Cenocorystes bretoni, Van Bakel & Guinot & Artal & Fraaije & Jagt, 2012

Van Bakel, Barry W. M., Guinot, Danièle, Artal, Pedro, Fraaije, René H. B. & Jagt, John W. M., 2012, A revision of the Palaeocorystoidea and the phylogeny of raninoidian crabs (Crustacea, Decapoda, Brachyura, Podotremata) 3215, Zootaxa 3215 (1), pp. 1-216 : 18-19

Cenocorystes bretoni n. sp.

( Fig. 2A–D )

Diagnosis. Carapace small, subhexagonal in outline, slightly longer than wide, maximum width at epibranchial spine one-third distant from front; posterior margin rimmed, strongly concave, shorter than orbitofrontal margin; dorsal regions weakly differentiated; cervical groove weakly defined, V-shaped; branchiocardiac grooves welldefined; epibranchial region with median oblique scar; cuticle granular. Pterygostome large, inflated, buccal margins concave, rimmed. Thoracic sternum narrow, elongated; sternites 1, 2 narrow, triangular; sternites 3, 4 separated by oblique grooves. Mxp3 elongated, coxae large, basis-ischium fused, grooved.

Derivation of name. In honour of Gérard Breton (Université de Rennes I, Rennes, France), who has substantially contributed to our knowledge of fossil decapods from France.

Material examined. Holotype: MAB k. 2899 (indeterminate sex); upper Cenomanian , precise level unknown; Vibraye, near Le Mans (Sarthe, northern France).

Description. Carapace small, subhexagonal in outline, slightly longer than wide (in absence of orbitofrontal margin, length measured from basis of extraorbital spine), widest approximately one-third of total carapace length from front; dorsal surface fairly convex in both directions. Orbitofrontal margin broad, front not preserved. Anterolateral margins arched, with 3 broadly based spines; 1 hepatic plus 2 epibranchial, decreasing in size posteriorly. Posterolateral margins converging backwards, sinuous, slightly concave anteriorly, slightly convex posteriorly, margin rounded; with small spine anteriorly. Posterior margin fairly concave, rimmed, narrower than orbitofrontal margin. Dorsal regions of carapace poorly differentiated. Cervical groove weakly defined, discernible at lateral margins, axially; medially interrupted by small gastric pits. Epibranchial region exhibits oblique scar in medial portion, arched muscle scars posteriorly running subparallel to cervical groove. Cardiac region large, bounded by rather deep branchiocardiac grooves. Dorsal surface of carapace densely, uniformly granular.

Pterygostome large, inflated; buccal margin concave, rimmed. Pleural suture well-defined as thin line.

Thoracic sternum narrow, not connected to pterygostome, narrowing backwards, sternites 1, 2 narrowly triangular, situated at lower level; sternite 3 crown shaped, well separated from sternite 4 by oblique grooves; sternite 4 trapezoidal, episternite 4 suboval, robust, slightly extending laterally; suture 4/5 crescent-shape; sternite 5 without lateral depression; suture 5/6 crescent-shape. P4 arthrodial cavity tilted; P5 reduced, (sub)dorsal.

Abdomen narrow, first somite broader. Mxp3 elongated, with large, flabelliform coxa; endopod basis-ischium elongated, merus shorter, strongly depressed axially.

Remarks. The new species presents the following features that allow placement in Cenocorystes : reduced length of the carapace, with length roughly equalling width; large orbits; weakly defined cervical groove; arched anterolateral margin with four spines directed forwardly and outwardly, gently rounded posterolateral margins. In absence of a completely preserved front in both species of the genus, the total length was taken from the base of the extraorbital spine, which is preserved in the holotypes of both. Cenocorystes bretoni n. sp. differs from the type species, C. fournieri , in having a more tumid dorsal carapace surface and in lacking clearly defined grooves around the hepatic anterolateral spine ( Collins & Breton 2009: fig. 6) as well as four protogastric tubercles, and in having a more concave posterior margin. In addition, C. bretoni n. sp. has more distinct branchiocardiac furrows, whereas the intestinal region is better defined in C. fournieri .

The new species, with partial ventral side preserved, reveals the thoracic sternum of Cenocorystes for the first time. It matches the general sternal configuration of Palaeocorystidae ; differences with other genera are the rather strong grooves separating sternites 3 and 4, sternite 5 lacking a lateral depression, and sutures are more closed.

Figures

FIGURE 2. Cenocorystes bretoni n. sp. (Palaeocorystidae), MAB k. 2899 (holotype; indeterminate sex), upper Cenomanian, Vibraye near Le Mans (northern France); A, dorsal view of carapace; B, ventral view, showing thoracic sternum, abdomen and mxp3; C, detail of thoracic sternum; D, left lateral view of carapace. 2, 3, 4, 5, thoracic sternites 2, 3, 4, 5; aP1, aP2, aP3, arthrodial cavities of P1, P2, P3; br, branchiostegite; cx5, P5 coxa; e4, e5, episternites 4, 5; cmxp3, coxa of mxp3; pt, pterygostome. Scale bars: A, B, D: 5mm; C: 2mm.