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1. Introduction
Hermit crabs are an interesting group of decapod crustaceans with a scattered and fragmentary fossil record (i.e., Via, 1959; Beschin et al., 2002, 2005, 2010, 2012; De Angeli et al., 2009; Garassino et al. 2009a, 2009b; Pasini and Garassino, 2010a, 2010b; Pasini and Garassino, 2011; Fraaije et al., 2011, 2015, 2020; Garassino et al., 2014; Schweigert et al., 2013; Fraaije, 2014; Hyžný et al., 2016; De Angeli and Caporiondo, 2017; Ossó, 2020; Pasini et al., 2020). The vast majority of this group has a very specialized morphology bearing a poorly mineralized and asymmetric abdomen, as a result of a life confined to empty molluscs (e.g., Walker, 1992), or in other cavities like sponges, corals or plant remains (e.g., de Forges et al., 2001; Lemaitre, 1989, 1990). This peculiar morphology favours rapid disarticulation and most fossil remains correspond to isolated propodi (see Klompmaker et al., 2017).
Here we describe two new taxa of paguroids from the Eocene of the south-central Pyrenees Basins (Spain). Previous records of decapod crustaceans from this area are relatively common (i.e., Via, 1969, 1973; Fraaye, 1995; Fraaije and Pennings, 2006; Artal and Castillo, 2005; Van Bakel et al., 2012; Artal et al., 2013a, 2013b; Ossó et al., 2014; Dominguez and Ossó, 2016; López-Horgue and Bodego, 2017; Artal and Van Bakel, 2018a, 2018b, 2020; Ferratges et al., 2019, 2020a, 2020b; Artal and Van Bakel, 2020); but paguroids are rare and fragmentary (e.g.Via, 1959, 1969; Solé and Via, 1989; Ferratges et al., 2020a), with the exception of the semi-articulated specimen described by Ferratges et al. (2020b).
2. Geological setting
The southern Pyrenean basins record one of the most complete Eocene marine sedimentary successions in Europe, with decapod crustaceans previously described from several outcrops (see above). These basins were located in the south-central Pyrenean zone (Figure 1), and paleogeographically, were located in tropical latitudes during the Paleocene-Eocene (e.g., Hay et al. 1999; Silva-Casal et al. 2017). This area corresponded to an elongated gulf connected to the west by the Bay of Biscay, on the southern limit of the axial zone of the Pyrenees (Hay et al., 1999).
Figure 1 Geological and stratigraphic setting of the studied material. A: Location of the Tremp-Graus and Jaca-Pamplona Basins in the South-Central Pyrenean zone (modified from Dreyer et al., 1999). The red stars show the location of the studied outcrops (B, C).
The material described herein comes from two outcrops (Figure 1A); one corresponding to lower Eocene (middle Ypresian) sandstones and marly beds, in the lower part of the Roda Formation (Cuevas-Gozalo et al., 1985) (Figure 1B); and the other one corresponding to upper Eocene (Priabonian) sandstones and marly beds, in the upper part of the Arguis Formation (Millán et al. 1994; Silva-Casal et al., 2017) (Figure 1C).
The lower Eocene material was collected from the “Barranco de Bacamorta” in the neighbourhood of the village of Bacamorta, in northeaster Huesca province, Aragón, Spain (coordinates 42º21’48”N, 0º24’56”E). The Roda Formation includes highly fossiliferous intervals, which have provided an important assemblage of decapods; Zanthopsis dufuorii (H. Milne Edwards in d’Archiac, 1850), Periacanthus ramosusArtal and Castillo, 2005, Eosymethis aragonensisVan Bakel, Guinot, Artal, Fraaije and Jagt, 2012, Collinscarcinus obliquesulcatusArtal and Van Bakel, 2020, Archaeoportunus isabenensisArtal, Ossó and Domínguez, 2013, Gaudipluma bacamortensisArtal, Van Bakel, Fraaije and Jagt, 2013, have been previously documented. The material was collected from the upper portion of the beds bearing Gaudipluma bacamortensis in limestones of the Roda Formation, near to Bacamorta.
