Eopolyonyx bretoni (Crustacea, Anomura), a new genus and species of Porcellanidae from the Late Eocene of northeast Italy

Abstract

Eopolyonyx bretoni (Anomura, Porcellanidae) new genus and species is described on the basis of a single, almost complete small carapace preserved in an upper Eocene built-up at Parona (Verona, NE Italy).

Riassunto

Eopolyonyx bretoni (Crustacea, Anomura), nuovo genere e specie di Porcellanidae dell’Eocene superiore dell’Italia nordorientale.

Viene descritto il nuovo taxon Eopolyonyx bretoni (Anomura, Porcellanidae) di cui è noto un unico esemplare, un minuscolo carapace quasi completo rinvenuto in una bioerma dell’Eocene superiore di Parona (Verona, Italia nordorientale).

Introduction

Late Eocene crustaceans recovered from small coral-algal domes cropping out near Parona (Verona, Veneto) have been recently studied by BESCHIN et al. (2009, 2019, 2021b). They represent an abundant and diversified fossil association indicative of shallow-marine environments in coastal patch-reefs; the over 630 considered remains are above all tiny carapaces or propodi of chelipeds of 111 taxa within the Gebiidea, the Anomura and the Brachyura. Even if the Brachyura constitute the most represented group with 80 species arranged in 29 families, the Anomura are richly diversified as well, with 29 taxa within seven families, a datum confirmed by the new studied material. In fact the small carapace herein described was found in one of the built-ups considered in BESCHIN et al. (2019) (fig. 1); it is referred to a new genus within the Porcellanidae showing clear affinities with the representatives of the extant genus Polyonyx Stimpson, 1858, adapted to live within the tube of polychaetes.

Geological setting

Parona, a neighbourhood in the municipality of Verona (NE Italy), is situated in the southern part of the Central Lessini Mountains, on the left side of the River Adige. The rocks that crop out north and south of the village and that constitute the core of the mountains, are particularly Eocene lithotypes; they were thoroughly described by BESCHIN et al. (2009, 2019).

During the Eocene in the nowadays Veronese foothill belt, shallow-water car- bonate sediments were deposited in a period of volcanic activity with varying intensity (DE ZANCHE et al., 1977; ZORZIN, 2017).

Lower Ypresian clayey limestones and bedded marls underlie the two most important and fossiliferous Eocene formations for this area: the “Nummulitic Limestones” (Calcari nummulitici) dated as Late Ypresian and Lutetian, with intercalations of volcanic rocks (particularly hyaloclastites and volcaniclastics), for a maximum thickness of about 150 m, and the “Priabona Marls” (Marne di Priabona) Priabonian in age whose residual thickness is about 80 m.

The latter formation with its fossil association rich in discocyclinids and nummu- lites, corals, bryozoans, serpulids, mollusks, echinoids, teeth of sharks (FABIANI, 1915) is typical of a neritic shallow-marine environment, probably not far from the coast, affected by abundant terrigenous supplies. Along the hillside many small coral-algal built-ups with diameters of few meters (2-3 m) heterotopic with stratified marls can be observed: they are constituted by organogenic unstratified micritic limestones/calcarenites very rich in crustacean remains. They are inter- preted as part of patch-reefs in a wide Late Eocene lagoon (BESCHIN et al., 2019).

Institutional abbreviation. MCZ: Museo Civico “G. Zannato” at Montecchio Maggiore, Vicenza, Italy.

Systematics

Order Decapoda Latreille, 1802
Infraorder Anomura MacLeay, 1838
Superfamily Galatheoidea Samouelle, 1819
Family Porcellanidae Haworth, 1825
Genus Eopolyonyx gen. nov.

Type species. Eopolyonyx bretoni sp. nov.

Etymology. Eopolyonyx (m.) is a combination of eo– (from the Greek ηως = dawn), to indicate an ancestral form, and Polyonyx Stimpson, 1858, an estant genus to which the new taxon shows apparent affinities.

Diagnosis. As the type species.

Eopolyonyx bretoni sp. nov.
(figg. 2.A1, 2.A2, 2.B)
2022 ?Polyonyx sp. – BUSULINI et al., p. 43, fig. E.

Holotype and sole specimen. MCZ 7950 (figg. 2.A1, 2.A2) housed in the Museo Civico “G. Zannato” at Montecchio Maggiore, Vicenza, Italy.

Type locality. Parona (Verona – NE Italy).

