Retroplumidae, Gill, 1894

GUINOT, DANIÈLE, TAVARES, MARCOS & CASTRO, PETER, 2013, Significance of the sexual openings and supplementary structures on the phylogeny of brachyuran crabs (Crustacea, Decapoda, Brachyura), with new nomina for higher-ranked podotreme taxa, Zootaxa 3665 (1), pp. 1-414 : 138-141

Family Retroplumidae

The male gonopore is coxal. The retroplumid P5 ( Fig. 44C ) are very much reduced, often filiform, fringed with particular setae, mobile, and “reduced to feather-like rudiments, arising close one another, high up, almost on the back” ( Alcock 1899b: 79, under Ptenoplax Alcock & Anderson, 1894 ), and thus markedly different from the ambulatory pereopods P2–P4. The small P5 coxae are reentrant (see Saint Laurent 1989: fig. 23a), located dorsally, and lie in a depression on the surface of the expanded sternite 7 ( Fig. 34 ). The long P5 basis-ischium is proportionally unusually longer than the other articles of the leg ( Fig. 44C ); the basis-ischium is normally short in the P2–P4. The wide thoracic sternum shows parallel, equidistant sutures 4/5–6/7 (in contrast to medially converging in eubrachyurans with a wide thorax), as confirmed by the regularly partitioned axial skeleton ( Fig. 45B ). Coxal apodemal platelets are dorsally visible only on the P4 coxae.

The true coxal condition of the male gonopore is immediately apparent. A thick penis is enclosed within the salient P5 coxo-sternal condyle and emerges from the extremity of the condyle, thus with condylar protection (see Modalities of penis protection: Condylar protection). The papilla is completely concealed by the laterally expanded abdominal somite 2, whereas the P5 condyle is exposed as does a small portion of sternite 8. The penis emerges close to the base of the G1. The modification of somite 8 involves the pleural, sternal, and appendicular portions. The last thoracic somite (somite 8) is strongly reduced. The P5 coxae, which are not aligned with the preceding coxae and lie on sternite 7, are, however, articulated on sternite 8 as usual. Sternite 7 is well developed and strongly tilted.

Sternite 8 greatly differs from sternite 7 in being markedly narrow and not aligned with the preceding sternites, as shown in Retropluma quadrata ( Fig. 34 ). Only a small portion of sternite 8 is visible dorsally, close to the P5 coxal condyle, between the lateral slit separating abdominal somites 1 and 2. The P5 thoracic pleurite (pleurite 8) is much reduced and partially encircles the P5 coxa, being only visible when the carapace is slightly lifted. Pleurite 8 is fused with sternite 8, without a recognisable suture between them. Pleurite 7 is large and almost aligned horizontally to sternite 7.

Retroplumids, which clearly show coxal gonopores ( Fig. 34 ), were nevertheless variously assigned. They were first thought to be aberrant Catometopa among the Goneplacidae ( Alcock & Anderson 1894: 179) . According to Alcock (1899b: 78, 79, as Ptenoplacidae ), who suggested homolid as well as dorippid affinities, retroplumids were true catometopes “but of an archaic type ”, notably on account of the typically catometope form and position of the male gonopores at the base of the P5 “on a tubercle embedded in a notch in the posterior border of the sternum” (the tubercle is actually the P5 coxo-sternal condyle). Alcock (1900b: 285, 455, as Ptenoplacidae ) saw a superficial resemblance to Macrophthalmus , but concluded: “though it [ Retropluma notopus ( Alcock & Anderson, 1894) ] has no look of Hexapus , yet shows an attraction to Hexapus ”. Borradaile (1907: 482) remarked that the male gonopore was not sternal but that the “duct passes along a sternal groove to the coxopodite”. The family was left in the Catometopa by many authors (e.g., Tesch 1918a: 29–33; Rathbun 1918: 15). Balss (1944: 604; 1957: 1633, 1662) placed Retroplumidae in the vicinity of Palicidae and indicated a coxal male gonopore inside a sternal groove. Sakai (1976: 322, 592) treated Retroplumidae (together with Palicidae ) within the thoracotremes. The Retroplumidae was included in Heterotremata by Guinot (1978a: 251, as a tentative inclusion in Dorippoidea ), Schram (1986: 308), Guinot & Bouchard (1998: 651), Guinot & Quenette (2005: 332), and McLay (2006a: 376), but in Thoracotremata by Schweitzer & Feldmann (2001b: 201).

Saint Laurent (1989) considered Retroplumoidea a separate superfamily among the Heterotremata. She did not state any particular relationships with Hexapodidae despite observing a shared character (i.e., the loss of the exopodites of the female pleopods 2, perhaps related to the narrowness of the female abdomen), and did not recognise connections with other brachyuran families. Martin & Davis (2001: 74), Števčić (2005: 105), and Ng, Guinot & Davie (2008: 181) concurred and did not group any other extant families within Retroplumoidea (see Affinities between Palicoidea , Retroplumoidea and Hexapodoidea ).

