Silvacarcinus laurae Collins & Smith, 1993

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 : 69-70

Silvacarcinus laurae Collins & Smith, 1993

( Figs. 20A–E, G ; 21A–F )

Material examined. Holotype and three paratypes, all from the Ypresian , lower Eocene , Forest , Brussels, Belgium ( IRScNB TCCI 6115 , 6117–6119 ). One additional specimen with preserved thoracic sternum ( MAB k. 2881, ex Tak Collection; indeterminate sex), lower Eocene (Ypresian), Egem, northwestern Belgium.

Emended description. Mxp3 coxae partially preserved; well separated by sternites 1‒3, large, flabelliform. Subhepatic region elongated, narrow, with granular crest; pleural suture raised, granular. Pterygostome tumid, with low, blunt crest; buccal frame wide, buccal margin concave, with smooth buccal collar; branchiostegite developed. Thoracic sternum elongated, suboval, with undivided, plain medial part, raised lateral portion of sternite 4, episternites 4‒6. Sternites 1, 2, or only 2, small, crown shaped, at lower level than other sternites; sternite 3 diamond shaped, distinguished from sternite 4 by lateral grooves; sternite 4 subtrapezoidal, anterior corners slightly wider than sternite 3, lateral margins weakly concave, sternite medially with clear depression, episternite 4 large, distinguished from sternite 4 by oblique, shallow groove, P1 gynglyme just posterior to groove; suture 4/5 only lateral, crescent shaped; episternite 5 nearly square, posteriorly with abdominal holding structure near P2 gynglyme; suture 5/6 sinuous, crescent shaped; sternite 6 elongated, laterally with acute groove marking low lateral depression; sternite 7 smaller than preceding sternites, anteriorly ridged, anteromedially with pitted, spherical bulge, medially with 2 tumid, vertical elevations, separated by medial line; sternite 8 small, strongly tilted, perpendicular to preceding sternites, sides separated creating medial pit. P1‒P3 gynglymes on the same level; P1 coxa large; arthrodial cavity of P4 of normal size, weakly tilted, arthrodial cavity of P5 markedly reduced, subdorsal.

Remarks. Examination of the type series and an additional specimen with ventral characters preserved necessitates an amendment of the description; the dorsal carapace was described by Collins & Smith (1993: 264). The original description of S. laurae by Collins & Smith (1993: 263, text-fig. 2 (not 3a, b); pl. 2, figs. 1, 2, 4–6 (not 3), from the lower Eocene of Brussels ( Belgium), was based on five specimens. As pointed out by Guinot et al. (2008: 32), re-examination of the thoracic sternum (IRScNB TCCI 611) ( Collins & Smith, 1993: text-fig. 3a, b; pl. 2, fig. 3) showed it was erroneously assigned to S. laurae , and used for the type description. The thoracic sternum is completely different from the sterna associated with the carapaces of S. laurae , and cannot be included in Raninoidia. To avoid further confusion, the type series has been re-examined and a new description of the ventral characters is given herein.

Some ventral characters present in the S. laurae material are useful to determine its taxonomic placement. Sternite 4 is subtrapezoidal; sutures 4/5 and 5/6 are lateral and crescent shaped, the medial portion pointing anteriorly. The P4 was not preserved; however, the arthrodial cavity is of normal size, indicating P4 was not reduced and in a normal position. The P5 arthrodial cavity is much reduced and tilted, which indicates the P5 was reduced and carried subdorsally, typical for Palaeocorystoidea . Sternite 3 is diamond shaped, sternite 4 shows a medial gutter, features that recall those in Orithopsis tricarinata . The carapace shows an acute, sinuous cervical groove and branchial crests which are concave and connected over the cardiac region (H-shaped elevation). The orbits are distinctly large, the supraorbital margin shows the broken bases of, presumably, long spines which protected the eye. All these features place Silvacarcinus in Orithopsidae .

The arrangement of the posterior sternites 7 and 8 is complex and cryptic, the sex of the examined material being difficult to determine. A sunken pit appears to be present between sternite 7 and the strongly tilted sternite 8 ( Fig. 21D ); no paired suture 7/8 is discernible but there is a medial line on sternite 7 ( Fig. 21F ). The apertures of the spermathecal apertures may be recessed and hidden in the sunken pit if the specimen is a female. More material is needed to verify the exact morphology, but it is apparent that the posterior sternites of Silvacarcinus and Orithopsidae are different from those of Cenomanocarcinidae and Palaeocorystidae .

Figures

FIGURE 20. A, E, Silvacarcinus laurae Collins & Smith, 1993 (Orithopsidae), IRScNB TCCI 6119 (paratype; indeterminate sex), lower Eocene (Ypresian), Brussels (Belgium); frontal view showing huge orbits, and dorsal view of carapace; B–D, G, Silvacarcinus laurae Collins & Smith, 1993, IRScNB TCCI6115 (holotype; indeterminate sex); dorsal view of carapace; frontal view; left lateral view, and detail of frontal region; F, Cherpiocarcinus rostratus Marangon & De Angeli, 1997 (Orithopsidae), middle Oligocene, northern Italy; reconstruction of carapace after type material (drawing by A. De Angeli). bs, branchiostegite; ia1, insertion of antennule; ia2, insertion of antenna; o, orbit; P1, P2, P3, pereiopods 1, 2, 3; ps, pterygostome. Scale bars: A- F: 5mm; G: 2mm.

FIGURE 21. Silvacarcinus laurae Collins & Smith, 1993 (Orithopsidae), lower Eocene (Ypresian), Brussels (Belgium). A, B, D, F, IRScNB TCCI 6115 (holotype; indeterminate sex); ventral view showing thoracic sternum; posterior view; detail of posterior view, and detail of thoracic sternum; C, IRScNB TCCI 6119 (paratype; indeterminate sex); ventral view showing thoracic sternum; E, MAB k. 2881 (ex Tak Collection; indeterminate sex), lower Eocene (Ypresian), Egem, northwestern Belgium; ventral view showing thoracic sternum. 2, 3, 4, 5, 6, 8, thoracic sternites 2, 3, 4, 5, 6, 7, 8; 4/5, 5/6, 6/7, 7/8, thoracic sternal sutures 4/5, 5/6, 6/7, 7/8; aP3, aP4, aP5, arthrodial cavities of P3, P4, P5; b, bulge; c, buccal collar of pterygostome; cmxp3, coxa of mxp3; cx1, P1 coxa; e4, e5, e6, e7, episternites 4, 5, 6, 7; in, intestinal canal; pt, pterygostome. Scale bars: 5mm.