Camarocarcinidae Feldmann, Li & Schweitzer, 2008

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 : 45-49

Family Camarocarcinidae Feldmann, Li & Schweitzer, 2008 emend.

Camarocarcinidae Feldmann, Li & Schweitzer, 2008: 1742 .

Type genus. Camarocarcinus Holland & Cvancara, 1958 .

Remarks. Cretacocarcinus Feldmann, Li & Schweitzer, 2008 , is transferred to Necrocarcinidae (see below).

Emended diagnosis. Carapace outlined hexagonally rounded to subcircular, widest at one-third from front; strongly convex in all directions; anterolateral margin short, slightly convex, bluntly rounded in cross section, with single spine. Epibranchial spine moderately strong ( Camarocarcinus obtusus ), strong ( C. quinquetuberculatus ) to conspicuously strong ( C. arnesoni ). Posterolateral margin long, arched, blunt in cross section, with 4 spines. Posterior margin clearly concave, as wide as ( C. arnesoni ) or slightly narrower ( C. quinquetuberculatus ) than orbitofrontal margin. Rostrum short, broadly triangular, sulcate, with downturned projection, bifid, with 2 distal, 2 subdistal teeth; shallow post-rostral slits may be present ( C. arnesoni ). Orbital margins raised, orbits small, deep, close set, obliquely arranged. Outer orbital corner prominent, supraorbital margin with 2 long fissures. Branchiocardiac grooves shallow, broad; branchial groove defined by scars, cervical groove indistinct to obsolete. Carapace surface with numerous pits on anterior regions ( C. arnesoni , C. obtusus ) or regions with indistinct tubercles ( C. quinquetuberculatus ). Dorsal carapace surface perforated, endocuticle with upright nodes.

Pterygostome grooved, with long blunt crest. Branchiostegite with strong rim along coxae ( C. arnesoni , C. quinquetuberculatus ), joining coxae of pereiopods, thus no exposure of pleurites. Mxp 3 in oxystomian condition, endopodite strongly elongated, merus longer than ischium. Exopodite relatively wide, longer than endopodite ischium. Mxp3 coxae not observed, but presumably not close to each other.

Chelae homochelous, lower margin spinose, palm surface with few tubercles ( C. arnesoni , C. quinquetuberculatus ). P2 and P3 articles oval in cross-section ( C. arnesoni ). P5 not preserved, supposedly (sub)dorsal, reduced.

Thoracic sternum strongly concave over complete length, sternum narrowing towards posterior, sterno-abdominal depression deep ( C. quinquetuberculatus ). Sutures short, lateral. Suture 4/5 crescent shaped: lateral part roughly horizontal, longitudinal part deep. Sutures 5/6, 6/7 arched, clearly divergent ( C. quinquetuberculatus ). Medial line not present at sternites 1‒7; sternite 8, spermathecal aperture unknown. Arthrodial cavities evenly spaced, ventrolaterally directed.

Male abdomen unknown; abdomen wide in females, occupying complete width of sterno-abdominal depression, thus in contact with coxae of pereiopods. Abdominal somite 6 long in females; medial part of abdomen raised; female telson ending between P1 and P2 coxae ( C. arnesoni ).

Remains of abdominal holding system observed on episternite 5 ( C. quinquetuberculatus ), insufficiently preserved to be described in detail.

Remarks. The Camarocarcinidae is the smallest family in terms of number of genera within Palaeocorystoidea , comprising but a single genus, Camarocarcinus , to which three species from Denmark, Greenland and North Dakota, all of Paleocene age, have been assigned. Camarocarcinus was initially placed in Raninidae ( Holland & Cvancara 1958: 499), subsequently transferred to Calappidae ( Glaessner 1969: R494; Collins & Wienberg Rasmussen 1992: 33; Schweitzer & Feldmann 2000: 241; Fraaije 2002: 914), to Leucosiidae ( Schweitzer et al. 2003a: 34) , included in the ‘assemblage Palaeocorystidae-Necrocarcinidae-Cenomanocarcinidae-Orithopsidae’ ( Guinot et al. 2008: 37) and, finally, designated as type genus of a distinct family by Feldmann et al. (2008: 1742, 1743).

