Serrablopluma, Artal, Pedro, Van Bakel, Barry W. M., Fraaije, René H. B. & Jagt, John W. M., 2013

Artal, Pedro, Van Bakel, Barry W. M., Fraaije, René H. B. & Jagt, John W. M., 2013, New retroplumid crabs (Crustacea, Brachyura, Retroplumidae Gill, 1894) from the Eocene of Huesca (Aragón, Spain), Zootaxa 3652 (3), pp. 343-352 : 344-348

Serrablopluma n. gen.

Type species. Serrablopluma diminuta n. sp.

Diagnosis. Small size carapace, subtrapezoidal in outline, much wider than long, widest anteriorly, maximum width at position of lateral spines; anterior margin wide, broadly sinuous, terminating in fairly widely projecting lateral spines; front conspicuously reduced, narrow; orbits wide, with sub-orbital spine; lateral margins straight, inclined, converging posteriorly; posterior margin wide, broadly convex; dorsal carapace surface flat in both directions, with 3 strongly marked ridges; anterior ridge continuous, intersecting anterior margin; medium ridge interrupted only by gastric grooves; posterior ridge continuous, straight. Thoracic sternum broad; sternites 3, 4 trapezoidal, sternites 5–7 subrectangular; vulvae large, subcircular, situated below extremity of sutures 5/6; abdomen subtriangular in both sexes; female abdomen much broader than males; thoracic sternites, abdominal somites notably ridged; thoracic sternite 5, abdominal somite 6 with distinct abdominal holding mechanism; chelipeds equal, robust, relatively large; P2–P4 long, flattened; P5 conspicuously reduced.

Etymology. From Serrablo, the mediaeval name of the area (Pyrenees, Huesca) and the ending pluma, which refers to the main character of the family.

Remarks. The new genus differs from all others currently included in the Retroplumidae (see De Grave et al. 2009: 41) in having a wide, broadly arched anterior margin, a conspicuously reduced front and an anterior dorsal ridge that intersects the anterior margin at the position of long, fairly projecting lateral spines.

Serrablopluma diminuta n. sp. ( Figs. 1–2 )

Diagnosis. As for genus.

Etymology. From the Latin diminutus, in allusion to the rather reduced size.

Material examined. MGSB75287, holotype; MGSB75289, MGSB75290a, b, MGSB78334a, b, and MGSB78335a, b, c, paratypes; MAB k. 3279–3281, MAB k. 3293, MAB k. 3295, paratypes, all from Fanlillo, municipality of Yebra de Basa, Huesca, and of late Eocene (Priabonian) age. In the holotype, the maximum carapace width and length (in millimetres) are 12 and 9, respectively.

Description. Carapace small (maximum carapace width 12 mm); subtrapezoidal in outline; wider than long, widest anteriorly at level of lateral spines; dorsal surface flat in both directions. Anterior margin sinuous, broadly arched, with projecting lateral spines. Front conspicuously reduced, narrow, directed forwards; orbits wide, with sub-orbital spine. Lateral margins straight, inclined, converging posteriorly. Posterior margin convex, wide, only slightly shorter than orbitofrontal margin. Dorsal surface with 3 ridges; anteriormost intersecting anterior margins laterally; median ridge defined by epibranchial swellings, interrupted by gastric grooves; posterior ridge straight, continuous, interrupted only by small indentations of subtle branchiocardiac grooves. Dorsal surface smooth except for horizontal ridges, shallow grooves. Mesogastric region defined by 2 faint swellings, bounded by weak posterior grooves. Cardiac region large, gently swollen, sub-rhomboidal in shape. Intestinal region flat.

Thoracic sternum much broader than long, sternites with subtle ridges; sternites 1,2 fused, small, subtriangular; sternites 3, 4 subtrapezoidal; sternite 4 with pronounced lateral extensions; small, weak indentation between sternites 3, 4; sternites 5–7 subrectangular, with subtle inclined ridges; sternite 8 conspicuously reduced, oblique; vulvae large, subcircular, situated below extremity of suture 5/6. Male abdomen subtriangular, with segments 3–5 fused, all segments with pronounced horizontal ridge; female abdomen much broader, all segments separated, with marked ridges. Abdominal holding mechanism with press-button on sternite 5, and lateral extensions of abdominal segment 6. P1 subequal, robust, relatively large, strong, inner portion of dactyli with numerous denticles. P2–P4 long, flattened. P5 conspicuously reduced.

