1. Introduction

Records of fossil homolids in the Eocene of Europe are not uncommon, for instance in northern Italy, where several genera and species of Homolidae ^{De Haan, 1839} have been described (^{De Angeli et al., 2019, p. 20}), as well as in the British Isles (^{Collins et al., 2020, p. 32}) and Denmark (^{Collins et al., 2005, p. 19-20}). Records of the Homolidae have never been reported to date from the Eocene of the Iberian Peninsula. Therefore, this is the first report for the family in the Eocene of the Iberian Peninsula. Here we report the first occurrence fossil homolid crabs in the Eocene of the Iberian Peninsula. The material is represented by a crab carapace fragmentarily preserved, decorticated, and preserving some cuticle remains and pereopods, recovered from early Eocene strata near the village of Roda de Isábena (Huesca province, Aragon, Spain). A proper mechanical preparation of the holotype, partially destroyed by a previous inadequate attempt of preparation, allowed us to reveal important elements, such as the complete left cheliped and partial ambulatory legs. The remains of the intralineal carapace and of the left extralineal flank, with partially preserved cuticle, and the peculiar morphology of the left cheliped, as well as the presence of the linea homolica, indicate a clear affinity with Homolidae.

Here we describe Paromola bretoni n. sp., a new fossil species of the extant genus Paromola, with only two other fossil species known: P. rosenburgensis^{Nyborg and Garassino, 2017}, and P. vetula^{Crawford, 2008} (see ^{Nyborg and Garassino, 2017}).

Extant species of Paromola are spread world-wide (^{Guinot and Richer de Forges, 1995, fig. 70}), usually inhabiting the upper bathyal zone common among homolids, albeit they can also inhabit shallower depths (ibid, p. 489-491, fig. 76), similar to the palaeoenvironment where the fossil specimen dwelt. Explanation on the former and current systematics of Homoloidea De Haan, 1839 are summarized by ^{Guinot (2019, p. 753)}.

2. Geological setting

The Tremp-Graus Basin (southern Pyrenees) records marine sediments from the Thanetian-lower Cuisian (lower Ypresian, lower Eocene). Paleogeographically, during the Paleocene-Eocene, these basins corresponded to an elongated gulf connected to the west by the Bay of Biscay, located on the southern limit of the axial zone of the Pyrenees, in tropical latitudes (^{Hay et al., 1999}). The Upper Palaeocene to Lower Eocene in this area consists in the following formations: Serraduy, Puebla Limestone, Roda, Morillo and San Esteban (Figure 1); and it is bounded by unconformities related to tectonic activity (^{Leren et al., 2010}).

Figure 1 Map location and explanation of formations. Red star indicates outcrop where Paromola bretoni n. sp. was recovered. 

The studied material comes from the exposed outcrop of prodelta marls corresponding to the Roda Formation (Figure 1), located near to Roda de Isábena (northeast of Huesca, Spain). This formation exposes a thick succession of marls and sandstones and has provided an interesting decapod assemblage (e.g. ^{Vía Boada, 1959}; ^{Vía, 1969}, ¹⁹⁷³; ^{Artal and Castillo, 2004}; ^{Artal et al., 2005}, ^2013a, ^2013b; ^{Schweitzer et al., 2007}; ^{Van Bakel et al., 2012}).

The Roda Formation, is coarsening and shallowing upwards (^{Molenaar and Martinius, 1990}), and show a general progradational pattern to the S and SW (^{Puigdefàbregas et al., 1985}; ^{Molenaar et al., 1988}; ^{Lopez-Blanco et al., 1991}, ²⁰⁰³; ^{Nio and Yang, 1991}; ^{Lopez-Blanco, 1996}; ^{Molenaar and Martinius, 1996}; ^{Leren et al., 2010}).

This formation has been biostratigraphically dated as lower Ypresian in age (^{Samsó et al., 1990}; ^{Tosquella et al., 1990}) and consists of a succession of up to 200 m thick formed of shallow-marine, mixed siliciclastic-carbonate deposits (^{Martinius and Molenaar, 1991}).

Abbreviations. MPZ: Museo de Ciencias Naturales de la Universidad de Zaragoza (Spain). ICMD: Institut de Ciències del Mar (ICM-CSIC), Barcelona (Spain).

3. Systematic palaeontology

Type species. Dorippe cuvieri^{Risso, 1815}, by original designation.