The carapace of camarocarcinids is of moderate size, yet strongly tumid both longitudinally and transversely. Camarocarcinids closely resemble necrocarcinids in size, carapace curvature and appearance. They can be, however, distinguished by having the carapace with regions non-areolated, lacking a distinct tubercle in Camarocarcinidae (areolated, clearly tuberculate in Necrocarcinidae ), in having an indistinct groove system in Camarocarcinidae (distinct in Necrocarcinidae ), and the anterolateral margin with one or two spines in Camarocarcinidae (four or five teeth in Necrocarcinidae ). Feldmann et al. (2008: 1743) remarked that the orbits were elevated and more upwardly directed in Camarocarcinidae , which resulted in a more projected orbitofrontal region than in Necrocarcinidae . Based on each of these differences described above, Cretacocarcinus Feldmann, Li & Schweitzer, 2008 , is here removed from Camarocarcinidae and transferred to Necrocarcinidae (see below, under Cretacocarcinus ).

The pterygostome of Camarocarcinus arnesoni is strongly sculpted, the innermost crest being acute, sharp and extending along the branchiostegite ( Holland & Cvancara 1958: pl. 74, fig. 13; Feldmann et al. 2008: 1746, fig. 5.1). This crest is found to be a plesiomorphic character, which also occurs, although less prominently, in Necrocarcinidae . This ‘pterygostomian rim’ ( Holland and Cvancara 1958: 501) is hypothesised to have functioned as an endostegal channel in the respiratory system (see Respiration in the Palaeocorystoidea below). A prominent subantennary lobe of the pterygostome cannot be verified, but is assumed to be present (probably broken off in C. quinquetuberculatus ; Collins & Wienberg Rasmussen 1992: 34, fig. 19C).

Remarks. The subgenus Notopocorystes (see Wright & Collins 1972: 73) was given generic rank by Tucker (1998), who also accorded specific rank to the three subspecies of N. stokesii . Records from England, France, Germany, Switzerland, Iran, Kazakhstan, China and Madagascar document a stratigraphic range from the lower Albian to the Cenomanian.

Ventral features are preserved in Camarocarcinus arnesoni and C. quinquetuberculatus . The proepistome, epistome, antennae or antennules are not retained in any of the specimens examined. The thoracic sternum is well preserved in the holotype of C. quinquetuberculatus , the description of which was completed by Guinot et al. (2008: 30). Neither the pereiopods nor the posterior sternites are preserved in any of the material examined; hence no conclusion can be drawn regarding the nature of the last pair(s) of pereiopods. Feldmann et al. (2008: 1743) suggested that the morphology of the sternum, coupled with characters of the dorsal carapace of Necrocarcinus spp. , differed to such an extent from those of Camarocarcinus and Cretacocarcinus that they concluded the latter two are ‘demonstrably unrelated’ to Necrocarcinidae . The morphology of the thoracic sternum of Camarocarcinus quinquetuberculatus and Cretacocarcinus smithi would document a morphology that is different from any known brachyuran group, according to Feldmann et al. (2008: 1743): ‘The general sternal architecture within these two species is one of a flat-floored axial element with nearly vertical lateral elements’. According to Feldmann et al., this ‘deep cavity’ would receive a narrow abdomen. The thoracic sternum of C. quinquetuberculatus is deeply excavated, which corresponds to a sterno-abdominal depression, rather than to a sterno-abdominal cavity. Feldmann et al. (2008: 1743) compared the thoracic sternum of Camarocarcinus and Cretacocarcinus with Necrocarcinidae by way of ‘ Necrocarcinus siouxensis’, of which they remarked that the species had recently been reassigned to Cenomanocarcinus ; ‘ Necrocarcinus siouxensis’ is here referred, with a query, to Orithopsis (see below). Furthermore, they compared a single specimen of Necrocarcinus labeschii (SM B23180), which has also been examined for the present study (see below). The thoracic sterna of C. quinquetuberculatus and N. labeschii are, as far as material is available and preservation enables this to be compared, closely similar. The description of the thoracic sternum of N. labeschii by Feldmann et al. (2008: 1743), who stated that ‘the margins of sternites 4–6 are parallel’, is incorrect. In both C. quinquetuberculatus and N. labeschii , sternite 4 is subtrapezoidal with concave sides, the surface being rather strongly excavated medially, episternites 4 being the widest in the sternum; sternites 5‒7 slightly decreasing in width posteriorly; the episternites are obliquely arranged, and elevated from a flat, undivided medial portion of the sternum, the sutures being only lateral (see Karasawa et al. 2011: fig. 10B, C). Only the anterior sternites (3 and 4, including episternite 4) of Cretacocarcinus smithi are preserved: sternite 4 is strongly excavated, the sides are concave and episternites 4 are distinctly elevated. This profile is stronger than, but not different from either that of C. quinquetuberculatus or N. labeschii . In addition, Guinot et al. (2008: 30) remarked that the thoracic sternum of ‘ Necrocarcinus ’ wrighti ( Feldmann et al. 1993: figs. 29.4, 29.5) was not substantially different from that of Camarocarcinus . It may be concluded, with reservation, that the thoracic sternum of Camarocarcinidae strongly resembles the sternum of Necrocarcinidae . Obviously, better-preserved material is needed to complete the diagnoses of both families.