Remarks. Serrablopluma diminuta n. gen., n. sp. shows the plesiomorphic characters of Retroplumidae (de Saint Laurent 1989: 111–112), namely a sinuous anterior margin and a narrow front; a set of dorsal transverse ridges; a thoracic sternum that is much broader than long; subtrapezoidal sternites 3, 4, with pronounced lateral extensions in sternite 4 behind a distinct notch in sternite 3; distinctly wide, sub-rectangular sternites 5–7; a conspicuously reduced, oblique sternite 8; distinct sexual dimorphism for abdomen; P2–P4 long, flattened; P5 conspicuously reduced, sub-dorsal. The thoracic sternum is typical of Retroplumidae (see Guinot & Quenette 2005: fig. 29A).

Genera that resemble Serrablopluma n. gen. are Loerentheya (see Glaessner 1969: fig. 338, 5) and Loerenthopluma , both of which have a sub-trapezoidal carapace, with the maximum width anteriorly and a wide anterior margin. Serrablopluma n. gen., however, shows distinct characters such as the anterior carapace ridge that intersects the anterior margin, a markedly long lateral spine and lateral margins that are uniformly straight.

The extinct genera Retropluma and Retrocypoda differ (see Beschin et al. 1996: figs 4, 5) in having a subrectangular carapace outline, a much narrower anterior margin, lateral margins broadly arched, not diverging posteriorly and an anterior dorsal carapace ridge without lateral spines. The carapace outline is sub-trapezoidal in Serrablopluma n. gen., the anterior margin conspicuously wide, clearly broader than the posterior margin;moreover, the anterior margins terminate in a long lateral spine intersecting the first anterior dorsal ridge, while lateral margins are straight and continuous and converge posteriorly.

Species of Costacopluma and Archaeopus , which were included in the Retroplumidae by De Grave et al. (2009: 41), exhibit much more inflated carapaces, with markedly convex dorsal surfaces. The dorsal surface in Archaeopus has in addition swollen regions (Rathbun 1908: pl. 47, fig. 5), rather than the thin ridges that define Serrablopluma n. gen.

Figures

FIGURE 1. Serrablopluma diminuta n. gen., n. sp., MGSB 75287 (holotype, indeterminate sex); A, dorsal view. MAB k. 3279 (paratype, male); B, ventral view. MGSB 75290 a (paratype, female); C, ventral view. MGSB 75289 (paratype, male); D, ventral view. MGSB 75290 b (paratype, female); E, ventral view. All from the upper Eocene (Priabonian) of Fanlillo (municipality of Yebra de Basa, Huesca, northeast Spain). Scale bars equal 5 mm.

FIGURE 2. Serrablopluma diminuta n. gen., n. sp., MGSB 78335 a (paratype, indeterminate sex; anterior carapace margin damaged); A, oblique frontal view, showing orbits; C, dorsal view. MGSB 78334 a (paratype, indeterminate sex; anterior carapace margin damaged); B, oblique frontal view, showing orbits. MGSB 78334 b (paratype, female partial thoracic sternum); D, ventral view showing vulva and reduced sternite 8. MGSB 78335 c (paratype, male); E, posteroventral view, showing reduced P 5. All from the upper Eocene (Priabonian) of Fanlillo (municipality of Yebra de Basa, Huesca, northeast Spain). Scale bars equal 5 mm (A – D), 2 mm (E). O, orbit; sos, sub-orbital spine; ls, lateral spine; V, vulva; 4 / 5, 5 / 6, 6 / 7, 7 / 8, thoracic sternal suture 4 / 5, 5 / 6, 6 / 7, 7 / 8; St 4, St 5, St 6, St 7, St 8, thoracic sternites 4, 5, 6, 7, 8; cx 4, cx 5, P 4, P 5 coxae; mP 4, mP 5, P 4, P 5 merus.