Type stratum. Priabonian (Late Eocene).

Ethymology. The new species is dedicated to the memory of our colleague, Gérard Breton (Le Havre – France), passionate researcher of fossil crustaceans who passed away in 2020.

Material and measurements. One carapace damaged on its left anterolateral portion: MCZ 7950 (maximum width: 7.8 mm; maximum length: 6.6 mm).

Diagnosis. Carapace transversely ovate, weakly wider than long, swollen. Front slightly protruding, flat and depressed, with convex and continuous margin. Orbits small, shallow, a little slanting. Lateral margins convex; narrow hollow concentric to lateral margins; posterior margin wide and mildly concave. Dorsal regions not defined by grooves. Branchial regions very large and swollen with one blunt large tubercle. Surface smooth.

Description. Carapace transversely ovate, weakly wider than long (width/length ~1.18), swollen. Front not completely preserved; anyway it appears broad (ratio between its width and maximum carapace width about 0.4), flat, a little depres- sed with a rear transverse hollow; it is slightly protruding beyond orbits, with a weakly convex, entire margin in dorsal view. Orbits small, shallow, anterolaterally directed. Fronto-orbital width about three fifths of maximum carapace width. Lateral margins regularly convex; a narrow hollow particular apparent on their hepatic portion runs dorsally concentric to lateral margins. Posterior margin wide, not completely preserved but mildly concave in its median part. Dorsal regions not defined by grooves. Branchial regions very large, swollen with one blunt large tubercle on each meso-metabranchial area. Median regions depres- sed; gastric area triangular, cardiac area narrow and long. Dorsal surface overall smooth, with damaged cuticle.

Discussion. The representatives of the Porcellanidae are characterized by a dorso-ventrally flattened carapace, often widest in posterior half, with various ratio between width and length, dorsal regions usually weakly defined, front with different width and shape, from short to mildly protruding beyond orbits, trian- gular or lobate or transverse, too, sometimes downturned, orbits anterolaterally directed, lateral margins continuous (sometimes with a notch at intersection of cervical groove) entire or spiny; pereiopods 2-4 not chelate and pereiopod 5 reduced and dorsally resting; pleon broad, symmetrical, folded under sternum; telson and uropods developed (HAIG, 1965; SCHWEITZER & FELDMANN, 2010). The specimen herein studied shows features that allows it to be referred to this family and precisely: carapace wider than long; front transverse in dorsal view, slightly protruding, depressed; lateral margins regularly convex; posterior margin wide; dorsal surface with not defined regions.

Porcellain crabs have been already reported with some genera and species from Paleogene coral-associated faunas in Veneto (NE Italy). Some of them are represented only by propodi of chelipeds (BESCHIN et al., 2016, 2018, 2019; DE ANGELI & CECCON, 2017; DE ANGELI et al., 2019), but the coral-algal do- mes here cropping out have yielded carapaces, too: Disipia sorbinii Beschin, Busulini  & Tessier, 2016, Montemagrelisthes prealpinus De Angeli & Ceccon, 2017, Pachycheles dorsosulcatus Beschin, Busulini, De Angeli & Tessier, 2007, Paraporcellana fabianii Beschin, Busulini & Tessier, 2016, Petrolisthes lineatus Beschin, Busulini & Tessier, 2016, Porcellana thomasi De Angeli & Ceccon, 2017 are Ypresian in age (BESCHIN et al., 2007, 2016, 2021a; DE ANGELI & CECCON, 2017); Beripetrolisthes mulleri De Angeli & Garassino, 2002, Disipia sorbinii, Eopetrolisthes striatissimus (Müller & Collins, 1991), E. levistriatus Beschin, Busulini & Tessier, 2018, Lobipetrolisthes blowi De Angeli & Garassino, 2002, Longoporcellana lobata De Angeli & Garassino, 2002, Petrolisthes bittneri De Angeli & Garassino, 2002, P. lineatus, P. punctatus Beschin, Busulini & Tessier, 2019, P. vallionensis De Angeli, Garassino & Pasini, 2019, Pisidia dorsosinuosa De Angeli & Garassino, 2002 are known from Priabonian built-ups (DE ANGELI & GARASSINO, 2002; BESCHIN et al., 2018, 2019; DE ANGELI et al., 2019); Petrolisthes vicetinus Beschin, De Angeli & Checchi, 2001 is lower Oligocene in age (BESCHIN et al., 2001; CECCON & DE ANGELI, 2019). The carapaces of all these species are longer than wide to equidimensional, have a protruding, overall triangular front and an apparent cervical groove (excluding Longoporcellana lobata): these features are not present in Eopolyonyx bretoni gen. nov., sp. nov. The new taxon shows most affinities with the porcelain crabs with a wider than long carapace of the extant genera Polyonyx Stimpson, 1858 and Raphidopus Stimpson, 1858.