Species. Paromola bathyalis^{Guinot and Richer de Forges, 1995 (extant)}; P. crosnieri^{Guinot and Richer de Forges, 1995 (extant)}; P. cuvieri^{(Risso, 1815) (extant)}; P. japonica^{Parisi, 1915 (extant)}; P. macrochira^{Sakai, 1961 (extant)}; P. rathbunae^{Porter, 1908 (extant)}; P. roseburgensis^{Nyborg and Garassino, 2017 (fossil)}; P. vetula^{Crawford, 2008 (fossil)}.

Figure 2 Paromola bretoni n. sp., holotype MPZ 2020/615. A) dorsal view; B) frontal view; C) left lateral view; D) ventral view; E) close-up of dactyli. Abbreviations: ba = possibly antennular basal article; lh = linea homolica; psr = pseudorostral spines; P2-P4 = ambulatory legs; r = rostrum; rl = rostral lobe; s1 = pleonal somite 1. Photographs by Isabel Pérez Urresti. Scale bar A, B, C, D, equal to 10 mm; E, equal to 5 mm. (Photographs by Isabel Pérez Urresti). 

Figure 3 Idealized reconstruction of Paromola bretoni n. sp., the shaded parts taken from the specimen, and light parts taken from extant Paromola spp. (without scale). 

Diagnosis. Dorsal carapace longitudinally subrectangular; regions fairly defined, marked by shallow incised grooves, some culminated by upward directed spines at anterior third of cara-pace; widest at level of the mesobranchial region, narrowing in anterior third. Long, pointed, and moderately upturned rostrum; pseudorostral spines directed upwards, originating just above upper margin orbits. Posterior margin concave axially with two lateral reentrants. Cheliped extremely long, subcircular in section, spiny; carpus very elongated; ambulatory legs long, spiny, subcircular in section.

Etymology. Honoring the late Gérard Breton, French palaeocarcinologist.

Material. Holotype MPZ 2020/615, decorticated crab carapace preserving few remains of cuticle, and complete left cheliped and remains of left ambulatory legs.

Extant comparative examined material. Fourteen samples of Paromola cuvieri from the Biological Reference Collections (CBR) at the Institut de Ciències del Mar (ICM-CSIC,Barcelona,Spain)(see ^{Guerrero et al., 2020}), under the catalog/accession number: ICMD002504, female, juvenile, Namibia; ICMD002505, male, Namibia; ICMD002502, male, juvenile, Barcelona; ICMD002023, female, Catalonia; ICMD119/1998, female, Namibia; ICMD002499, female, Namibia; ICMD002498, male, juvenile, Namibia; ICMD002501, male, juvenile, Namibia; ICMD002500, male, juvenile, Namibia; ICMD002503-1, male, juvenile, Namibia; ICMD002503-2, female, juvenile, Namibia; ICMD120/1998, male, juvenile, Catalonia; ICMD121/1998, male, juvenile, Catalonia; ICMD122/1998, male, juvenile, Catalonia.

Locality Type. Roda de Isábena, Huesca, Spain. Stratigraphic position and age. Roda Formation, late Ypresian, early Eocene.

Measurements. Holotype MPZ 2020/615. Carapace length = 36.05 mm: carapace width: 35.44 mm (without lateral spines; half carapace 17.72 mm x 2). Left cheliped (P1): merus = 52.39mm x 6.17mm; carpus = 27.72 mm x 6.7mm; chela = 59.52 x 10.16mm (propodus + dactyli).

Description. Carapace longer than wide, subrectangular, longitudinally ovate in outline; widest at metabranchial region, narrowing in anterior third; fairly defined carapace regions; remains of cuticle of dorsal surface thin, appears gently granulate smooth, bearing presumably large upward directed acute spines (scars). Convex lateral margins of dorsal carapace, weakly vaulted longitudinally and weakly vaulted transversely in intralineal areas. Rostrum simple, pointed, rounded in section, presumably well developed and moderately upturned, rostral lobe axially sulcate; pseudorostral spines present on anterior margin, directed upward. Orbital cavity rounded, upper margin rimmed.

Anterolateral margin not preserved. Posterolateral margin convex, with at least two laterally-projecting spines. Posterior margin rimmed, consisting of three concave arcs, almost equal in length: one axial, and two lateral. Gastric regions not well defined, Epigastric region with two anterior swellings. Protogastric region with four transversely aligned spines (broken). Urogastric region poorly defined, weakly depressed. Cardiac region sub-triangular, with three swellings, two anterior separated by a shallow depression, and one posterior. Intestinal region poorly defined, depressed. Mesobranchial regions positioned on either side of urogastric region, weakly inflated, with at least one small spine (broken); metabranchial regions weakly inflated, with at least two small spines (broken).

