Rogeryon Schweigert & Audo, 2017

Audo, Denis, Schweigert, Günter, Charbonnier, Sylvain & Haug, Joachim T., 2017, Systematic revision and palaeobiology of Rosenfeldia triasica and Rogeryon oppeli gen. et comb. nov. (Eucrustacea, Polychelida), European Journal of Taxonomy 367, pp. 1-23 : 9-11

Genus Rogeryon Schweigert & Audo gen. nov.

urn:lsid:zoobank.org:act:77B95F27-FB43-43C1-9085-88176C919CFF

Eryon – Woodward 1866: 500 (pro parte: E. oppeli ); 1881: 529 (pro parte: E. oppeli ); 1911: 307 (pro parte: E. oppeli ).— Glaessner 1929: 166 (pro parte: E. oppeli ).

Rosenfeldia – Schweigert 2004a: 70 (pro parte: R. oppeli ); 2004b: 329 (pro parte: R. oppeli ); 2015: 273 (pro parte: R. oppeli ). — Schweigert & Frattigiani 2005a: 198 (pro parte: R. oppeli ); 2005b: 328 (pro parte: R. oppeli ). — Garassino & Schweigert 2006: 30 (pro parte: R. oppeli ). — Schweitzer et al. 2010: 43 (pro parte: R. oppeli ). — Audo et al. 2016: 13, figs 1h–k (pro parte: R. oppeli ).

Type species

Eryon oppeli Woodward, 1866 .

Included species

Rogeryon oppeli gen. et comb. nov.

Diagnosis

Extremely thin exoskeleton; ovoid, slightly pear-shaped cephalothorax longer than wide, wider in its anterior half; very narrow frontal margin; small, shallow cervical and postcervical incisions; cervical groove strongly marked near cervical incision and near lateral margin, shallower between these two areas; postcervical groove strongly marked from postcervical incision toward median line, longer than ⅓ of carapace width; telson and uropods rounded; eyes with hexagonal ommatidia; median margin of third maxilliped ischium with a few proximal large teeth and numerous more distal small teeth (serrated aspect), first pereiopods about as large as succeeding ones.

Etymology

Dedicated to Roger Frattigiani, an enthusiast amateur fossil collector who co-operates with researchers since several years. Gender of the genus is masculine.

Remarks

Rogeryon gen. nov. is represented by a single, very rare species: Eryon oppeli Woodward, 1866 , from the Late Jurassic of Germany. The study of this species is complicated due to the preservation of many specimens: (1) the holotype does not preserve its carapace, but can fortunately be reliably associated with other specimens by the peculiar shape of tail fan and pereiopods; (2) most known specimens have a very thin carapace that was variably deformed; (3) some specimens (SMNS 66004/2, SMNS 70102), whose carapaces are divided across median line, probably correspond to exuvia (see Audo 2016). We remark that the thinness of the carapace is unlikely directly linked to the preservation of exuviae in the case of some specimens. Exuviae occur abundantly in Solnhofen-type outcrops ( Schweigert 2007a), but in polychelidans they do not present a particularly thin carapace, not as thin as that of E. oppeli or Knebelia bilobata (Münster, 1839) (see Audo et al. 2014b).

Since its first description, this species has never been studied in detail. Schweigert (2004a, 2004b), Schweigert & Frattigiani (2005a, 2005b) and finally Garassino & Schweigert (2006) questioned the original generic assignment to Eryon Desmarest, 1817 and proposed an assignment to Rosenfeldia based upon the denticulate margins of s4–s6, uropodal exopod and endopod and telson. Unfortunately, these characters cannot be considered as typical of Rosenfeldia since they occur in several other species of polychelidans. Indeed, a telson with spiny margins occurs in Coleia boboi Garassino & Gironi, 2006 , Proeryon hartmanni (Meyer, 1836) , Coleia barrovensis M’Coy, 1849 , Proeryon giganteus (Van Straelen, 1923) , Palaeopentacheles roettenbacheri (Münster, 1839) and Tethyseryon campanicus Bravi, Garassino, Bartiromo, Audo, Charbonnier, Schweigert, Thévenard & Longobardi, 2014 . A spiny uropodal endopod occurs also in Proeryon giganteus (Van Straelen, 1923) , Proeryon hartmanni and Palaeopentacheles roettenbacheri ; a similar uropodal exopod occurs in Proeryon hartmanni (with a diaeresis in this latter case). Actually, all these margins are often fringed with small spines and setae in extant species; indeed, these structures increase the surface of the telson and uropods. We therefore consider these characters to be only marginally informative. Rosenfeldia triasica and Eryon oppeli also share a rounded telson, but this character, although rarer, also occurs in Tetrachela raiblana (Bronn, 1858) . The rounded telson may therefore be a convergence between these three taxa.

Moreover, the redescription of Eryon oppeli allows the recognition of several characters almost unique to this species among other polychelidans: (1) an extremely thin exoskeleton, reminiscent of the thinness of the exoskeleton of Knebelia bilobata , K. schuberti (Meyer, 1836) and that of some extant species, but unlike that of other fossil species, which are distinctly thicker; (2) an ovoid, almost pear-shaped exoskeleton wider in its anterior half, distinct from other polychelidans (clearly ovoid, subcircular or pear-shaped, larger in posterior half in almost all other species); (3) a rounded telson, as in Rosenfeldia , rare in polychelidans (generally triangular); (4) a first pereiopod about as large as succeeding ones, generally larger in other polychelidans, except in the poorly preserved Wrangelleryon perrates Feldmann, Schweitzer & Haggart, 2013 ; (5) finally, as indicated by Audo et al. (2016), Eryon oppeli possesses hexagonal ommatidia, contrary to all other polychelidans, for which preserved quadratic ommatidia were documented. Considering only these points, E. oppeli could resemble W. perrates , but these two species do not seem to be closely related as they differ in many aspects: W. perrates is narrower, has petaloid uropodal endopod and exopod, and its P5 is larger in proportion. Their resemblance is therefore superficial and possibly linked to similar constrains on the shape of their pereiopods.

For these reasons, we consider that Eryon oppeli cannot be ascribed to any currently described genera of Polychelida and we propose the erection of Rogeryon to accommodate it.

The familial assignment of Rogeryon is not straightforward. Indeed, its morphology is very different from all other polychelidans. Its relatively wide carapace and the lack of a diaeresis on uropodal exopod could suggest an assignment to Eryonidae ; however, both these characters occur regularly within polychelidans. Furthermore, its hemicircular Mxp3 is clearly different from the subtriangular one of Eryonidae (and narrow one of Polychelidae ). Rogeryon bears some resemblance to the older Adamanteryon Audo, Schweigert, Saint Martin & Charbonnier, 2014 (uncertain Polychelida family): both have a very thin carapace marked by strong cervical and postcervical grooves, shallow cervical and postcervical incisions and a carapace wider in its anterior half. Nevertheless, Adamanteryon distinctly differs from Rogeryon by its slender P1 far longer than P2–P4 and a much broader, more angular carapace. In our current state of knowledge it is, therefore, impossible to assign Rogeryon to an existing family of Polychelida , and describing a new family would only complicate the systematics of polychelidan lobsters (which already comprise two monogeneric and monospecific families: the Tetrachelidae and the Palaeopentachelidae Ahyong, 2009 ).