Cenomanocarcinus vanstraeleni Stenzel, 1945

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 : 56-57

Cenomanocarcinus vanstraeleni Stenzel, 1945

( Fig. 18B, C, E )

Material examined. MAB k. 2885 (indeterminate sex), carapace, MAB k. 2895 (female), partial carapace with remains of abdomen, thoracic sternum and base of mxp3, MAB k. 2909, (indeterminate sex), partial carapace with remains of P1 and thoracic sternum; all from the upper Albian‒lower Cenomanian (Pawpaw Formation), Tarrant County (Texas, U.S. A).

Remarks. The morphology of this species has been described by Stenzel (1945: 447), Vega et al. (2007a: 414) and Guinot et al. (2008: 14). A large specimen of C. aff. vanstraeleni was described ( Guinot et al. 2008: 18) with episternite 5 showing ‘a pair of prominences that is blunt, weathered, but well-preserved and recognisable on left side (crab seen by ventral view). This pair of prominences is assumed to be a part of the abdominal holding system’. Two new specimens, smaller in size but with well-preserved but partial thoracic sterna, retain such prominences and allow a more detailed description.

One specimen (MAB k. 2909; Fig. 40C, E ) is an incomplete carapace with well-preserved proximal portion of mxp3, thoracic sternites 3‒5, P1 and P2 bases, abdominal somite 6 and telson. The relatively wide, flat thoracic sternum and wide abdomen suggest a female. Abdominal somite 6 and the telson are disarticulated and posteriorly displaced, exposing episternites 5. On either side, near the P2 gynglyme, a prominent double peg is present. The structure is raised, the distal (posterior) peg being slightly larger and slightly more admedially positioned than the subdistal (anterior) one. The anterior corners of abdominal somite 6 are rounded and swollen, which suggests a socket was present ventrally.

The second specimen (MAB k. 2895; Fig. 40D, F ), a left side of a well-preserved carapace, retaining elements of P1 and P2 and a small portion of the thoracic sternum. Sternites 4 and 5 are partially preserved, episternites 4 and 5 complete. Episternite 5 shows well the P2 gynglyme in ventral view. A well-preserved double peg, separated by a weak depression is present, the distal peg being larger, more posterolaterally placed than the subdistal one.

In another, rather small, specimen (MAB k. 2922), it is seen that the blunt crest on the pterygostome becomes more acute anteriorly, terminating below the orbits in a prominent, pointed subantennary lobe of the pterygostome.

A comparison of the available material permits better differentiation of C. beardi , C. inflatus and C. vanstraeleni . Cenomanocarcinus inflatus and C. vanstraeleni are closely similar, but both species are highly variable with regard to tuberculation and granulation. Both possess an anterolateral margin with five or six teeth anterior to the epibranchial spine. The cervical groove notches the anterolateral margin more strongly and acutely in C. vanstraeleni than in C. inflatus . The nature of the epibranchial spine differs between the two species, being strongly developed in C. inflatus , but similar to or only marginally better developed than the anterolateral teeth in C. vanstraeleni . There are also differences in the posterolateral margins: a conspicuously strong spiniform tooth halfway the otherwise smooth and rounded posterolateral margin in C. inflatus , whereas C. vanstraeleni has a posterolateral margin which is sharper in cross section and bears many small spinules, of which one may be slightly larger, but by no means similar to the spine of C. inflatus . The most obvious difference between the two species is the width of the cardiac region, which is much greater in C. inflatus ( Fig. 18A, B ).

Cenomanocarcinus beardi is distinguished from its congeners by the lack of a tooth between the strong fourth anterolateral tooth and the epibranchial spine. It therefore has five, instead of six or seven, anterolateral teeth (epibranchial included, outer orbital excluded). The epibranchial spine is strongly developed, and the posterolateral margin is smooth and rounded; these characters are similar to C. inflatus and thus differentiate C. beardi from C. vanstraeleni . The strong spiniform tooth halfway the posterolateral margin, characteristic of C. inflatus , is nevertheless absent in C. beardi . In addition, the posterior spine at the base of the branchial crest is stronger. The three crests have fewer but stronger tubercles and the transversely vaulted connection (referred to as ‘H’-shape by Schweitzer et al. 2003a: 36) is clearly seen, whereas it is indistinct or obsolete in C. inflatus and C. vanstraeleni . Characters are summarised in Table 5.

TABLE 5. Characters of Cenomanocarcinus beardi , C. inflatus and C. vanstraeleni .

Figures

FIGURE 18. A, D. Cenomanocarcinus inflatus Van Straelen, 1936 (Cenomanocarcinidae), MNHN J08587 (lectotype), photograph of MAB k. 2968 (cast), upper Cenomanian, Butte de Gazonfier, Le Mans (northwestern France); dorsal and left lateral views (direction of photography in D indicated by arrow in A); B, C, E, Cenomanocarcinus vanstraeleni Stenzel, 1945, MAB k. 2909 (B, C; indeterminate sex), upper Albian-lower Cenomanian (Pawpaw Formation), Tarrant County, Texas (U.S.A); dorsal and left lateral views (direction of photography in C indicated by arrow in B); MAB k. 2885 (E; indeterminate sex), dorsal view of carapace; F, Cenomanocarcinus beardi Schweitzer, Feldmann, Fam, Hessin, Hetrick, Nyborg & Ross, 2003a, MAB k. 2880 (indeterminate sex), Cenomanian (Woodbine Formation), Wagner Park, Texas (U.S.A), dorsal view of carapace. ebs, epibranchial spine; pbs, posterobranchial spine. Scale bars: 10mm.

FIGURE 40. Double peg in Palaeocorystoidea; A, B, Silvacarcinus laurae Collins & Smith, 1993 (Orithopsidae), IRScNB TCCI 6115 (holotype), lower Eocene (Ypresian), Brussels (Belgium); ventral view showing thoracic sternum, and detail of thoracic sternum; C–F. Cenomanocarcinus vanstraeleni Stenzel, 1945 (Cenomanocarcinidae), MAB k. 2909 (C, E), upper Albianlower Cenomanian (Pawpaw Formation), Tarrant County, Texas (U.S.A); details of thoracic sternum; MAB k. 2895 (D, F); same provenance, detail of abdomen and thoracic sternum, and ventral view showing thoracic sternum, abdomen and base of mxp3. 4, 5, 6, thoracic sternites 4, 5, 6; 4/5, 5/6, thoracic sternal sutures 4/5, 5/6; a6, abdominal somite 6; cmxp3, coxa of mxp3; cx2, P2 coxa; dp, distal peg; e4, e5, episternites 4, 5; g2, gynglyme for P2 coxa; p, peg (undifferentiated); sp, subdistal peg; t, telson. Scale bars: A-E: 5mm; F: 10mm.