HEXAPODIDAE, Miers, 1886

Guinot, Danièle, 2006, Rediscovery of the holotype of Paeduma cylindraceum (Bell, 1859) and description of a new genus of Hexapodidae (Decapoda, Brachyura), Zoosystema 28 (2), pp. 553-571 : 566-568

FOSSIL HEXAPODIDAE

The family Hexapodidae includes a relatively small number of extant genera, distributed throughout Indo-West Pacific, eastern Pacific, and eastern Atlantic regions, some being monospecific, and several species endemic to South Africa. This represents a more restricted distribution than in the fossil record and suggests that hexapodids are a highly specialized group that has remained conservative, and has adapted to a small number of specialized ecological niches ( Schweitzer & Feldmann 2001: 332). With several robust fossil records, the Hexapodidae is one among several eubrachyuran families known from pre-Eocene occurrences, and one of the families known with certainty from the Cretaceous. According to Schweitzer (2005: 291) such a conservative specialization suggests that the Hexapodidae is not a likely candidate to contain the ancestral xanthoids. Th e hexapodid lineage presumably survived as refugium species.

A key of extant and fossil Hexapodidae and similar genera, including both Paeduma and Stevea , was provided by Schweitzer & Feldmann (2001: 337, 344, 345; see also Schweitzer 2005: 289). The distinctive characters of Stevea given by Schweitzer & Feldmann (2001: 337) were according to the erroneous diagnosis of Manning & Holthuis (1981), as previously explained. Most references to Paeduma in the literature (except for the mention of the type species, P. cylindraceum ) correspond to Hexalaughlia n. gen.

Stevea cesarii Beschin, Busulini, De Angeli & Tessier, 1994 (p. 191, fig. 8, pl. 10), from the Eocene of Italy, shows a well preserved thoracic sternum, which is typically hexapodid, and a male abdomen without constriction. The abdominal somites 2-6 are fused (De Angeli pers. comm.) but traces of sutures are visible, which corresponds to extant Stevea (rather than to Paeduma ). There was no mention of striae on the pterygostomian region in the description by Beschin et al. (1994: pl. 10, fig. 4b), and no striae can be discerned from their figures. A sketch and examination of a specimen, kindly provided by A. De Angeli, have nevertheless confirmed the presence of a row of stridulating striae, which are typical of Paeduma and Stevea . The hexapodid figured as S. cesarii on the same plate (Beschin et al.: pl. 10, fig. 2) from limit Ypresian-Lutetian (instead of Middle Lutetian), and with a different carapace shape, is not S. cesarii , and instead belongs to another new species (De Angeli pers. comm.).

A fossil without oblique pterygostomian striae, from the Miocene of central Japan was assigned to the genus Paeduma by Karasawa (1990: 25, pl. 8, fig. 14, as Paeduma sp. ), who followed Manning & Holthuis’ (1981) diagnosis. A stridulating apparatus is nevertheless diagnostic of Paeduma and Stevea , so the generic status of this Japanese hexapodid needs to be reviewed.

The nearly complete loss of P5 is a problem in recognizing fossil hexapodids because the legs, in particular P5, are often lost during fossilisation. Among the criteria that could be used are the shape of sternites 5-7 (well developed, equal and subrectangular) and the strong reduction of sternite 8, and perhaps even without visible trace.

Several fossil crabs truly lacking P5 (instead of lost during fossilisation) have been attributed to the Hexapodidae . The first verified occurence for this condition was for the Paleocene Goniocypoda rajasthanica Glaessner & Rao, 1960 . More recently, G. tessieri Rémy & Tessier, 1954 , from the Maastrichtian, has been confirmed as another fossil member of the Hexapodidae (see Crane 1981; Crane & Quayle 1986; Schweitzer & Feldmann 2001; Feldmann & Schweitzer 2004; Schweitzer 2005).