The upper Eocene material comes from Yeste, a small village in north Huesca province, Aragón, Spain (coordinates 42º23’09”N, 0º40’42”W). The Arguis Formation includes highly fossiliferous intervals, which have provided an important assemblage of decapods (see Ferratges et al., 2020a). The material was collected from the last carbonatic levels of sequence IV in the rail cut of the train going from Huesca to Canfranc. This corresponds approximately with the last levels of coral rudstones just below the Yeste-Arrés Formation, represented in section 11 of Morsilli et al. (2012).
From an environmental point of view, the Roda Formation represents a prodelta environment and the last intervals of the Arguis Formation are the last episodes of reef influence within the mesophotic ecological zone.
3. Material and methodology
Parapetrochirus robustus n. gen., n. sp. was collected from a single block of limestone that after preparation contained two chelipeds which that probably correspond to the same individual. Evidence supporting that both chelipeds correspond to the same individual include 1) close proximity of both chelipeds within the same rock and 2) morphological features of both chelipeds including a concavity without ornamentation in the upper inner portions and similar granulation pattern 3) there are a left and right cheliped, its sizes fit one specimen. E. gerardbretoni n. sp. was instead recovered from a carbonate block, with no other remains of crustaceans.
The specimens were prepared using a Micro Jack 2 air scribe (Paleotools) and binocular magnifying, and later photographed dry and coated with ammonium chloride sublimate. Detailed photography of the cheliped surfaces was made using a Nikon d7100 camera (Nikon, Tokyo, Japan) with a macro 60-mm-lens.
The specimens are deposited in the Museo Geológico del Seminario de Barcelona (MGSB) and the Museo de Ciencias Naturales de la Universidad de Zaragoza (Spain) (MPZ). Eocalcinus gerardbretoni n. sp. was collected under permit EXP: 032/2018 from the Servicio de Prevención, Protección e Investigación del Patrimonio Cultural (Gobierno de Aragón). Parapetrochirus robustus n. gen., n. sp. was collected in the early 80s and is housed within the historic collection of the Seminario de Barcelona.
4. Systematic Paleontology
Order Decapoda Latreille, 1802
Infraorder Anomura MacLeay, 1838
Superfamily Paguroidea Latreille, 1802
Family Diogenidae Ortmann, 1892
Genus Parapetrochirus n. gen.
Zoobank ID code: urn:lsid:zoobank.org:pub:92F-D0E58-ED01-438B-8417-85F13C7819B6
Figure 2 Parapetrochirus robustus n. gen., n. sp. (MGSB68569) from the Ypresian Roda Formation (Bacamorta, Huesca, Spain). A: lateral view of inner side of left chela; B: lateral view of outer side of left chela; C: frontal view of left chela; D: lateral view of outer side of right chela; E: frontal view of right cheliped; F: upper view of right cheliped; G: lateral view of inner side of right cheliped. Specimen whitened with ammonium chloride sublimated.
Figure 3 Idealized reconstruction of Parapetrochirus robustus n. gen., n. sp. The parts not preserved in the fossil material have been tentatively inferred from closely related modern taxa.
Type species: Parapetrochirus robustus n. gen., n. sp., by monotypy.
Diagnosis: Heterochelid chelipeds strongly robust, left largest. Left propodus large, palm subtrapezoidal, margins inclined; fixed finger notably large, fairly strong; dorsolateral surface of propodus covered with scale-shaped tubercles; lower margin laterally compressed, keel-shaped. Right palm subrectangular, triangular in cross-section; carpus subtriangular; axis of carpal-propodal articulation oblique in relation to sagittal plane. Except for the upper mesial portions, all segments are densely covered with a characteristic squamose ornamentation.
Etymology: The name refers to the similarity of the new genus to PetrochirusStimpson, 1858 due to its characteristic squamose ornamentation on the outer surface of the palms and dactylus.
