudinales 24–26 magnae, convexae; intervallis brevissimis separatae; transverse, praecipue ad umbones, rugosae: lunula antica parva, parum profunda: umbones recurvi obtusi: margo pallearis profunde crenatus; dens cardinalis in utraque valva brevis, in valva sinistra solidior; dentes laterales tenues, breves " (Shell heart-shaped, subquadrate; anterior margin convex, posterior margin obtusely angulate, ventral margin poorly convex: 24 – 26 radial ribs, wide, convex; separated by very narrow interspaces; commarginally rugose, especially near umbo: lunule anterior, small, slightly deep: umbos incurved, obtuse: ventral margin deeply crenulated; cardinal teeth in both valves short, stronger in left valve; lateral teeth weak, short).

The valve illustrated by de Stefani (1888) ( Fig. 3K ) appears more elongate and triangular than the present material, but the size reported in the description (16.5 mm in length, 16.0 mm in height) points to a more equidimensional shape, in agreement with the morphometric trend recorded in the study material ( Fig. 4A ). Growth is mostly isometric, with only a slight increase in elongation with growth, as indicated by a low correlation coefficient (~ 0.5) of L/H vs L ( Fig. 4B ). The maximum size is 23.2 mm in length. Width is 6.3 – 7.5 mm in 19.0 – 21.0 mm long valves.

Cardita revoluta , with which the present species was confused at first, is from the lower Pleistocene of Reggio Calabria, southern Italy ( Seguenza 1877: 10; 1879: 280, pl. 15, figs 40, 40a). The species reported under this name by Cerulli Irelli (1907: 135, pl. 12, fig. 29 – 36) and Gignoux (1913: 404, pl. 19, fig. 3), from the lower Pleistocene of the Monte Mario area (Rome), seems to match the original description and illustration by Seguenza. Based on this identification, C. revoluta differs from A. subrevoluta in several respects: much more convex, markedly inequilateral, with a larger, strongly prosogyrate umbo. The radial ribs are poorly convex, bearing scattered pointed nodules and separated by narrow, shallow interspaces. It is particularly similar to Cardita rhomboidea ( Brocchi, 1814) , from the Italian Pliocene ( Brocchi 1814: 523, pl. 12, fig. 16; Rossi Ronchetti 1952: 55, fig. 21A–F).

Actinobolus antiquatu s var. rhodiensis (Fischer, 1877) , as reported by Sacco (1899: 18, pl. 5, figs 11, 12) from the Astian of Piedmont, is Akardita subrevoluta . He also reported the species from Stazzano, a Tortonian locality near Turin, but this record needs confirmation.

Cardita rhodiensis Fischer, 1877 , from the Pliocene of Rhodes ( Fischer 1877a, b), is a markedly different species, much more similar to C. revoluta and C. rhomboidea in shape and sculpture as seen from the type material (https:/ /science.mnhn.fr/taxon/species/ cardita / rhodiensis ).

Akardita subrevoluta shares most shell characters with A. monodi . The living species has a slightly more pointed umbo and a wider anterior part. As described above, the radial ribs on the posterior slope of A. monodi are only slightly finer than the others, and the posterior-dorsal beads are somewhat scaly and pointed. Both species are also similar in size, being the West African species close to 20 mm in maximum shell length ( Huber 2010; von Cosel pers. comm. 19.10.2017).

Figures

FIGURE 3. Akardita subrevoluta (de Stefani, 1888). A–C. LV, Tavernelle (Siena), Italy, lower Pliocene (MZB 47006). D–F. RV, Tavernelle(Siena), Italy, lowerPliocene (MZB 47006).G–I.LV,Tavernelle (Siena), Italy, lowerPliocene (MZB 47006).J. RV, Tavernelle(Siena), Italy, lower Pliocene (MZB 47006). K. Original illustration, after de Stefani (1888: pl. 10, figs 11–13). Scale bars:10 mm (A, B, D, E, G, H, J), 4 mm (C, F, I).

FIGURE 4. Akardita subrevoluta (de Stefani, 1888). A. Scatter plot H vs L (64 valves from Tavernelle) Open circle indicates the size in the original description.B. Scatter plot and least square regression of L/H vs L.