Necrocarcinidae Förster, 1968 emend.

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 : 58-60

Family Necrocarcinidae Förster, 1968 emend.

Necrocarcininae Förster, 1968: 169 .

Type genus. Necrocarcinus Bell, 1863 , by original designation.

Included subfamilies. Necrocarcininae Förster, 1968 , and Paranecrocarcininae Fraaije, Van Bakel, Jagt & Artal, 2008 .

Genera included. Cretacocarcinus Feldmann, Li & Schweitzer, 2008 , Glyptodynomene Van Straelen, 1944 , Necrocarcinus Bell, 1863 , Paranecrocarcinus Van Straelen, 1936 , Protonecrocarcinus Förster, 1968 , and Shazella Collins & Williams, 2005 .

Diagnosis. Carapace outline hexagonally or pentagonally rounded to subcircular, widest at mid-length or slightly anterior to mid-length. Carapace convex in all directions ( Necrocarcininae ), or arched ( Paranecrocarcininae ), with hepatic regions inflated. Anterolateral margin blunt, relatively long, convex, with conical teeth ( Necrocarcininae ) or sharp, with granular teeth ( Paranecrocarcininae ). Epibranchial corner with normal to moderately strong conical tooth ( Necrocarcininae ) or without prominent epibranchial tooth ( Paranecrocarcininae ). Posterolateral margin arched, with posterior (subdistal), and possibly medial tooth ( Necrocarcininae ); or smooth to granular ( Paranecrocarcininae ). Posterior margin concave, as wide as or narrower than orbitofrontal margin. Rostrum triangular, sulcate, tip sharp, with 1 or 2 pairs of subdistal teeth ( Necrocarcininae ); or rostrum trapezoidal, bifid, with pair of subdistal teeth ( Paranecrocarcininae ). Post-rostral slits absent ( Necrocarcininae ) or distinct ( Paranecrocarcininae ). Orbitofrontal margin about half carapace width, orbits elevated, small, subcircular, closely spaced, arranged obliquely; supraorbital margin simple, with 2 fissures, inner ones well closed. Branchiocardiac grooves distinct, straight to divergent; branchial groove defined by scars; cervical groove distinct, sinuous with 2 rounded deflections ( Necrocarcininae ), or branchial, cervical grooves indistinct ( Paranecrocarcininae ). Carapace surface areolated, with strong tubercles often in 3 rows ( Necrocarcininae ), surface may be granular to smooth, less areolated ( Paranecrocarcininae ). Endostome relatively narrow, anteriorly elongated with margins raised, laterally shaped into well-developed exhalant channels. Dorsal surface with upright nodes, cuticle with endocuticular pillars extending up to or through exocuticle surface.

Pterygostome deeply grooved, with blunt ( Necrocarcinus ) or tubercular ( Cretacocarcinus ) crest. Prominent subantennary lobe of pterygostome sharp, slightly elongated ( Necrocarcinus labeschii , Cretacocarcinus smithi ). Branchiostegite joining coxae of pereiopods, no exposure of pleurites; surface with moderate rim along coxae ( Cretacocarcinus smithi ), which may be weak ( N. labeschii ) or absent ( N. wrighti ). Mxp 3 in oxystomian condition ( N. wrighti , Cretacocarcinus smithi ). Mxp3 coxae large, flabelliform, not close to each other ( N. wrighti ). Exopodite wide, shorter than endopodite ischium ( Cretacocarcinus smithi ).

Chelae in Necrocarcininae with several strong tubercles on outer surface; upper margin tilted, with flattened teeth; lower margin tubercular. Fingers slender, gaping ( N. labeschii ). Chelae in Paranecrocarcininae : upper margin with flattened triangular teeth, lower margin granular. Outer surface tumid, tubercular or granular, inner surface flat, smooth; fingers closing, with few rounded teeth ( Paranecrocarcinus quadriscissus , P. vanbirgeleni ). Pereiopod articles oval to subcircular in cross section ( Necrocarcinus wrighti ). P5 unknown, supposedly reduced, (sub)dorsal (see below).

Thoracic sternum strongly concave over complete length, sternite 4 medially strongly excavated, lateral sides of sternites 4‒8 (with gynglymes) raised ( Necrocarcinus labeschii , N. wrighti , Cretacocarcinus smithi ). Sternal plate weakly narrowing towards posterior ( Necrocarcinus labeschii ). Sternites 1 and 2 at distinctly lower level than sternite 3. Distinct notch for insertion of mxp3 coxa, followed by short, yet deep groove on sternal plate ( N. labeschii ). Anterior side of sternite 4 wider than sternite 3 ( N. wrighti ). Sternite 8 strongly tilted, same size as sternite 7 ( N. labeschii ). Sternal sutures 4/5, 6/7 short, lateral, suture 4/5 crescent shaped: lateral part roughly horizontal, longitudinal part deep ( N. labeschii ). Suture 7/8 long, straight ( N. labeschii ). Spermathecal apertures not observed. Arthrodial cavities evenly spaced, ventro-laterally directed. Medial line not present, at least for sternites 1‒6 ( N. labeschii , N. wrighti ).

Abdomen sexually dimorphic (after Carter 1898: 27, pl. 1, fig. 9; ‘abdomen of the female is half as wide again as that of the male’), all somites free, somites 1‒5 with, ‘mesonotum raised into a sharp transverse rib’, somite 6 longer than preceding ones; telson rather long, triangular.

Abdominal holding system unknown.

Remarks. Necrocarcinids are generally small in size. The family is the oldest of the palaeocorystoid groups known, with the first representatives appearing in the Hauterivian (Lower Cretaceous, 136–130 Ma; Paranecrocarcinus hexagonalis Van Straelen, 1936 ). Our knowledge of necrocarcinid ventral features is unfortunately scant, a fact already stressed by Feldmann et al. (2008: 1743) and Guinot et al. (2008: 24). The only thoracic sternum of Necrocarcinus labeschii examined here is only of mediocre preservation; no locking structure could be detected. The presence of such structures is to be expected, however. Only one other thoracic sternum of a necrocarcinid has ever been illustrated and described, namely that of N. wrighti ( Guinot et al. 2008: 28) . No necrocarcinid abdomen was available for the present study; the specimens on which Wright & Collins (1972: 64) may have based their observations could not be traced. The description of the abdomen in our diagnosis therfore relies on Carter (1898: 27). His illustration shows an abdomen that matches the palaeocorystoid pattern well and likely represents a female. Sternite 8 in N. labeschii is strongly tilted, indicating a (sub)dorsal placement of the P5.

Based on carapace, thoracic sternum and epistome, Necrocarcinidae is here considered to represent the most basal condition within Palaeocorystoidea . Modifications for respiration whilst buried under the sediment are present, but are comparatively primitive.

Karasawa et al. (2011: 551) stated that necrocarcinids appeared to have a complete sternal suture 6/7. However, we consider this to be a preservational artefact, the suture in fact being only lateral. Those authors also opined that Necrocarcinidae and Cenomanocarcinidae differed from other families of Raninoidea by possessing spines on the posterolateral margin; however, cenomanocarcinids may also have a strong posterolateral spine and, additionally, a subdistal tooth.