Acasta martillacensis Carriol

Carriol, René-Pierre, Cahuzac, Bruno & Lesport, Jean-François, 2011, New species of Acasta and Balanus (Balanoidea: Acastinae, Balaninae) from the Early Miocene (Burdigalian) of the Aquitaine Basin, France, Zootaxa 3109, pp. 60-68 : 63-65

Acasta martillacensis Carriol sp. nov.

Figs. 2A–D , 3

Type locality. Martillac ditch section, near 'Couvent de la Solitude', Département Gironde, Aquitaine Basin, southwest France.

Type stratum. Early Burdigalian, fine grey-blue sands in the lower portion of the section exposed, level GA 39191, “Sables à Mactres”.

Material examined. Holotype (no. 85-12-68, Cahuzac Collection), a shell (basis with wall plates), retaining the right scutum. Paratypes, (no. 85-12-69, Lesport Collection) a rostrum; (no. 85-12-70, Lesport Collection) half a basis with carina, a carinolateral, lateral and half rostrum.

Additional material (Lesport Collection). 2 rostra, 2 carinae, 1 basis, 1 fragmentary basis with a partial lateral and carinolateral.

Measurements. Holotype: opercular aperture, carino-rostral diameter 3.5 mm; carina at mid-height 4.4 mm; rostrum at mid-height 3.9 mm, sheath height 1.9 mm; basis, carino-rostral diameter 4.1 mm; scutum, occludent margin 3.1 mm, articular margin 2.0 mm, articular ridge 1.4 mm. Paratype no. 85-12-69: rostrum at mid-height 6.3 mm. Paratype no. 85-12-70: basis, carino-rostral diameter 7.0 mm, opercular aperture, carino-rostral diameter 5.5 mm. Additional material: 2 rostra, at mid-height 5.1 and 6.6 mm, respectively; 2 carinae, at mid-height 5.6 and 7.1 mm, respectively; basis, larger diameter 6.0 mm.

Diagnosis. Shell with internal longitudinal ribs; cellular space between sheath and wall; carinal sheath adhering to the wall in the centre of its basal edge; basis cup shaped; scutum externally with radial striae, and internally with adductor ridge, not prominent.

Description. Shell white, occasionally with pink hue towards apex and on radii (original coloration visible on holotype). Parietes internally with well-developed ribbing extending up to sheath or nearly so. Between ribs, in parts of plate where they are close, fine longitudinal spaces present. These tubes do not show an aperture between the ribs near basal margin of plates (see Fig. 3 ); only in broken plates can tubes be demonstrated. Parietes externally with growth ridges crossed by very fine longitudinal ribs, and with, hollow, upward-curving spines sloping downwards, spines occasionally bifurcated. Radii and alae not very wide, upper edge oblique, sutural edge of radii with striations. Sheath broad, occupying about half length of compartment, transversally striated with basal edge free from wall except in carina, where central part of basal edge turns down, adhering to wall. Cellular space, secondarily calcified, between sheath and wall. Basis cup-shaped, calcareous, non porous, concentrically ringed outside but smooth inside, with margin bearing small pits for reception of teeth formed by ends of internal parietal ribs. Scutum wide, taller than wide, slightly bowed between apex and basal margin; occludent margin slightly concave, tergal margin straight, basal margin convex; exterior ornamented by growth increments crossed by incised radial striae; interior, with articular ridge not prominent barely extending over the tergal margin, and with adductor ridge not prominent, of same length as tergal edge of adductor muscle pit; lateral depressor muscle scar rounded, rather deep; adductor muscle pit shallow, rather wide with its posterior extremity not clearly discernible.

Etymology. In reference to Martillac, Aquitaine Basin (southwest France).

Remarks. The specimens of Acasta martillacensis sp. nov. have ribbed parietal plates as do other numerous species of Acasta . No apertures of the tubes are visible between these ribs near the basal margin of the plate. It is along breaks of certain specimens that these tubes appear and these specimens, which seem rather old in view of their size, are only partially tubiferous. Tubes are present only in particular parts of the plates, in the parts where ribs are numerous and in contact, as if the ribs by contacting had closed a longitudinal space between them. These tubes are not the same that those present in Perforatus perforatus and are different to those in Balanus crenatus . They are not tubes in the sense we usually understand and the presence of solid paries remains a characteristic of Acasta .

Affinities. Among the 26 extant species retained in the genus Acasta by Kolbasov (1993), one species has fossil representatives: Acasta spongites , the extant species from the Atlantic and the Mediterranean Sea, which has been described from the Pliocene of France ( De Alessandri 1907) and Italy (Moroni 1952). There are eight extinct species ( Carriol 2008). The Acasta with fossil representatives, except A. vesiculosa Carriol, 2008 (see complete description in Schneider et al. 2009), may be distinguished from Acasta martillacensis sp. nov. in the lack of a vesicular sheath. Acasta martillacensis sp. nov. is close to A. vesiculosa in having the vesicular sheath and a carinal sheath adhering to the wall in the centre of its basal edge. However, the former can be distinguished by the internal ribbing of the wall, its cup-shaped basis and its scutum, in which the adductor ridge is not prominent. The claim that A. vesiculosa from the middle Miocene of France ( Carriol 2008) showed internal ribbing of the wall is erroneous, due to the poor preservation of a small set of specimens available.

Figures

FIGURE 2. A – D, Acasta martillacensis sp. nov., holotype (no. 85 – 12 – 68), level GA 39191, a shell (basis, plus wall plates) with right scutum which was inside of the shell: A, right side of shell; B, external side of basis; C – D, external and internal side of scutum. E – H, Balanus aculeatus sp. nov., holotype (no. 19 – 12 – 23), carina with a fragment of basis in connection: E, basis magnified showing septed tubes; F, external side; G, internal side; H, lateral view.

FIGURE 3 A – B. Acasta martillacensis sp. nov., paratypes (no. 85 – 12 – 69 and no. 85 – 12 – 70, respectively): A, fragment of paries transversally cut; B, fragment of paries longitudinally cut at sheath level. p, paries; r, rib; sh, sheath; sp, septae; t, tube.