Diogenes Dana, 1851

Genus Diogenes Dana, 1851 View in CoL

Type species: Pagurus miles Fabricius, 1787, by subsequent designation of Stimpson (1859); Eocene–Recent.

Diogenes augustinus sp. nov.

Figs. 1 View Fig , 2 View Fig .

Zoobank LSID: urn:lsid:zoobank.org:act:FFF586B9­DBC5­455B­A38A­DD3011628FB9

Etymology: In honour of Agustín Gálvez (Zaragoza, Spain) who found and donated the specimen.

Type material: Holotype: MPZ2020 View Materials /54, articulated specimen, partial carapace, plus chelipeds and walking legs . Paratypes: MPZ2020 View Materials /55 and MPZ2020 View Materials /56, isolated chelipeds. From the type locality and horizon .

Type locality: Village of Lúsera , Huesca Province, Spain .

Type horizon: Arguis Formation , upper Bartonian, middle– upper Eocene .

Material.— Type material only. Diagnosis.—Shield longer than wide, length/width ratio 1.32, slightly convex transversely and longitudinally; rostrum not prominent. Orbital cavity concave, shallow antennal cavities. Regions of shield poorly marked; massetic region elongated, with perpendicular striate. Anterior branchial area globose, subtriangular. Eyestalks smooth, shorter than shield, diameter 1.42 mm. Chelipeds markedly unequal, left largest; palm quadrate, without stridulatory mechanism on mesial face of palm. Carpus triangular; axis of carpal­propodal articulation oblique in relation to sagittal plane. P2 merus depressed on outer side. Dorsolateral faces of propodus covered with small granules. Dactyls of P2 and P3 about same length as propodus, margin smooth.

Measurements (in mm).— MPZ 2020/54: shield length 10.74; shield width 8.08; eyestalk length 7.63; left manus length 15.12; left manus height 14.29; right manus length 8.01; right manus height 7.05. MPZ 2020/55: major cheliped height 19.35; length 17.74. MPZ 2020/56: major cheliped height 17.33.

Description.—Shield longer than wide, slightly domed; surface roughened by shallow grooves, except for most of gastric region which is smooth with a subtle longitudinal ridge; rostrum weakly developed; anterior margin weakly convex; orbital cavity shallow; post­ocular projection acute; post­antenal projection slightly rounded; central gastric furrow present, but weakly developed; massetic groove weakly marked; massetic region elongated, with perpendicular stretch marks; keraial region poorly differentiated; gastric regions flat; U­shaped cervical groove; posterior carapace not well preserved ( Fig. 1A View Fig ). Eyestalks slightly longer than half length of shield. Chelipeds markedly unequal, with the left considerably larger; without stridulatory mechanism developed on the mesial face of the palm. Major cheliped: palm longer than broad, lateral surface of carpus covered with densely small granules; upper margin with pointed conical granules; weakly granulated dorso­ventrally ( Fig. 1 View Fig ). Minor cheliped: palm covered by small and conic wide­spaced granules, upper margin with five pointed teeth. Merus and carpus of both chelipeds less granulated than palm; subtriangular fixed finger broad at base, with rounded distal extremity; fingers curved ventromesially; moveable finger longer than mesial margin of palm. Ambulatory legs symmetrical; merus subrectangular and elongate, upper margin convex, less ornamented than carpus and propodus; outer side of P2 merus depressed to accommodate P3; carpus subtriangular; carpus and propodus of P2–P3 with slightly serrated dorsal margins. Dactylus of ambulatory legs as long as propodus, broadly curved ( Figs. 1 View Fig , 2 View Fig ).

Remarks.—Reconciliation of fossils and neontologist classifications are problematic, because most of the taxonomic features to differentiate between modern genera are not preserved in extinct material (e.g., Jagt et al. 2006; Fraaije 2014; Fraaije et al. 2014). This drawback is exacerbated by the fact that the majority of fossil paguroids are found as disarticulated elements or, in the best cases, they are hidden inside gastropod shells; and this hampers our ability to provide a complete description of specimens. This explains why the systematic placement of fossil paguroids is fraught with difficulties ( Fraaije and Polkowsky 2016). Fortunately, the material described herein includes both the chelipeds and the carapace and represents one of the few examples in which all of the anterior portion of the animal is available.

Despite these drawbacks, McLaughlin (2003) listed important morphological characters to distinguish between different modern taxa that can also be recognised in fossil species. For instance, Fraaije et al. (2019), among others, included detailed data on shields of extant taxa that can be seen in fossils.

The material studied retains delicate diagnostic characteristics, such as the morphology of the shield, cheliped ornamentation, ocular peduncles and ambulatory legs. This allows its definite assignment to the family Diogenidae . We include the new taxon in Diogenes on the basis of similarities in features of chelipeds, pereiopods and distribution of dorsal carapace regions. Our material also shows corrugated and poorly defined massetic regions, a gastric region without Y­line and eyestalks that are long and thin ( Fig. 2 View Fig ). Diogenids of Eocene age include nine genera that have been described on the basis of isolated chelae only ( Garassino et al. 2009; De Angeli and Caporiondo, 2017), making comparison with our material difficult.

The modern genera Dardanus Paul’son, 1875 and Calcinus Dana, 1851 , show similar characteristics to those observed in the studied specimen. However, Diogenes augustinus sp. nov. differs from both genera in having a near­straight frontal area, a less well­developed rostrum, a well­defined anterior branchial region (triangular in shape), much larger ambulatory dactyli; in addition, it lacks a Y­line on the posterior part of the shield. The genus Petrochirus Stimpson, 1858 has a characteristic squamose ornamentation; moreover, the anterior branchial region is not clearly defined.

Stratigraphic and geographic range.— Type locality and horizon only.