Some fundamental traits of Retroplumidae must be taken into account, in particular the plesiomorphic eyes and cephalic appendages, the absence of orbits, the marked pterygostomial lobe clearly figured by Saint Laurent (1989: fig. 4), and the wide junction of the thoracic sternum with the branchiostegite posterior to the chelipeds (sternum/branchiostegite junction), thanks to a lateral extension of episternite 4. On the contrary, a sternum/ pterygostome junction in front of the chelipeds does not exist, so the Milne Edwards openings are contiguous to the chelipeds in retroplumids as in most eubrachyurans. These two characters are shared with Palicoidea (see Affinities between Palicoidea , Retroplumoidea , and Hexapodoidea ; Carcinisation and its outcomes: Cephalic condensation).

Fossil Retroplumidae . The Retroplumidae , represented in the living fauna by a reduced number of representatives (only two genera and ten species; see Ng, Guinot & Davie 2008: 181), is known from many fossils, an indication of an old flourishing group since the Cretaceous (see Vega & Feldmann 1992: fig. 7), from soft bottoms similar to that of extant members ( Saint Laurent 1989; Artal et al. 2006). For most palaeontologists, the Retroplumidae is a catometope family close to Ocypodidae and even part of Ocypodoidea ( Beurlen 1930: 350– 352; Glaessner 1969: R531; Collins & Morris 1975: 823; Via 1957: 553; Vía Boada 1969: 322–328; 1980: 4, fig. 1, table 1; 1982: 115, figs. 1, 2), namely a thoracotreme family ( Vega & Feldmann 1992: 139; Collins et al. 1994: 29; Feldmann & Martins-Neto 1995: 610; Vega et al. 1995: 347; Feldmann et al. 1995: 16; Feldmann et al. 1997: 126). Beschin et al. (1996), Collins in Collins et al. (2003), Schweitzer, Feldmann, Fam, Hessin, Hetricks, Nyborg & Ross (2003), De Grave et al. (2009), Schweitzer et al. (2010), Van Bakel, Artal, Fraaije & Jagt (2010), and Hyžný & Müller (2010) followed Saint Laurent (1989) in considering a separate superfamily Retroplumoidea .

The weakly specialised ornamentation of the carapace of † Archaeopus Rathbun, 1908 (type species: † Archaeopus antennatus Rathbun, 1908 [ Rathbun 1908: 346, pl. 47, figs. 4–7, pl. 48, pl. 49, figs. 2–4; from the Upper Cretaceous of North America and Japan]), has been regarded as representing the ancestral retroplumid morphology ( Bishop 1983b: 428; Beschin & De Angeli 2003: 12). According to Vega & Feldmann (1992; p. 148), † Archaeopus gave rise to † Costacopluma Collins & Morris, 1975 , and is considered one of the oldest known retroplumid genera ( Vía Boada 1980: 64; Schweitzer & Feldmann 2001b: 202). † Archaeopus has provoked much discussion, and its status is not yet resolved (Artal et al., unpublished). The distinctive dorsal and reentrant P5 of † A. antennatus suggested a retroplumid condition to Rathbun (1908). Glaessner (1969: R531) considered † Archaeopus Rathbun, 1908 , and † Retrocypoda Via, 1957 , to be possible members of the Palicidae . The shape of the thoracic sternum and male abdomen of † A. antennatus did not support such an assignment according to Saint Laurent (1989: 148), who excluded the genus from Retroplumoidea , without recognising, however, any relationships with the Palicidae . McLay (2006a: 387, 389, table 3), a neontologist, who regarded the Retroplumidae as “an enigmatic group whose proper place amongst the Brachyura is uncertain”, excluded from the Retroplumidae several species of † Archaeopus , at least † A. antennatus , † A. ezoensis (Nagao, 1941) , † A. lunicarina Schweitzer & Feldmann, 2001 , † A. rathbunae Beurlen, 1965 , and † A. vancouverensis (Woodwards, 1896) , plus other retroplumid genera (see below), and included all of them in Palicidae . McLay (2006a) also suggested the establishment of a new family for † Archaeopus . Thus, † Archaeopus has been either included in or closely related to the Retroplumidae ( Beurlen 1930; Vía Boada 1969; Collins & Morris 1975; Vega & Feldmann 1992; Beschin & De Angeli 2003; Schweitzer, Lacovara, Smith, Lamanna, Lyon & Attia 2003; De Grave et al. 2009; Schweitzer et al. 2010) or removed from it ( Saint Laurent 1989; McLay 2006a).