Material: Two chelipeds of large size, with four articulated segments, merus, carpus, propodus (propodus comprises palm plus fixed finger) and dactylus. The number of the only specimen known and holotype is MSGB68569. Measurements: left propodus length: 51.93 mm; left manus length: 28.75 mm; left manus height: 26.58 mm; right manus length: 29.84 mm; right manus height: 20.04 mm.
Type locality: Bacamorta, Huesca Province, Spain.
Type horizon: Roda Formation, lower Eocene (middle Ypresian).
Parapetrochirus robustus n. sp.
Zoobank ID code: urn:lsid:zoobank. org:act:EB43F1F0-76DD-4B9F-AB0D-7837682A5462
Diagnosis: As for the genus, by monotypy.
Etymology: From the Latin word “robustus”, meaning robust.
Description: Paguroid of large size with markedly heterochely. Left propodus largest and conspicuously robust, palm subtrapezoidal, with oblique margins, converging towards the carpus; palm longer than high, higher distally, at the movable finger junction. Upper margin straight, fairly inclined, lower margin straight, also inclined. Both margins with dentiform tubercles. Inner and outer surface of palm densely tuberculated, covered with closely spaced squamose granules. Fixed finger extremely robust, distal tip of finger directed upwards; lower margin arched in the first portion; extremely keeled in the distal portion; occlusal edge bearing some few large denticles. Dactylus unknown.
Right cheliped large. Palm subrectangular, fairly longer than high, with the upper and lower margins straight, parallel, bearing small denticles. Carpo-propodial articulation sinuous. The ornamentation of the inner and outer surfaces consists of squamose closely spaced tubercles. Fixed finger long, straight, incomplete; the occlusal edge bearing some few strong teeth. Dactylus long, with semi-circular section, distal tip of finger curved, directed downwards; occlusal edge bearing few teeth. Both fingers covered with dense squamose granules. Inner and outer surfaces bearing scarce, small, setal pits. Carpus stout, subtriangular, with a strong extension for the articulation with the merus; mesial portion without ornamentation; upper margin with denticles; outer surface densely granulated. Merus subtrapezoidal, stout, with a large rounded extension and a notable concavity for the articulation with the carpus. Mesial portion nearly flat, somewhat concave, without ornamentation, lower margin strongly ridged. All segments in both chelipeds exhibit a notable concavity, without ornamentation, and a strong ridge, in the upper portion of the mesial view.
Remarks: The main characters of the new taxon fit the general diagnosis of the extant and extinct Diogenidae (McLaughlin, 2003; McLaughlin et al., 2007; De Angeli and Caporiondo, 2017). The general morphology of chelipeds, like shape and size of the propodus, the ornamentation, and the notable heterochely confirms affinities with different genera within the family. Thus, we can assign with confidence Parapetrochirus n. gen. to Diogenidae (see also McLaughlin et al., 2010).
Nevertheless, diagnostic characters of the studied specimen justify the description of a new genus. The combination of a robust subtrapezoidal palm, the extraordinarily large size and stout aspect of the fixed finger in the left propodus; with appreciable concavities in the upper mesial portions of the left and right chelipeds constitute an important difference with all other extinct and extant genera assigned to the Paguroidea (McLaughlin et al., 2010; De Angeli and Caporiondo, 2017; Schweitzer and Feldmann, 2001).
DiogenesDana, 1851, seems a close genus to Parapetrochirus n. gen. due to the notable differences in shape and size between the two chelipeds, being the left one the larger. The right cheliped presents important similarities, such as the general shape and the dense ornamentation (McLaughlin, 2003; Ferratges et al., 2020b). However, the new taxon exhibits important differences in the left cheliped including a robust propodus, with an especially large and broad fixed finger, extremely keeled lower margin and a concave upper margin.