Considering only parts that can be preserved as fossils, Polyonyx (type species P. gibbesi Haig, 1956 [= Porcellana macrocheles Gibbes, 1850] – fig. 3) has a weakly ovate carapace, wider than long, convex, smooth usually with transverse striae on posterior lateral part. In the specimens of extant species the front is mildly wide, downfolded, generally trilobate but often straight in dorsal view. HAIG (1965) highlights that P. transversus (Haswell, 1882), a species widespread in Indian and western Pacific Oceans, has a transverse front with an only slightly developed me- dian rostrum. In Eopolyonyx bretoni gen. nov., sp. nov. the frontal margin appears slightly convex, almost transverse and probably a little directed upwards; it is not possible to determine if a very tiny median rostrum could be seen in frontal view (fig. 2.A2). Till now Polyonyx has been reported in the fossil record only with P. arcuatus Müller and Collins, 1991 (Hungary, Late Eocene); its carapace is appa- rently wider than the one of the new taxon, has transverse striae on the posterior branchial regions, its front is strongly downfolded (MÜLLER & COLLINS, 1991).

Dealing with Raphidopus, its carapace is ovate, wider than long with a short trilobate front, only weakly downfolded, lateral margins with a deep anterior notch and spiny branchial part, branchiocardiac grooves sunk, branchial regions with transverse striae (STIMPSON,1858; HAIG, 1965; NG et al., 2012). Most of these characters lack in the new taxon. The four species referred to this genus live in the Indo-Pacific (NG et al., 2012); R. ciliatus Stimpson, 1858, the type species, has been recently found as fossil in Middle Pleistocene rocks from Japan (KATO & KITAMURA, 2020).

The studied carapace  has more affinities with Polyonyx for the general shape of carapace and its size, but is characterized by a continuous transverse front, the lack of ornament on the posterior part of the carapace and the presence of a branchial tubercle: all these features suggest its accommodation in a new different genus within the Porcellanidae.

Conclusions

The family Porcellanidae, is represented today by over 300 species within 31 ge- nera. Porcelain crabs are worldwide spread above all in intertidal and sublittoral habitats of tropical and subtropical regions, and can be found in small cavities under stones or in sponges, coral reefs, or even in worm tubes (RODRIGUEZ et al., 2005; ROBINS & KLOMPMAKER, 2019). Up today they are known as fossils from the Late Jurassic with Vibrissalana jurassica Robins & Klompmaker, 2019 (Ernstbrunn, Austria).

They were reported from the Paleogene in Veneto (Italy) for the first time by BESCHIN et al. (2001) and DE ANGELI & GARASSINO (2002); during the last two decades new findings of carapaces have allowed the identification nowadays of 17 species arranged in 12 genera (the findings of chelipeds are not considered); they make Paleogene porcellanid faunas from NE Italy the richest and the most differentiated considering that now the known fossil genera within the Porcel- lanidae are 17 (DE ANGELI & GARASSINO, 2002; BESCHIN et al., 2019, 2021a; DE ANGELI et al., 2019; ROBINS & KLOMPMAKER, 2019; KATO & KITAMURA, 2020). The known Eocene representatives of the family appear to have already developed a mutualistic relationship with corals as seen in many extant species.

Eopolynyx bretoni gen. nov., sp. nov. shows apparent morphological affinities with the representatives of Polyonyx, which can be found today among coral debris or underside of rocks and many species live in tube of polychaetes (HAIG, 1965; OSAWA & NG, 2016). The new taxon probably was already adapted to a similar environ- ment, considering the presence of tube of worms and of corals in Parona area.

Acknowledgements

We wish to thank Dr. Roberto Ghiotto, Director of the Museo di Archeologia e Scienze naturali “G. Zannato” at Montecchio Maggiore (Vicenza, Italy) for permission to study a specimen under his care. Many thanks to Andrea Checchi for providing photographs and Matteo Calvagno for preparing the line drawing of the studied carapace.

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