The male abdominal somite 6 of † A. antennatus in fact distally lacks the lateral expansions for abdominal locking that are characteristic of extant retroplumids ( Guinot & Bouchard 1998: 651, fig. 17A–C) and clearly visible in fossils, for example in the Campanian † Costacopluma concava Collins & Morris, 1975 ( Collins & Morris 1975: pl. 97, only fig. 4) and the Maastrichtian † C. mexicana Vega & Perrilliat, 1989 ( Vega & Perrilliat 1989: fig. 2h). Moreover, † A. antennatus has free abdominal somites (some are fused in extant retroplumids) and a narrower thoracic sternum than in modern retroplumids. Lastly, its male abdomen ( Rathbun 1908: 47, fig. 6) differs from that of known retroplumids in having a non crescent-shaped abdominal somite 6 and from that of the Palicidae in being shorter (long abdomen, almost reaching mxp 3 in the Palicidae ). A species such as † A. lunicarina has indeed palicid carapace and chelipeds, and could belong in Palicidae , but evidence provided by the ventral surface is needed. See Fossil Palicidae .

The dorsal carapace of † Archaeopus , which has a less flat body than other retroplumids, shows wide variation: poorly developed ridges and developed regions in † A. lunicarina , from the Late Campanian- Maastrichtian of Alaska ( Schweitzer & Feldmann 2001b: 202, figs. 17, 18); quadrate outline and well-developed transverse swellings in † A. mexicanus Schweitzer, Feldmann, Gonzáles-Barba & Vega, 2002 , from the Upper Cretaceous ( Schweitzer et al. 2002: 15, figs. 17, 18); defined regions, sometimes with thick transverse ridges in more recent species, such as † A. antennatus ( Schweitzer et al. 2002: 16; Schweitzer, Feldmann, Fam, Hessin, Hetricks, Nyborg & Ross 2003: 47, 50), † A. ezoensis , from the Late Cretaceous of Japan (Collins et al. 1993: 304; Karasawa & Schweitzer 2004), † A. vancouverensis from the Campanian-Maastrichtian of British Columbia and Vancouver I. ( Bishop 1986b: fig. 10C, D; Schweitzer & Feldmann 2001b: 202, fig. 19; Schweitzer, Feldmann, Fam, Hessin, Hetricks, Nyborg & Ross 2003: 49, fig. 16.3), and † A. schencki (Van Straelen, 1939, as † Martinezicancer schencki ) from the Paleogene ( Karasawa & Kato 2003b: 145, table 9).

† Archaeopus rathbunae Beurlen, 1965 ( Beurlen 1965: 271, fig. 4), from the Middle Cretaceous (Albian) of Brazil, not considered to be a retroplumid ( Vía Boada 1980: 54, 64), with a carapace resembling that of a carcineretid ( Vega & Feldmann 1992: 147), was suggested to probably belong in † Carcineretidae Beurlen, 1930 ( Schweitzer & Feldmann 2001b: 202) but was not listed by Schweitzer et al. (2010). It remains enigmatic, despite its supposedly dorsal P5. † Archaeopus senegalensis Rémy in Gorodiski & Rémy, 1960, from the Eocene of Senegal, first attributed to Palicidae , was included in † Costacopluma ( Collins & Morris 1975: 827, fig. 1; Feldmann & Martins-Neto 1995: 610; Feldmann & Portell 2007: 91, fig. 2F; Ossó-Morales et al. 2010: 222, table 2), but not mentioned by Schweitzer & Feldmann (2010d) in their revision of Remy’s type material deposited in the MNHN.

True retroplumids are crabs often with strong transverse ridges on the carapace, sternum and abdominal somites, a characteristic of certain fossil species (Collins et al. 1993: 306). Examples are: † Retropluma eocenica Via, 1959 , from the Eocene of Italy and Spain, with characteristic carapace ridges ( Artal et al. 2006; Beschin et al. 2009: 75: pl. 3, fig. 6; Van Bakel et al. 2010: 46); † R. laurentae Collins in Collins, Lee & Noad 2003, from the Miocene of Indonesia, with a suboval carapace and a discontinuous posterior ridge (Collins in Collins et al. 2003: 209, pl. 7, figs. 3, 4) [the extant Retropluma laurentae McLay, 2006 , from the Western Pacific, is a junior homonym of † R. laurentae Collins in Collins, Lee & Noad 2003, and was renamed R. solomonensis McLay, 2006 ; see McLay 2006b]; † R. borealis Fraaije, Hansen & Hansen, 2005 , from the late Miocene of Denmark, with straight dorsal ridges ( Fraaije et al. 2005: 56, fig. 4, pl. 1, figs. 5, 6); † Loerenthopluma lata Beschin, Busulini, De Angeli & Tessier, 1996 , from the Middle Eocene to Oligocene of Europe, with a wide carapace and a discontinuous posterior ridge ( Beschin et al. 1996; see Hyžný & Müller 2010); † L. danielae Van Bakel, Artal, Fraaije & Jagt, 2010 , from the Lower Eocene of Belgium, with a long rostrum and well-preserved P5 merus and ventral surface, thus with complete thoracic sternum and abdomen and a typical locking system ( Van Bakel et al. 2010: fig. 1). † Goneplax ? craverii Crema, 1895 , from the Pliocene of Italy, with a long rostrum, three transverse ridges on carapace (the median one being interrupted), cited twice in Schweitzer et al. (2010: 100, as † Retropluma craverii ; 135, as † Goneplax craverii ), is presently assigned to Retropluma ( Vía Boada 1969: 324, 339, 342; Beschin et al. 1996: 95; De Angeli et al. 2011: 39, figs. 1–4; see also Garassino et al. 2013).