Morris & Collins (1991), who described Prepaeduma decapoda Morris & Collins, 1991 from the Pliocene, considered Prepaeduma to be an ancestor to Paeduma in which P5 was not yet fully suppressed ( Schweitzer et al. 2000: 55). Beschin et al. (1994) justifiably doubted the placement of Prepaeduma in the Hexapodidae . Finally Schweitzer & Feldmann (2001: 335, 339) clearly demonstrated that the type material of the Pliocene P. decapoda was a composite. The holotype ( Morris & Collins 1991: fig. 56), without preserved pereopods, exhibits the seven sternites that are diagnostic of the Hexapodidae (in particular the well developped and similar sternites 5-7). Conversely, the paratype, which has a small but visible P5 ( Morris & Collins 1991: fig. 57), has been referred to Orthakrolophus bittneri ( Morris & Collins, 1991) , of the Chasmocarcininae Serène, 1964 ( Collins et al. 2003: 218, pl. 7, fig. 2). Collins et al. (2003: 220, pl. 7, fig. 8) provided a revised description of the true Prepaeduma decapoda , and referred it to Hexapus . Hexapus decapodus is a true hexapodid, with well visible seven thoracic sternites only, and shows five or six long stridulating ridges on the pterygostomian regions, typical of Hexapus (e.g., H. sexpes ).

For a discussion of the fossil representatives of Thaumastoplax , see Imaizumi (1959) and Collins & Morris (1976, 1978).

REMARKS ON THE HEXAPODID STRIDULATING APPARATUS

The stridulating apparatus consists in Paeduma of a prominent, narrow, and oblique row of rather thick striae on the pterygostomian region ( Fig. 2A, B ), and thin striae on the inner surface of the dactyl of both chelipeds.The same kind of stridulating mechanism is present in Hexaplax Doflein, 1904 ( Doflein 1904; Tesch 1918; see also Guinot-Dumortier & Dumortier 1960: 122, plectrum erroneously indicated on the propodus). The pars stridens and plectrum are well differentiated and consist of specialized striae, although in Hexaplax the distinction between these two structures lacks precision. In Stevea williamsi , thick and spaced pterygostomian striae are rubbed by thin and closed striae on the inner part of the dactylus of both chelipeds, as in P. cylindraceum . The fossil Stevea cesarii shows a similar stridulating pterygostomian row of striae.

A slightly distinct stridulating mechanism exists in a few other Hexapodidae . In Hexapus sexpes for instance, there is an area with several oblique and elongated striae disposed near the anterolateral angles of the buccal cavity; they are rubbed by thin, closed striae on the inner surface of the dactyl of both chelipeds (A. Milne-Edwards 1873: 254, pl. 12, fig. 1a; Tesch 1918: 240; Guinot-Dumortier & Dumortier 1960: 130, fig. 9; Manning & Holthuis 1981: fig. 32b; Manning 1982: 159, fig. 1d). Fossil Hexapus species probably show an area of oblique pterygostomian striae, as in Hexapu s pinfoldi Collins & Morris, 1978, from Eocene of Pakistan, with 14 oblique and postero-laterally directed pterygostomial striae, sometimes irregularly arranged ( Glaessner & Secretan 1987: 8, pl. 1, figs 5b, 6; Beschin et al. 1994: 194). A similar area exists in the extant Lambdophallus sexpes Alcock, 1900 , but the striae seem to be fewer and thicker ( Alcock 1900: 330; Alcock & McArdle 1903: pl. 62, fig. 1a). Hexalaughlia orientalis n. comb. and H. chuenensis n. comb. lack a stridulatory apparatus.

A stridulating apparatus was described as a prominent ridge in Goniocypoda edwardsi Woodward, 1867 and probably also in G. quaylei Crane, 1981 , both from the Upper Eocene of Hampshire ( Crane 1981: 6, 7, fig. 8D). This is an additional character supporting assignment of Goniocypoda Woodward, 1867 to the Hexapodidae .

Figures

FIG. 2. — Paeduma cylindraceum (Bell, 1859), holotype ♂ 15.3 × 23.5 mm, dry, “Gallapagos [sic] Mr Cuming” (OUMNH 15693): A, frontal view; B, different views of thoracic sternum and abdomen; C, mxp3 and stridulating striae; D, proximal articles of left P4, with apodemal platelet; E, posterior view of thoracic sternum to show reduced sternite 8. Abbreviations: a.p., apodemal platelet; a1-a6, abdominal segments 1-6; bc, carapace border; bi, basis-ischion; c, carapace; cd, articular condyle of cx4 on thoracic sternite 7; co, constriction at level of abdominal somite 6; cx1-cx4, coxae of P1-P4; e4-e7, épisternites 4-7; m, merus; p, projection of sternite 4; s8, exposed portion of sternite 8; t, telson; 1-7, sternites 1-7; 4/5-6/7, thoracic sternal sutures 4/5-6/7. Scale bars: 2.5 mm.