Akardita, Perna & Brunetti & Bella, 2018

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 : 216-219

Genus Akardita n. gen.

Type species. Cardita subrevoluta de Stefani, 1888 . Lower Pliocene , area of Siena, Italy.

Diagnosis. The new genus is uniquely characterised by its combination of the following shell characters: shape ovate-subquadrate, slightly elongate, inequilateral, weakly truncate posteriorly; length up to ca. 40 mm; approximately 20–25 slightly convex radial ribs, beaded in early stage, narrow and shallow interspaces, weak commarginal sculpture; hinge robust with three right and two left cardinal teeth, lateral dentition obsolete; ligament external, opisthodetic; inner shell margin crenulated, pallial line entire.

Description. Shell small to moderately large, up to ca. 40 mm in length, robust, equivalve, moderately inflated. Shell shape slightly elongate, ovate, more or less subquadrate, inequilateral, well rounded anteriorly, poorly to moderately convex ventrally, slightly truncate posteriorly. Umbo relatively small, prosogyrate. Lunule small, slightly concave; escutcheon elongate, deeply sunken.

Main sculpture of weakly convex radial ribs, approximately 20 to 25, with narrow, shallow interspaces. Posterior slope distinct, with weaker radial ribs. Commarginal sculpture weak, consisting of closely set, irregularly spaced growth striae crossing ribs and interspaces and producing a fine, somewhat irregular decussate pattern. Early radial ribs beaded, with slightly wider and deeper 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, subtrigonal, slightly oblique; posterior tooth larger, elongate, parallel to posterior-dorsal margin. Lateral dentition only consisting of a small, tubercle-shaped anterior lateral tooth. Posterior lateral dentition almost totally obsolete.

Ligament external, elongate, opisthodetic. Inner margin crenulate, especially anteriorly and ventrally. Pallial line entire, moderately convex. Anterior adductor muscle scar slightly larger, reniform; posterior scar roughly triangular; both well distinct.

Etymology. The name was created in assonance with Cardita , type genus of the family Carditidae . Gender feminine.

Included species. Cardita subrevoluta de Stefani, 1888 , Akardita iberica n. sp., Cardita (Venericardia) monodi Nicklès, 1953 .

Distribution. Lower Pliocene of the Mediterranean (northern Italy) and adjacent Atlantic Ocean (Guadalquivir Basin, southern Spain) to Recent (West Africa).

Remarks. None of the extant European carditids, Cardita calyculata (Linnaeus, 1758) , Cardites antiquatus (Linnaeus, 1758) , Glans trapezia (Linnaeus, 1767) , Centrocardita aculeata (Poli, 1795) , Coripia corbis (Philippi, 1836) , and C. jozinae (van Aartsen, 1985) , mostly well-known species, shows significant similarities with both fossil species treated in the present work. Conversely, the diverse carditid fauna of West Africa ( Dautzenberg 1912; Nicklès 1950, 1953 Pasteur-Humbert 1962; Bernard 1984; Cosel 1995; Ardovini & Cossignani 2004) includes Cardita (Venericardia) monodi Nicklès, 1953 , which is rather similar in sculpture and shape to the two fossil species.

“ Cardita ” monodi does not fit easily in Cardita or Venericardia [cf. type species C. calyculata and V. imbricata (Gmelin, 1791) in La Perna et al. 2017 , fig. 2A, B, and 3A–F]. It was recently referred to Megacardita by Huber (2010) but, as discussed by La Perna et al. (2017), this European Miocene genus includes large (up to 100 mm in length), sturdy and markedly inequilateral species. Also, its position in Cyclocardia Conrad, 1832 ( Gofas & Rosenberg 2017) is questionable, as discussed below.

“ Cardita ” monodi shares most shell characters with the two Pliocene species herein treated, being also particularly similar to the type species of Akardita n. gen. The specimen here studied ( Fig. 1A–F ), from off al- Dakhla, Western Sahara, is only slightly larger (13.3 mm in length) than the type material, which consists of two valves from distinct specimens (11.5 mm and 12.0 mm in length, respectively), referred to as holotype in the original description ( Nicklès 1953: 5, pl. 1, figs 3–6). The present specimen differs by being slightly truncate posteriorly and with a lower number of ribs (17 instead of 22).

In Cardita monodi , the posterior ribs are flatter and slightly narrower than the others ( Fig. 1D ), while they are notably finer in both fossil species, though of similar convexity. In addition, the posterior-dorsal beads are somewhat pointed and scaly ( Fig. 1C, F ). These sculptural differences, the only remarkable ones between C. monodi and Akardita n. gen., are not deemed to be significant. Therefore, it is assigned to the new genus as the sole living representative known so far. Akardita monodi ranges from Atlantic Morocco (35° N) to Mauritania (Baie du Lévrier, 20° N), 30–100 m depth ( Ardovini & Cossignani 2004; Huber 2010, von Cosel, pers. comm. 19.10.2017).