Some Italian fossil records appear to be superficially close to the new genus, as Calcinus agnoensisBeschin, De Angeli, Checchi and Zarantonello, 2005 or Petrochirus saviiBeschin, De Angeli, Checchi and Zarantonello, 2012 (De Angeli and Caporiondo, 2017). The former exhibits a subcircular left chela, with arched margins, whereas Parapetrochirus n. gen. has a subtrapezoidal palm, with the margins inclined. The latter, is surprisingly assigned to the genus Petrochirus, indicating that has subequal chelipeds, while Petrochirus is diagnosed by having unequal chelipeds.
Other Italian representatives recovered from Eocene levels, as Pagurus valdagnensisBeschin, De Angeli, Checchi and Zarantonello, 2012 or Eocalcinus cavusBeschin, Busulini, De Angeli and Tessier, 2002 (Beschin et al., 2012). These fossils look superficially similar because of the shape of the propodus, with a robust fixed finger. Nevertheless, the large concavities in both chelipeds, and the peculiar left propodus provide important morphological differences. Petrochirus mezi Lörenthey, 1909 (Beschin et al., 2006, p. 99, fig. 9a, b) is larger and more robust than Parapetrochirus n. gen. In addition, it has a very different outline and the lower and upper margins do not have keels.
We can exclude the belonging of the new genus to PetrochirusStimpson, 1858 having the right cheliped bigger than the left (McLaughlin, 2003). The right chela seems similar. However, the fossil species of Petrochirus need to be revised (Bermúdez et al., 2017; De Angeli and Caporiondo, 2017; Vega et al., 2008) since most of them were erected based upon very fragmentary material or just because a superficial appearance due to the dense and characteristic squamose ornamentation.
North American representatives of the super-family, with very complete chelipeds also look superficially similar, as Paguristes hokoensisSchweitzer and Feldmann, 2001 or Pagurus malloryi Schweitzer and Feldmann, 2001. The former can be easily distinguished due to the subequal chelipeds, with thin and elongated fixed fingers; the latter exhibits a right cheliped larger than the left, as diagnostic in the genus PagurusFabricius, 1775.
Other families as Coenobitidae with terrestrial habitats, and peculiar left chelae or Xylopaguridae, which presents a notable asymmetry in the chelipeds and an especial habitat, are easily distinguished despite the common shared characters within the superfamily.
Family Calcinidae Fraaije, Van Bakel and Jagt, 2017
Genus EocalcinusVia, 1959
Type species: Eocalcinus eocenicusVia, 1959, by original designation.
Species: Eocalcinus albusBeschin, Busulini and Tessier, 2010; E. gerardbretoni n. sp. (herein); E. cavusBeschin, Busulini, De Angeli and Tessier, 2002; E. eocenicusVia, 1959.
Eocalcinus gerardbretoni n. sp.
Zoobank ID code: urn:lsid:zoobank. org:act:73E19EE8-9C1E-4C5A-8095-85347BA99F8F
Figure 4 Eocalcinus gerardbretoni n. sp. (MPZ 2020/617) from the Priabonian levels of the Arguis Formation (Yeste, Huesca, Spain) A: lateral view of outer side of left chela; B: lateral view of inner side of left chela; C: frontal view of left chela; D: upper view of left chela; E: inferior view of left chela. Specimen whitened with ammonium chloride sublimated.
Diagnosis: Left propodus hemicircular, stout. Palm higher than long; lower margin sinuous both in lateral and lower view. Fixed finger with occlusal edge sinuous, obliquely oriented.
Etymology: The species is dedicated to the French paleocarcinologist Gérard Breton, for a lifetime dedicated to the study of geology and paleontology and special interest in decapod crustaceans.
Material: Single specimen corresponding to the left chelae. Holotype MPZ 2020/617. Propodus length 23.30 mm; manus length: 20.76 mm; manus height: 20.02 mm.
Type locality: Yeste, Huesca, Spain.
Type horizon: Last carbonate level of the Arguis Formation in section 11 of Morsilli et al.,(2012).