Karasawa et al. (2011: 550, 551) complicated matters by stating that, ‘ Camarocarcinidae is the sister to the remainder of the raninoids; Necrocarcinidae and Cenomanocarcinidae are sister to Palaeocorystidae , ...’, ‘... separate Camarocarcinidae from its most similar taxon, Necrocarcinidae’ and ‘ Necrocarcinidae and Camarocarcinidae appear as sister taxa’. Camarocarcinus and Necrocarcinus have a similar thoracic sternum, with a flat axial depression and lateral episternites strongly raised. The angle of the raised episternites is not substantially different in the two families, as stated by Feldmann et al. (2008: 1743, 1744) and Karasawa et al. (2011: 550). While Fraaye (1994) suggested that Camarocarcinus was derived from the Necrocarcinus stock, Feldmann et al. (2008: 1741) stated that, ‘details of the morphology […] falsify the contention that Camarocarcinus was derived from a necrocarcinid ancestor’. Karasawa et al. (2011: 533) remarked that, ‘ Guinot et al. (2008) thought that Camarocarcinus belonged to Palaeocorystidae’, but, in fact, Guinot et al. (2008: 706) assigned Camarocarcinus to the Raninoidea , close to the Palaeocorystidae .

Feldmann et al. (2008: 1743) were of the opinion that the carapace ornament of Cretacocarcinus differed from that of Necrocarcinidae , in that the ‘nodes’ [tubercles] on the dorsal surface of Cretacocarcinus smithi were not aligned in rows. An axial row of tubercles (one cardiac, one urogastric and two mesogastric), a tubercle on each branchial portion and two tubercles transversely arranged on the protogastric region in C. smithi perfectly match the arrangement in the type of Necrocarcinus , N. labeschii . Tuberculation of regions is absent or indistinct in the species of Camarocarcinus ; in C. quinquetuberculatus it is only axially defined ( Fig. 15A–D ). Feldmann et al. (2008: 1743) noted that pillars of the endocuticle ‘extend up to or through the exocuticle surface’. Waugh et al. (2009: 35) found that ‘ Camarocarcinus arnesoni has cuticle that is most similar to that of Symethis and Cretacoranina because the nodes on its surface are somewhat enlarged or bulbous at their tops’, whereas Feldmann et al. (2008: 1743, fig. 5.5) had earlier described the outer surface as ‘smooth to granular’. The examined cuticles of C. arnesoni and C. quinquetuberculatus ( Fig. 15A–G ) show the endocuticle to have upright nodes, overlapped by one or two layers of cuticle with numerous fine perforations. The preserved cuticular surface near the orbitofrontal region and near the posterior margin comprises fine granules with sparse perforations. These perforations most likely contained setae during life ( Waugh et al. 2009: 16).

The Camarocarcinidae appears to be closely related to Necrocarcinidae , their differentiation currently being weak. Until additional ventral characters can be compared, features of the dorsal carapace seem to be the most reliable for distinguishing both families.

Figures

FIGURE 15. A–D, Camarocarcinus quinquetuberculatus Collins & Wienberg Rasmussen, 1992 (Camarocarcinidae), MGUH 21.609 (holotype), middle Paleocene, central Nûgssuaq, western Greenland; dorsal view of carapace; frontal view showing elongate mxp3; ventral view showing thoracic sternum, P1 coxae and mxp3, and right lateral view; E–G, Camarocarcinus arnesoni Holland & Cvancara, 1958, MGSB unregistered, Paleocene, North Dakota; frontal; left lateral, and dorsal views of carapace. iol, infraorbital lobe. Scale bars: 5mm.