Cervical groove pronounced, directed to the flanks of the carapace. Branchiocardiac groove shallow, interrupted medially. Linea homolica sinuous. Left cheliped extremely long, subcircular in section; merus strongly spinose; carpus very elongated, both, carpus and propodus, covered with small spines. Dactyli subequal, dark colored, with several setal pits; dactylus with one proximal tooth in occlusal margin; polex edentulous. Meri of P2 to P4, very elongated, with three to six large spines in the upper margin, and small spines in evenly spaced linear arrangement along the whole length of the lower margin.

4. Discussion

Although the studied specimen is partially preserved, some diagnostic characters, such as the well-preserved and extremely long left cheliped, the pointed simple rostrum, the pseudorostral spines, and the linea homolica, allow to assign it within the Homolidae with confidence.

The combination of these characters is similar to that of Paromola, and fits perfectly with the diagnosis for the genus provided by ^{Guinot and Richer de Forges (1995, p. 358-359)}, in particular the acute simple rostrum, developed pseudorostral spines, and the long carpus of an extremely long cheliped. Although some other homolids also possess a simple rostrum and long chelipeds, they differ substantially from the studied specimen. For instance, some species of Homola^{Leach, 1816}, possess acute rostrum, and long chelipeds, but they are shorter than in the new species (see ^{Guinot and Richer de Forges, 1995, p. 335-338, fig. 11a-b}). As well, species of Moloha^{Barnard, 1947}, possess long pseudorostral spines, and dorsal carapace with long and sharp spines, which are absent or not preserved in the studied specimen (see ^{Guinot and Richer de Forges, 1995, p. 376-391, figs. 28-33}). Dagnaunus pritchardi (^{Jenkins, 1977}), from the early Oligocene of South Australia, formerly described as Paromola pritchardi, and subsequently transferred to the former genus by ^{Schweitzer et al. (2004, p. 134)}, has longer pseudorostral spines than P. bretoni n. sp. (see ^{Jenkins, 1977}).

Based upon the above mentioned characters, and the relatively large size, the placement of that specimen within Paromola, seems appropriate until new material confirms the systematic position.

The state of preservation of the carapace of Paromola bretoni n. sp., precludes us to compare it further with the fossil species above indicated. Extant species of Paromola possess a thin carapace (P. Abelló, pers. comm.), so it can be inferred that the fossil species had the same fragile cuticle, as the studied specimen, which does not help to preserve the entire carapace. However, P. bretoni n. sp. differs from P. roseburgensis from the early Eocene of Oregon (USA) in having larger size, apparently shorter pseudorostral spines, and different shape of cheliped, which is shorter in P. roseburgensis (see ^{Nyborg and Garassino, 2017, p. 140-142, figs. 3, 4}). Moreover, Paromola bretoni n. sp. differs from P. vetula, from the Oligocene of Río Negro (Argentina) by its larger size, and its remains of dorsal carapace preserved are less granulated than in P. vetula (see ^{Crawford, 2008}). Londinimola williamsi^{Collins and Saward, 2006}, from the lower Eocene of Essex (UK), draws attention by its size, and rostral and pseudorostral features, both similar to those of Paromola bretoni n. sp. Apparently, Londinimola williamsi is the closest fossil species to our species, by sharing a simple rostrum, pseudorostral spines above upper orbital margin, dorsal regions fairly defined, some culminated by upward directed spines, and linea homolica well defined, but it differs from Paromola bretoni n. sp. in having a downturned rostrum instead of upturned; as well, the preserved cheliped and remains of ambulatory legs of P. bretoni n. sp. are clearly longer and robust than in L. williamsi. In any case, the poor preservation of the new species, does not permit a more detailed comparison (see ^{Collins and Saward, 2006, p. 69-70, pl. 1, figs. 1a-c, 2a, 2b}).

Similarly, comparing P. bretoni n. sp. with the extant species of the genus Paromola is difficult. However, despite the crushed carapace of Paromola bretoni n. sp. could disguise the true length of it, none of those extant species seem to have such left cheliped span with respect to the carapace length. In this sense, examination and measurements of the extant specimens of P. cuvieri above listed, show different ratios than P. bretoni n. sp., for instance the carapace length-merus P1 length ratio is 0.600, whereas in P. bretoni n. sp. that ratio is 0.688, which means that the new species has longer left cheliped with respect to the carapace size than extant P. cuvieri, which would favour the specific differentiation between them. Other comparative measurements regarding the carpus of P. bretoni n. sp., such as carpus length-merus and length-ratio, do not show substantial differences: 0.505 in P. cuvieri vs. 0.529 in P. bretoni n. sp. In any case, the temporal span separating P. bretoni n. sp. from the extant species, justify the erection of a new species for the Aragonese specimen.