The two retroplumid subdivisions suggested by Saint Laurent (1989: 149), Retropluminae Saint Laurent, 1989, and †Costacopluminae Beschin, Busulini, De Angeli & Tessier, 1996 [a nomen proposed conditionally by Saint Laurent in 1989, thereby not available according to the International Code of Zoological Nomenclature, Art. 15.1 ( Anonymous 1999)], have been recognised by Beschin et al. (1996) but not by other palaeontologists, e.g., Schweitzer & Feldmann (2001b: 201), and they are not listed by De Grave et al. (2009: 41) and Schweitzer et al. (2010: 9).

According to Mclay (2006a: 389, table 3) † Costacopluma must be regarded as “undoubtedly the oldest known genus of retroplumids, in both the Americas and Africa”, with only four species that should be considered true retroplumids. For example, † C. salamanca Feldmann, Fernanda Rodriguez, Martinez & Aguirre-Urreta, 1997 , may be a species of Bathypluma , a suggestion not followed by Schweitzer et al. (2010); † C. bishopi Vega & Feldmann, 1992 (Late Cretaceous, Mexico), † C. binodosa Collins & Wienberg Rasmussen, 1992 (Late Cretaceous, Greenland), and † C. mexicana Vega & Perrilliat, 1989 (Late Cretaceous, Mexico) could be very likely members of Palicidae , but all these species were left as referable to Retroplumidae by Schweitzer et al. (2010: 99; see discussion in Armstrong et al. 2009; Nyborg et al. 2009). According to Ossó-Morales et al. (2010: table 2), the 14 species of † Costacopluma that have been reported may be distributed in two main groups, either with an ovate carapace (from the Cretaceous to the Eocene of Africa and America), or with an inverted subtrapezoidal carapace (from the Paleocene of America), without doubting that they are true retroplumids.

Figures

FIGURE 34. Coxal condition in Retroplumidae, with penis emerging from extremity of P5 coxo-sternal condyle. Retropluma quadrata Saint Laurent, 1989, male 10.4 × 12.0 mm, Philippines (MNHN-B19535). A, ventral view; B, posteroventral view; C, D, schematic representations of A and B. c, coxo-sternal condyle; cp, carapace (indicated by dotted line in C); cx4, cx5, P4, P5 coxae; p, penis; p5, fifth pereopod; pl7, pleurite of thoracic somite 7; pl8, pleurite of thoracic somite 8 fused to sternite 8; 6–8, thoracic sternites 6–8; 5/6–7/8, thoracic sternal sutures 5/6–7/8.

FIGURE 44. P5 ischium, long in Palicidae (A) and Retroplumidae (C), short in Crossotonotidae (B). A, Palicoides longimanus (Miyake, 1936), female 14.7 × 17.0 mm, New Caledonia (MNHN-B26791); B, Crossotonotus spinipes (de Man, 1888), male 36.9 × 43.3 mm, New Caledonia (MNHN-B26803): penis emerging from P5 coxo-sternal condyle and pulled out from sternite 8 invagination; basis-ischium concealed under coxa. C, Retropluma quadrata Saint Laurent, 1989, with attached ciliates (after Saint Laurent 1989: fig. 9; unknown size). b, basis; c, coxo-sternal condyle; cx5, P5 coxa; i, ischium; p, penis; pa, papilla. Scale bars: 3 mm (A); 1 cm (B).

FIGURE 45. Skeletons with parallel endosternites and narrow sella turcica. A, Pseudopalicus declivis Castro, 2000 (Palicidae), male, New Caledonia (MNHN-B30492): medially attenuated endosternites, remaining lateral portions covered by pleurites; marked median plate; B, Bathypluma spinifer Saint Laurent, 1980 (Retroplumidae), male, Philippines (MNHN-B37017): cx5, P5 coxa; m, median plate; p, pleurite; P5, pereopod 5; s, sella turcica; s.c., convexity of sterno-abdominal cavity; t, endosternite.