Cardita (Venericardia) matheroni Mayer, 1871 , sensu Dollfus & Cotter (1909: 46, pl. 4, figs 21–26) from the Pliocene of the Tagus Valley, Portugal, may also belong in Akardita n. gen. As discussed below, it is more similar to the fossil species described herein than to Cardita matheroni Mayer, 1871 , and may represent an undescribed species.

Megacardita ? redoniana La Perna, Mandic & Harzhauser, 2017, from the Redonian (upper Messinian–lower Pliocene) of northwestern France, is another potential member of our new genus. La Perna et al. (2017: fig. 24A–M, 26A–G) provisionally assigned it to Megacardita , remarking its resemblance to C. monodi .

Cardita zelebori Hoernes, 1865 , from the lower Burdigalian of central Paratethys (La Perna et al. 2017: fig. 27A–I), differs from the new genus mainly by its markedly rectangular outline with a sharper posterior truncation and a steeper posterior slope.

More data on these species and better knowledge of the European Neogene carditids would provide a sounder base for their systematic position, either in the new genus or in other taxa.

The only genus showing a rather close resemblance to Akardita n. gen. is Cyclocardia Conrad, 1832 . According to recent authors, the genus contains about 30 living species ( Huber 2010; Bouchet 2011), and several fossil species have been assigned to it (Janssen & Van der Slik 1972; Popov 1983; Janssen & Moerdijk 2004; Marquet 2005; Pérez & Del Río 2017). However, the wide morphological range encompassed by these species and an almost world-wide distribution of the group, including polar and tropical waters, suggest that too many taxa have been lumped in this genus.

The type species of Cyclocardia is Cardita borealis Conrad, 1831 , from the Northwest Atlantic ( Canada and northern USA) ( Fig. 2A–D ). Several species similar to C. borealis occur in the Atlantic and Pacific, mostly at high latitudes ( Coan 1977; Coan & Valentich-Scott 2012; Huber 2010), such as the Alaskan C. crassidens (Broderip & Sowerby 1829) ( Fig. 2E–G ), forming one of the few carditid groups with cold-water affinity. The easternmost record of Cyclocardia s.s. is from the Pliocene of Iceland ( Vermeij 2005). Other records from the upper Cenozoic of Europe (Janssen & Van der Slik 1972; Janssen & Moerdijk 2004; Marquet 2005) are based on a few species, such as Cardita scalaris J. Sowerby, 1825 , C. orbicularis J. Sowerby, 1825 , and C. chamaeformis J. Sowerby, 1825 , all subtriangular in shape and with finely beaded ribs, notably dissimilar from Cyclocardia s.s. Cardita scalaris is the type species of Scalaricardita Sacco, 1899 , a disregarded genus which could provide a good systematic position for this group.

The main differences between Akardita n. gen. and Cyclocardia s.s. concern the shell shape ( Table 1). In addition, Cyclocardia s.s. has a white shell surface beneath a thick brown periostracum, while Akardita monodi has creamy to reddish blotches and chevrons on a whitish base, and its periostracum is thin and light-coloured (" cuticule mince, jaune pâle " in the original description) as could be confirmed by the examination of other material (von Cosel, pers. comm. 19.10.2017).

As a component of the Northwest African fauna, Akardita n. gen. could be considered a warm-water genus. However, the latitudinal range of A. monodi is within the coastal area with strong cold upwelling (von Cosel pers. comm. 19.10.2017) that existed at least since the middle Pliocene ( Vermeij 2012). It is thus warm eurythermal rather than truly tropical. Its disappearance from higher latitudes appears to be related to the mid Pliocene–Pleistocene cooling trend ( Monegatti & Raffi 2001; Head & Gibbard 2005; Snyder 2016). The biogeographical history of Akardita n. gen. is similar to that of several genera whose latitudinal ranges shifted southward following the Pliocene–Pleistocene climate changes, as recently discussed for three cardiid genera ( La Perna 2016, 2017; ter Poorten & La Perna 2017). The carditid genus Lazariella shows a similar shift from the Miocene Aquitaine Basin ( Cossmann & Peyrot 1912) and Mediterranean ( Sacco 1899) to its current occurrence along West Africa ( Cosel 1995).

Figures

FIGURE 1. Akardita monodi (Nicklès, 1953), off al-Dakhla, WesternSahara, 50–60 m (La Perna coll., 026-2017). Scale bars: 5 mm (A, B, E), 2 mm (C, D, F).

FIGURE 2. Cyclocardia spp. A–D. Cyclocardia borealis (Conrad, 1832), northern Gulf of St. Lawrence, Canada, photo and permission C. Nozères, Fisheries and Oceans Canada (A, LF; B–D, RV).E–G. Cyclocardia crassidens (Broderip & Sowerby I, 1829), off Gay Head (Aquinnah), Martha's Vineyard, Massachusetts (Yale Peabody Museum of Natural History, YPM IZ 033525), photo E. Lazo-Wasem (http://collections.peabody.yale.edu/search/Record/YPM-IZ-033525) (E, LV; F, RV). Scale bars:10 mm (A, B, D, E, F), 5mm (C, G).