Description: Stout left propodus planoconvex and subcircular. Lower margin sharp, strongly sinuous, in lateral and lower, inferior view; outer surface strongly convex; inner surface concave. Palm slightly higher than long, with a strong concavity at the base of the fixed finger. Fixed finger short, robust, with the first portion of the occlusal edge concave, and after strongly arched (strongly convex). Dactylus very robust, triangular in cross-section, with the occlusal edge concave, obliquely oriented, smooth. Ornamentation, on the fixed finger and lower portion of the palm, is densely covered with small granules, very close together (pavement-like) and very uniform. Upper portion of palm and dactylus is covered with circular, small granules of similar size alternated with some larger granules.
Remarks: The studied specimen can be assigned to Eocalcinus based upon the general outline of the left chela, being hemicircular in shape; the lower margin of the propodus that is concave in the middle portion; the upper margin of the palm which is broadly arched; the fixed robust finger, without teeth in the occlusal edge, joining tightly the movable finger; the dactylus, that exhibits a broadly arched upper margin; and the whole chela is densely ornamented with small granules.
We justify the description of the new species E. gerardbretoni based upon a more subcircular general outline; the palm is somewhat higher than long; a lower margin convex in the proximal portion and concave in the middle; an occlusal margin of the fingers sinuous; a fixed finger notably short, stout, a lower margin with a strong concavity in the first portion and a strong convexity in the median portion; and granulation in the lower portion of the palm with elongated small granules, obliquely oriented.
The type species, E. eocenicus, shows some differences with E. gerardbretoni n. sp. with a chela more hemielliptical in shape; propodus longer than high, the palm being elongated; the lower margin of the propodus is nearly straight, only slightly concave in the middle portion; the fixed finger has straight margins; the dactylus is nearly straight in the occlusal edge, and exhibits notable small teeth in the upper margin; the ornamentation in the lower portion of the palm consists of very small circular granules, very uniformly distributed (Via, 1959).
Eocalcinus cavusBeschin, Busulini, De Angeli and Tessier, 2002 has a more elongated chela; the palm longer than high; the lower margin of the propodus is nearly straight, only weakly concave; the fixed finger is much more elongated, and the occlusal margin is only somewhat arched; the lower portion of the palm bears larger granules (Beschin et al., 2002; De Angeli and Caporiondo, 2017). Comparison with E. albus, Beschin, Busulini and Tessier, 2010, is almost impossible because this species was described based upon only a single dactylus. However, this dactylus has a totally straight occlusal margin, and the upper margin is gently denticulated (Beschin et al., 2010).
All other species of Eocalcinus with the exception of the type species are represented by the left chelae (or a single dactylus of the left chelae in the case of E. albus). Recent finds of the right chelae of E. eocenicus allowed the assignment of this genus to the family Calcinidae (Ossó, 2020). The strong heterochely of Eocalcinus and its comparison with similar morphology in modern taxa (i.e. Coenobita), suggests the use of the left chela as an operculum that fitted tightly with the aperture of the host shell (Figure 5A).
Figure 5 Idealized reconstruction of Eocalcinus gerardbretoni n. sp. A: reconstruction of the defensive position and the opercular function of the left cheliped; B: reconstruction of the walking position. The right cheliped has been based on Eocalcinus eocenicusVia, 1959, recently described by Ossó (2020). The parts not preserved in the fossil material have been tentatively inferred from closely related modern taxa.
5. Final remarks and conclusions
The record of Eocene paguroids in Spain is poor and often fragmentary. Via (1959) described Pagurus marceti, Pagurus marini and Eocalcinus eocenicus based on very fragmentary material. Ferratges et al. (2020b) recently described a new and very complete specimen of Diogenes augustinus comprising both chelae, locomotory legs and the cephalic shield which clearly provide important morphological information. The description of Parapetrochirus robustus n. gen., n. sp. and Eocalcinus gerardbretoni n. sp. increases our knowledge and known number of paguroids in the Spanish Eocene. The former represents an interesting find in prodelta marls of the Roda Formation. The latter appears in the last levels of reef influence of the Arguis Formation and increases the spatial and temporal distribution of Eocalcinus to the western Pyrenees.
